The biggest thing with transistors is that they no longer follow ohm's law. We will now start working with elements that follow relationships not governed by simply V=IR. The relationships between our voltages and currents will be determined by transistor configuration. KVL and KCL will still hold, but where going to have to be careful when we use them.

The Theory
Transistors are made possible by semiconductors. A semiconductor is a material that can be in a conducting state, or an insulating state based upon the voltage applied to it and its intrinsic properties.

Charged Silicon
Silicon by itself, is neither a good conductor or insulator. Its just a pretty decent resistor. One of the things we can do is to "dope" silicon. doping is when we add impurities to the silicon to change its conductive properties a little bit. What gives silicon it's conductive properties is the fact that silicon likes to bond to itself and form a crystal lattice. In this lattice, the silicon's valance electrons are all shared with neighboring silicon atoms. Since all the spare electrons are help up in this bond, any extra electrons (your electric current) have a hard time jumping from silicon to silicon. However, the energy needed to bump an electron out of its orbit, isn't that much, so there still is a flow of current through regular silicon, it just isn't that much.

When we dope silicon we take atoms which have one extra valance electrons, or one fewer electrons. What this does is either gives us a material with an overall positive charge (P type silicon) with excess holes (positive charged nucleus) or overall negative charge with excess electrons (N type silicon) This excess of movable charge allows current to flow. So p type and n type silicon are themselves decent conductors. How conductive a piece of p-type or n-type silicon is, is determined by carrier concentration and charge mobility (along with other factors). We can choose carrier concentration by how much we dope the silicon and mobility is determined by the majority charge carrier (electrons for n-type and holes for p-type). Naturally, electrons are more mobile so n type is usually more conductive than p type with similar doping. This is all fine and dandy, and is great science for making really good resistors, but to make a silicon device we have to look at the barrier between a piece of p-type silicon and n-type silicon.

Band Theory for dummies
I am not going to go in depth into band theory, but I will go over as much as you need to know to have an understanding of our devices. We know that like charges repel and opposite charges attract. Charges in our silicon are mobile and can move around. When we put a piece of nSilicon and a piece of pSilicon we form what is called a depletion region. The holes combine with the electrons and we get neutral charged silicon. This forms a barrier between between the pSilicon and nSilicon and charge no longer flows. Because we have a separation of charge, we have a built in potential and this is the key to silicon devices.

PN junction diode
The simplest device we can have is just a PN junction and with that junction we form a diode. A diode is a device that allows current to flow in one direction.

We discussed how a PN junction has a natural build in electric field caused by the separation of electric charge due to the creation of a depletion region. If we are to sketch this as voltage across the diode we get something like:

When we apply a voltage to out diode, that voltage will affect our built in potential. If it is the same polarity as the built in potential, our diode's potential hill increases and doesn't allow current to flow:

However, when we apply a voltage, opposite polarity of our built in potential, we create a potential hill that allows our current to flow. However we lose the built in potential to our diode and the voltage across our circuit is the voltage of our source minus the built in potential voltage:

Thus, a diode allows current to flow only in one direction, the one which opposes the built in potential and creates a potential hill that our electrons may pass through.

So here we see how a PN junction in silicon creates a useful device. This is the basis for all silicon devices. By taking advantage of the geometries and modifying the band structure of a crystal we can build silicon devices to do almost anything.

The transistor.
A transistor is another type of silicon device. For all intensive purposes there are two transistors that we are concerning ourselves with, the Bi-polar Junction Transistor (BJT) and the Metal-Oxide-Semiconductor transistor. Both transistors have the same general principles (they can both act as switches and both are amplifiers) however, they operate on vastly different mechanics and each have their own advantages.

The BJT
A BJT is made with three layers of semiconductors either NPN or PNP. The arrangement determines it's polarity. For the purpose of the rest of this document, we will be working with NPN transistors, for PNP everything is the same except we invert the biasing. (i.e. we now need -.7v to “turn on” the transistor). BJTs were the first practical transistors (however not the first theorized) BJTs have great transconductance (the ability to control output current with input voltage) and have some nice features which make them very practical to model. However, BJTs are trickier in the fact they have finite input and output resistances which must be take account for. All BJTs have three terminals, the base collector and emitter.

