understanding transitors

[Dylfish]

hey

just wondering what resources there are that you guys can recomend on understanding how transistors work in a circut. if it covers biasing and the like it would be good too. im really lost on how they work and how i can impliment them myself in a circut.

my mate whos a EE tried to help me last night using the trotsky drive as an example but i think he went into too much detail for my poor head to handle at once.

cheers dylan

[Zben3129]

"There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice.

Question

How does a transistor work?

Asked by: Tony Whelan

Answer

The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.

Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts – OFF.

Semi-conductive materials are what make the transistor possible. Most people are familiar with electrically conductive and non-conductive materials. Metals are typically thought of as being conductive. Materials such as wood, plastics, glass and ceramics are non-conductive, or insulators. In the late 1940's a team of scientists working at Bell Labs in New Jersey, discovered how to take certain types of crystals and use them as electronic control devices by exploiting their semi-conductive properties.Most non-metallic crystalline structures would typically be considered insulators. But by forcing crystals of germanium or silicon to grow with impurities such as boron or phosphorus, the crystals gain entirely different electrical conductive properties. By sandwiching this material between two conductive plates (the emitter and the collector), a transistor is made. By applying current to the semi-conductive material (base), electrons gather until an effectual conduit is formed allowing electricity to pass The scientists that were responsible for the invention of the transistor were John Bardeen, Walter Brattain, and William Shockley. Their Patent was called: "Three Electrode Circuit Element Utilizing Semiconductive Materials."

Reference:
Who invented the transistor?
How do transistors work?


Answered by: Stephen Portz, Technology Teacher, Space Coast Middle School, FL


There are two main types of transistors-junction transistors and field effect transistors. Each works in a different way. But the usefulness of any transistor comes from its ability to control a strong current with a weak voltage. For example, transistors in a public address system amplify (strengthen) the weak voltage produced when a person speaks into a microphone. The electricity coming from the transistors is strong enough to operate a loudspeaker, which produces sounds much louder than the person's voice.

JUNCTION TRANSISTORS

A junction transistor consists of a thin piece of one type of semiconductor material between two thicker layers of the opposite type. For example, if the middle layer is p-type, the outside layers must be n-type. Such a transistor is an NPN transistor. One of the outside layers is called the emitter, and the other is known as the collector. The middle layer is the base. The places where the emitter joins the base and the base joins the collector are called junctions.

The layers of an NPN transistor must have the proper voltage connected across them. The voltage of the base must be more positive than that of the emitter. The voltage of the collector, in turn, must be more positive than that of the base. The voltages are supplied by a battery or some other source of direct current. The emitter supplies electrons. The base pulls these electrons from the emitter because it has a more positive voltage than does the emitter. This movement of electrons creates a flow of electricity through the transistor.

The current passes from the emitter to the collector through the base. Changes in the voltage connected to the base modify the flow of the current by changing the number of electrons in the base. In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector.

Manufacturers also make PNP junction transistors. In these devices, the emitter and