Principle of operation
The way a BJT works is that a signal on the base of the transistor allows a larger current to flow from the collector to the emitter.

A transistor is a device that is not linear, and it doesn't follow one equation the way, say a resistor does.

Depending on several variables (Base Current, Collector-Emitter Voltage) we get different Collector currents.

Now, there are three modes of operation worth looking at:

-Cutoff: This mode is where there is no base current present and the transistor is off, no current flows from collector to emitter

-Saturation: This mode is where the transistor is fully conducting, this is the same as a logical 1.

Switching VS Amplification
The most used aspect of a transistor is its ability to be an electronic switch. The philosophy behind this is that we can use a small signal from say a microprocessor to control large electric objects like DC motors (For more on this check out mcuplace.com). However, for this blog we will be looking at transistors as amplifiers and because of that, the most useful region of operation is the forward active mode.

Forward Active
Typically to be in the forward active mode both the base-emitter diode is forward active (>.7v) as well as the base-collector diode. We also have the constraint of collector-emitter voltage >.3V. As long as we satisfy those three conditions we are in forward active region. When the transistor is in this mode, it behaves by the following formula

Where Beta is a parameter of the transistor.

Transistors are complicated devices in that there's no simple way to figure out whats going to happen with a circuit. When designing a circuit, we generally have goals we'd like to accomplish and we use the following above as assumptions to find the collector current. Once we know the collector current, we can see how variations in base current (voltage) produce variations in collector current (voltage) allowing us to amplify signals. Before we can do that, we have to model the transistor in a static DC case and figure out some parameters.

Q-Point and biasing
Biasing is the act of choosing our DC base current so that we can get proper signal amplification. We can bias our transistor in many ways. The result however is to find our Q-point base current giving us our Q-point collector current and allowing us to calculate parameters for signal analysis.

So lets start with a circuit:

In this circuit, we have a voltage divider hooked up to the base, and we have a collector resistor and an emitter resistor. The voltage divider allows us to set the base voltage, which determines our base current, and our collector current.

Quick Dirty Analysis

Analyzing a circuit like this can be a nightmare, if we take out time and stick to what we know, we can analyze every aspect of this circuit and find the exact currents and voltages... OR we can make a bunch of assumptions and get a good enough approximation.

First thing we know, our VCC is 5V, with our voltage divider we get a VB of 2.5V. Since the voltage drop across VBE is .7V we know VE is 1.8V. The current that goes through RE is the same (approximately) as RC so 1.8/1k is 1.8ma. Thus Ic is 1.8ma. Now we know our collector current we can go on to do signal analysis.

Checking our approximation

The easiest thing we can do, is use PSPICE to verify our circuit.

As you see, our guestimations are pretty close to what Pspice decided to spit out. So lets start analyzing what happens when we put a signal on our circuit.

Signal Analysis

For signals, we use a different type of model, one that allows us to not worry about recalculating every parameter of our circuit at every instance of our changing signal. For this type of model, we simply find the q point characteristics and shove it into our model. Lets take our circuit and modify it a little bit.

We've added quite a bit to our circuit but all will be explained in a moment. First off, we added an AC source which will be our signal. Like all sources it has a finite output resistance RS. We add CC1 as a Coupling Capacitor, this isolates our AC and DC parts of our circuit so that current from our biasing circuit doesn't flow into the source. This works because fundamentally, capacitors act as a DC block, they only allow AC to flow. Next, we add CE, this capacitor will increase our gain of the circuit and will be more apparent once we build our model. Finally we add a load resistor and another coupling capacitor, again we want to separate the AC and DC portions of our circuit.

Hybrid-Pi Model

What our model does, is take our complicated transistor and reduce it to a few resistors and a current source like such:

(photo curiosity of wikipedia)

This model allows us to replace the transistor in our circuit with the model above (note the Base Collector and Emitter nodes)

To build our model with have to define a few parameters:

: This is our imput resistance defined as where Vt is the thermal voltage and Ib is base current

Ro: Output Resistance, this is defined as where Va is the Early voltage(intrinsic parameter) and Ic is the collector current.

Gm: Transconductance, is defined as

Thermal Voltage is a relationship derived from Boltzmann's constant that correlates to the intrinsic voltage in a semiconductor junction. It is dependent on temperature, but for most cases it is 26mv.

Early Voltage is a theoretical voltage, that relates to our output resistance. For the transistor in the circuit the 2n2222 the early voltage is around 100v (you can look this up in the data sheet hopefully)

Ib, the base current should be . is approximately 150, but we can just use our PSICE values.

Ic, was shown to be approx 1.8ma.

With those parameters we find:

gm ~ .07 a/v

So lets construct our signal circuit with the above parameters. Now, we are building a circuit for signals and thus there are a few things to be aware of:

With respect to a signal in circuits, power and ground are the same thing, thus in our above circuit with the voltage divider, those resistors are actually in parallel with respect to a signal, thus we can convert them to a common resistor to ground.

Capacitors are a low impedance connection with respect to AC, thus they are shorts to our signals. The purpose of CE becomes a little more clear, rather than connecting to ground through RE, we can connect R pi directly to ground increasing the voltage drop across it and the signal gain.

With those considerations our circuit becomes:

First thing we want to calculate is our voltage gain AV

For our circuit Av = -35.

Thus any input signal should be amplified -35 times. Note the input voltage is after the signal source, with our current circuit the output resistance of the source dwarfs the input resistance of the amplifier, thus we have a large voltage drop across the source resistance and very little across our amplifier... oops...

Three types of amplifiers.

The amplifier we just analyzed is called a common emitter amplifier. We call it this since the emitter shares a connection to the input and output (namely the ground) There are two other types of amplifiers, Common Base and Common Collector. All can be analyzed using similarity techniques and all have different properties.

Common Emitter: Large voltage gain but worst bandwidth

Common Collector: Large current gain, Large input impedance, low output impedance, unity voltage gain.
Common Gate: Best frequency response, low input impedance, unity current gain, low voltage gain.

Now, each amplifier excels at one task, but they are most useful when combined together. The idea behind cascade amplifiers is to use buffers and these amplifier configurations to maximize performance and gain while no suffering the drawbacks of any one design.

MOS transistors
Metal-Oxide-Semiconductor Transistors are very similar to BJTs in the fact they act as both amplifiers and switches. Even the design methodologies are very similar. However the design principles are completely different. MOS transistors, like BJTs have three leads. Gate (base) Drain(Collector) and Source(emitter). However the similarities stop there. Due to the sheer size of this article, I will hold off on talking to much about MOS transistors.

I hope this article served as a nice guide into doing design work with transistors, I've barely scratched the surface when it comes to using these devices. Note we've only covered one mode of operation and barely at that. There is much more to learn about these devices but unfortunately, I don't have the time or knowledge to do so at this time. I look forward to posting much more on this topic as well as exploring more theoretical topics and possibly even device fabrication.

Typesetting vs WYSIWYG:

When you create a document you have two choices, you can use a program like Microsoft Word and how you type the document and format it on the screen is how it comes out (WYSIWYG) or you can use a language like HTML where you use tags to get desired formatting (a Typesetting language)

:

Latex is a typesetting program which is widely used in the physics and engineering community, it uses pre-formatted packages to adhere your document to certain guidelines but also allows you to customize every setting. Instead of trying to explain what Latex is, i am just going to provide a sample document in which i used Latex to make.

Sample Lab Document

Here is the .tex document that we will be referencing through out the post.

Sample tex document

As you can see, Latex makes good looking documents, fairly quickly. Latex has a whole host of useful typesetting features for mathematical formula, Greek letters, operators, figures, symbols and operations. It is a very powerful tool which will help you in any situation where you need to make a professional looking document. How it works: Texmaker is what i use to write my Latex documents, however a basic text editor is all you need. After you write your tex file (with .tex extension) you use Latex to "compile" it into a .dvi, .ps or .pdf format. Just like with any program any syntax errors will result in a compile error. So lets get into the code.

The Preamble:

The preamble of the latex document sets up the basic guidelines the document will follow. i.e. the author information, the font packages to use, other misc packages to use and the document type.

\documentclass[10pt,a4paper,titlepage]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{color}
\usepackage{graphicx}
\usepackage{epsfig}
\author{Anthony Tricarichi:106718074}
\title{Lab 09: Radioactive Decay}
begin{document}
\maketitle

In Latex we precede each command by \. Document class tells Latex what font to use, the type and other document settings. Usepackage tell latex what packages to use (use the ones i use as default). Author sets the author information for the document, Title sets the title. Like HTML uses tags, latex has something similar.

Environments:

Latex uses environments to guide the flow of a document. When in an environment Latex follows certain formatting guidelines. We start our document with the document environment, we enter an environment with \begin and exit it with \end In the document environment we denote section with \section{} and subsections with \subsection{}

Tabular Environment:

To create tables in Latex we use the tabular environment.

\begin{tabular}{|l|l|l|l|}
\hline
Column 1& Column 2 & Column 3 & Column 4 \\
\hline
\end{tabular}

We enter a tabular environment with \being{tabular} it accepts a parameter which tells latex how many columns the table will have {|l|l|l|l|} | denotes a horizontal line and l denotes left-justified. r,c,l are all valid parameters. \hline denotes a horizontal line, & denotes a column break and \\ is a new line. That makes the following table.

Math environments: One of latex's most powerful features is it ability to create very elegant and professional math equations. There are several ways to enter math mode in latex. If you want to use math environment tags in your regular document you can temporarily use $ to enter math mode and then another $ to terminate it. A more permanent environment is the equation array environment. It is very similar to the equation environment but allows you to align your equations like a tabular environment.

Math commands:

In math mode, you have access to commands that let you build your equations. +,-,/, and * all have their own symbols and formatting.

^{} is for superscript

_{} is for subscript

\frac{a}{b} is for fractions of the form

There are many more commands which i will outline at the end of the document.

\begin{eqnarray}
P_{abs} &=& P_{atm} + \rho gh \\ \nonumber
&=& 1.01*10^5 + 10*1000*.2 \\ \nonumber
&=& 103000 Pa \\ \nonumber
m_{slope} &=& \frac{\Delta x}  {\Delta y} \\ \nonumber
&=& \frac{0-100}{91807-129549} \\ \nonumber
&=& 377.42 \\ \nonumber
Percent Error &=& \frac{Experimental - Actual}{Actual} * 100 \\ \nonumber
&=& \frac{-243 - -273}{-273} * 100 \\ \nonumber
&=& 10\% \\ \nonumber
\end{eqnarray}

Generates the following -

End note and other commands:

Latex is a fairly simple typesetting language, if you take a look at my sample .tex document you should have all you need to make a Latex document.  This has only been a crash course into latex, more in-depth support on the working of Latex can be found almost anywhere on the internet.

COMMANDS!

\\ - Line break

\footnote{}

\(insert greek letter name) - Insert a greek letter

Kirchoff's Voltage Law:

Voltage is defined as the potential difference between two points due to an electric field, because of this field there is potential energy to do work (Voltage).  Like all other potential energy this one is also related to position or location. To help realize this we can use an analogy to gravity.

With gravity, potential energy is directly related to how high an object is, as you move an object higher it gains potential energy. However, if you take an object at some height, raise it up and bring it back to your starting position the potential energy is the same, despite the fact you had a gain in potential energy, you had the same amount in loses to return the object to the starting position. The net of this motion is NO change in potential energy.

Voltage works the same way. KVL states that if you have a circuit, and you go around a loop in that circuit, the net change in voltage is zero. We express this with the following formula:

Lets make use of this law, consider the following circuit

What we do is define some variables, we let the current in out circuit be denoted by I.

Here we sum all the voltage drops, because the voltage source provides a voltage and we are summing over the drops we make it negative. Of course due to Ohm's law (eqn 1) the voltage across the resistors is I times their resistance.

Mesh analysis:

If we take KVL one step further we can solve some pretty advanced problems involving many currents and resistors. Mesh analysis is all about taking the loops and assigning each of them their own current (called mesh currents) and we keep track of each current in each loop using KVL.

Ill try and make this as clear as possible, but unfortunately my paint skills are limited. Consider the following circuit with three mesh current, I1,I2 and I3. Each current is positive in the counter clockwise direction.

Phew! If you plug all the currents back in however, they all check out. The work looks alot more tedious than it actually is, Mesh analysis is a very useful way of making good use of KVL.

Kirchoff's Current Law:

The second most important law in EE is KCL or Kirchoff's Current Law. This law like KVL has an analogy in physics. Its very common knowledge that energy and mass are always conserved in a physical system, simply you cannot create or destroy energy (it can only be transferred one place to another). With circuits, we have conservation of charge or simply that the charge into a node has to leave that node. This can be expressed as:

Or that all the current into a node = 0 and all the current out of a node = 0 Consider the following:

KCL states that the current coming from the left of the node has to add up to the current going through both resistors, in this case its trivial, each resistor draws 100mA of current and I1 = 200mA and I2=I3=100mA.

Nodal Analysis:

Nodal Analysis is where we define each node in a circuit. With these definitions and KCL we can find all the node voltages and from there we can define the circuit. Lets take our previous example, instead of using mesh currents we use nodal analysis.

As we look at our drawing, we can make some pretty clear distinctions, V1 is 5V, V3 is 10V and the current through R3 is 5mA. Using KCL at V2 node we make the following conclusion.

You can see the power of nodal analysis, for this circuit it is much easier to do a nodal analysis than it is a mesh one.

Recap:

KVL is that the sum of the voltages in a loop must equal zero (Change in potential in a loop is zero)

KCL says that charge is conserved so, the current into a node must also exit that node.

Mesh analysis is an application of KVL where we define mesh currents and find them using KVL

Nodal analysis is an application of KCL where we define node voltages and use KCL to find the remaining variables.

With these tools in hand, you can analyze almost any electrical circuit, even if you cannot follow the math, having these principles in mind make circuitry more intuitive.

We will be continuing with op amp circuits in the next upcoming posts and then moving onto more advanced topics, if you would like to see a certain derivation or any specific topics email me at Jfkfhhfj@gmail.com

An op-amp typically has 5 inputs:

+ : The non inverting input
- : The inverting input
V+: Positive rail supply voltage (not shown)
V-: Negative rail supply voltage (not shown)
V_out: Output Voltage

Operation

As said earlier, the op-amp is a simple device, it applies a voltage gain on the voltage difference between the + and - pins. Gain is denoted by A and is typically around the scale of 10^6. Now, this might be a little confusing, does this mean the op-amp will output 10^6 volts when a 1 volt difference is applied across the + and - pins? No, you are bound by the positive rail and negative rail supply, you can never surpass those.

So if that's the case, wont a few millionths of a volt cause the op amp to go completely to one rail? This is true however we must make a few idealized assumptions about op-amps so that we can properly analyze op-amps and once we do that we can understand what feedback is.

The idealization and analysis of an op-amp

The best way to go about analyzing an op-amp is to break it up into two parts:

The input: The input of the op-amp are the inverting and non-inverting inputs, here we make our first assumption. The resistance between these two points is infinite. This means that no matter what voltage is on either pins no current flows between the two points. This is our second assumption, no current flows into or out of the input pins.

The output: Taking our idea on how the op-amp works we get the following formula:

We can turn the Vout pin into a dependent voltage source (i.e. it is a voltage source which is dependent on a voltage somewhere else in our circuit) going through a resistor. The output voltage is governed by the equation above.

Our final assumption is that, this resistor is 0 ohms.

Our ideal op-amp

Taking our four assumptions:
-Resistance between + and - pins is infinite
-No current flows into the + and - pins
-Vout = A*Vin
-Output resistance is zero

After we make these assumptions we get the following circuit

Now what?

A simple amp using feedback

Now the greatest power an op-amp has is its ability to use feedback to create some neat effects.

Lets say we input a voltage to the + terminal and then have a resistor from the output feedback to the - input.

Analysis

Lets start off by reminding ourselves of our assumptions, We know that the resistance is infinite between the inverting and non-inverting inputs and because of that no current flows. Because of that, the + and - pins are held at the same potential.

So lets look at our circuit. We see that the output voltage is hooked up in a voltage divider configuration with the - input. We recall for a voltage divider that:

or that....

Taking our original formula

We find for this circuit

This circuit represents controllable gain, which is extremely useful.

Many basic signal amplifiers work off of this very practical circuit.

Where do we go from here?

Operational amplifiers have many other applications to signal processing beyond just a simple controllable gain circuit.

Many other uses of op amps include:

-Signal Buffers (Zero gain amp circuit)
-Summers
-Differentiators
-Integrators
-Oscillators and Waveform Generation
-Much more!

We'll be going over uses of op amps as we use them.

A Step back

In the LDR post i posted about how you could use an operational amplifier to get more precise triggering of the LED light. Ill go over this briefly.

If you apply a voltage to the inverting input and another to the non inverting:

If V+ > V- the output will be the positive supply rail.

If V- > V+ the output will be the negative supply rail.

Thus if we hook up the LDR to the + terminal (in a divider configuration) and apply a voltage the the - input (via a divider or voltage source) we can get a binary output from the LDR sensor.

Formula sheet

Ideal Op amp:

Non-inverting amplifier output voltage:

I would like to demonstrate a very simple yet powerful circuit, a light sensor using an LDR.

A LDR is a Light Dependent Resistor, this circuit element varies its resistance based upon the amount of light that hits the device.

When we hook up this element in a voltage divider configuration, we can make a high and low signal based upon how much light hits the sensor. This is further filtered by using an NPN transistor in an inverter configuration to generate a purely digital signal.

This circuit allows you to produce a 1 or a 0 that correlates to whether the sensor is subjected to light or is in shadow. The amount of light that triggers the sensor is based on resistor R. (the resistor hooked up in the voltage divider with the LDR)

The circuit:

The schematic above was drawn with EAGLE, a CAD software for designing electric circuits as well as Printed Circuit Boards(PCBs)

Eagle is a very powerful tool and can make designing complicated circuits a breeze. Its much easier to draw a circuit out on eagle than to fumble around with a breadboard guessing and checking along the way.

The first thing you want to do is to create a new project file and make a new schematic.

The Blank Eagle Screen:

This is what comes up once you create a new schematic. Your first job is to start adding devices to your circuit. This is done by pressing the add devices button.

Once you do that the following screen comes up:

You can begin typing what part you need, first thing we're going to do is assemble the power supply so we have a steady potential across our circuit. We want a positive voltage regulator in a TO-220 package. A TO-220 package is fairly common package as its cheap and very easy to work with and we don't have to worry about overheating too much because the package can handle a lot of heat. Most of the time as well the package has a mounting hole for a heat sink a swell.

Once we select the part we want we can add it to our circuit. For the power supply were going to need two capacitors and a voltage regulator as well as inputs for our voltage from our source.

Once we place all the parts we have to hook everything up. For this, we use the wire hookup tool. We link all the parts like we normally would in a circuit with wire. We have to take one more step however, we have to use the node tool at any junction where two devices connect. Just because a wire and a device touch, doesn't mean they're linked. We must use the node tool to link the devices and make a connection. This is very important as unlinked nodes will produce an improper PCB design when we eventually get to that stage.

One thing you'll notice is the GND and VCC "devices." These are symbols we use to note common points in the circuit. We use a GND symbol to denote the ground. Therefore, any point in the circuit that connects the ground, connects to this symbol. This way of designing our schematic makes things look cleaner and allows us to separate functioning parts of our circuit into blocks that can be modified separately.

What the power supply does:

The power supply provides us with a known potential. When building this circuit we are going to use a 5 volt positive voltage regulator, which on the output gives us a steady 5 volts irregardless of the input voltage (assuming its DC and within specs).

The two capacitors act as filters, filtering out noise that might be introduced on our line. The capacitors in that configuration are called decoupling/bypass capacitors and are a type of low pass filter.

The LSP1/2 devices are nothing more than pads to attach our voltage source.

Once we have a steady source of power we can begin doing analysis.

The voltage divider.

The first part of our circuit is a voltage divider.

This allows us to output a voltage that is proportional to the voltage source.

Since

And

Thus

And

Then

From that relationship, we see how the voltage is proportional to the resistor R we choose and the resistance the LDR provides.

Using a multimeter we can find that the LDR puts out a resistance of about 200 ohms when in light and around 5Kohms in the dark.

We want to have a system where the change in V is the greatest for on an off conditions.

Assume we use a 1Kohm resistor for R, That gives us:

And for the off state:

This gives us a nice kick to switch our NPN transistor to solid on/off states.

The Inverter.

BiJunction Transistors are a type of "switch" they amplify current by allowing a large amount of current to flow via a small trigger.

The most important character of a BJT is its current gain, which is how much the BJT will amplify current before it reaches its limit.

For this setup we are going to be using the TIP120 NPN BJT.

Data sheet: http://www.learn-c.com/tip120.pdf

BJT usually have 3 leads; a base, collector and emitter.

The way a NPN BJT works is that a small current on the base will amplify a current from the collector to the emitter.

Now, in reality, the BJT is much more complicated than that, but for this post, that information should suffice.

The way our transistor is hooked up, is in an inverter configuration, which means when the transistor is OFF the device receives power, when the power is turned on, the transistor passes the load to ground, giving the device a 0 potential.

When our LDR is ON, there is a large amount of current going into the NPN transistor, this saturates the gate, and the LED doesn't light. When the LDR is off however, there is very little current going into the base of the transistor, thus there is very little voltage drop across the transistor and the LED gets power just as if it were hooked up to the resistor alone.

Operating Pictures:

We setup the project on a breadboard, which allows us to prototype the circuit. Instead of permanently soldering components to a PCB, we can just insert the leads of the components into the breadboard and use the internal network to connect our circuit.

The parts list for this circuit are:
2X - 2.2Kohm Resistors
1X - Blue LED
1X - TIP120 NPN Transistor.
2X - 15uf 50V electrolytic capacitors
1X - LM7805 5v pos Voltage regulator
1X - Diode
1X - 9v (or any DC voltage source)
- Some hookup wire.

The LED is off, which means the voltage divider is high, and the NPN transistor is ON. Current is being drained to ground, instead through the LED.

In the next setup, we cover the LDR, producing a low on the voltage divider, turning the transistor off, allowing current to flow to the LED.

Conclusion:

This is a very useful circuit and allows you to understand basic circuit elements. Light sensors are very useful in robotics in object detection and any other circuit where your application is sensitive to light.

- Alternative designs using op amps!

AC: Alternating current, circuit that deal with a cyclic harmonic change in potential over time. Us outlets are 110 VRMS 60HZ which means the average voltage put out is 110 volts and it oscillates as a 60HZ sine wave.

DC: Direct current, current is steady and a fixed potential is across two points. We use DC for most circuits.

Voltage: Potential difference across two points, Voltage is the work a circuit is capable of doing and is the product of an electrostatic attraction between two points.

Current: Is the charge that actually "flows" in a circuit. Voltage is the work capable current is what does that work.

The Three Fundamental components:

The Resistor: The resistor allows you to control current in a circuit, it is probably the most essential component. (V=IR)

Capacitor: Two plates separated by some sort of dielectric, Holds a charge and useful in storing energy and filtering signals.

Inductor: Coil of wire that holds energy in a magnetic field. Any current flowing through a wire produces a magnetic field around it.

The Fourth fundamental component?:

Resistors, Capacitors and Inductors are all useful things but it wasn't until semiconductors that we truly were able to create high speed circuits of today.

I've already done a writeup on semiconductors here:

http://mcuplace.com/mcu/blog4.php/2008/08/20/semiconduct-this

What is circuitry?

Circuitry is the process of taking these components and making circuits that do useful things. We can make sensors that record data, Counters, Timers, Computers, Calculators, Robots. We can make pretty much anything you want to with these components.

What we will be covering in this blog is a hodgepodge of information from digital circuits, to circuit analysis to schematic design and introduction to software suites.

Basic Symbols:

Part of reading a schematic is understanding what the symbols mean and how to interpret them.

Voltage Source / EMF

A battery or DC voltage source is denoted by parallel lines of varying size. The longer line denotes + and smaller line denotes -

Passive Components:

The symbol for a resistor is a zig-zag line.

Potentiometers or variable resistors are a resistor symbol with an arrow through it

The symbol for a capacitor is two separated parallel lines

Electrolytic capacitors may have a curved line or a + sign to denote polarity

The symbol for an inductor is a half coil

Semiconductors

The symbol for a diode is a triangle with a line at the tip, the triangle *points* where the current flows and the line is usually on the negative part of the diode.

The symbol for an LED is a diode symbol with arrows coming out from the triangle denoting it produces light.

Transistors

NPN Bi-polar Junction Transistors are denoted by the following symbol

PNP transistors are denoted by

NPN can be remember because the arrow is Not Pointing iN, and PNP is the complement of NPN

MOSFET transistors share a different symbol than BJT transistors.

NMOS transistors are denoted by:

PMOS are denoted by a NMOS symbol with a complement dot in the front.

MOSFET transistors also have different symbols but all use the same basic shape.

There are many other symbols out there for electric components, however these are the most common. As we demonstrate more circuits we will go over the symbols as we get to them.

We will be going over some basic circuits so we can get the hang of designing circuits and doing analysis.

Faraday's law of induction states that the EMF produced is the result of a change in magnetic flux with respect to time.

An inductor is nothing more than a piece of copper wire wound as a coil which makes it more permeable to magnetic flux.

An inductors job in a circuit is very similar to a capacitor it stores energy for later use. As current flows through an inductor, the current builds a magnetic flux around the inductor. This magnetic flux was the product of induction and will try and resist any change in current due to Lenz's law.

An inductor is a very simple and crude device, it is used in very similar applications to a capacitor.

LRC Circuits

a very popular application to an inductor is the creation of an oscillator using a capacitor and an inductor. Putting these two components in series with one another will create an oscillator at a certain frequency, this was primarily used in radio tuning as you had an inductor hooked up to a variable capacitor to turn into certain frequencies. LRC circuits can get much more complicated, but that is for another time.

Transformers

The principle role for an inductor is the transformer, a device that is used to modulate voltage across two circuits.

A Transformer is simply two inductors coupled via a piece of iron.

A transformer only works with an alternating current as a constant DC current will not produce a changing magnetic flux and thus wont create an EMF.

The voltage change across the transformer is proportional to the number of turns difference across the transformer.

N = Number of turns

Note about Inductors

Now, just as a capacitor is any break in your circuit, an inductor is ANY connection in your circuit. This is another problem that plagues analog circuits. Every wire is going to be creating a magnetic flux and every wire is going to be inductively coupled some how. This wont destroy your circuits, but it is something to be keen of when designing a circuit.

Formula Sheet

Faraday's Law

Transformers

The capacitance is a unit of how much a capacitor can "hold" which is a product of permittivity and area by the distance of the plates. Thus as area increases capacitance increases and as distance between the plates increases, capacitance decreases. Capacitors are measured in farads with one farad being 1 coulomb of charge across a capacitor with a potential of 1 volt across it. A coulomb is a large amount of charge so capacitors are usually measured in microfarads

The permittivity is a product of the material between the plates, in simple cases it is air, other cases it can be a sophisticated dielectric.

Now capacitance is a product of any two points in a circuit with a potential across them, separated by any distance. Although when d is a large value, capacitance is low, this still has an effect on a circuit. It is an important consideration how capacitance will affect your circuit since everything is a capacitor. Besides causing some unwanted affects, a capacitor can be a useful component.

Conservation of charge:
A Capacitor holds charge, unlike potential which is the product of an electrostatic force, charge is a "physical" thing, you cannot lose charge. This is very important since charge is a principle part of a capacitor.

Capacitor in Series:

Capacitors in series the capacitance add as the inverse so:

One advantage to using capacitors in series is that you can pass higher voltages across lower voltage capacitors (I.e you can pass 200V through 2 100V capacitors in series)

Capacitors in Parallel:

Capacitors in parallel the capacitance adds across the capacitors.

Capacitance is determined by the area of the capacitor, naturally more capacitors in parallel will have more area, which is why capacitance adds.

Work and Energy:

Capacitors can hold a charge, which means they can store energy. The energy associated with a capacitor is defined as:

This is useful in finding how much energy is stored in a capacitor, Power is the rate consumption of energy, thus by finding the energy stored in a capacitor you can find how much power it has.

Timing:

The largest use of a capacitor is in timing. It takes time to charge the capacitor, which can be used to your advantage as a timing source. The most simple resonator is a RC circuit. Capacitors are used in oscillating circuits which are used to produce specific frequencies. Also capacitors can modulate frequency due to the fact that charging a capacitor is time dependent.

This is a basic intro to the capacitor, I hope to go over more in the future.

Formula Sheet:

Capacitance:

Capacitor in series:

Capacitor in parallel:

Potential energy of a capacitor: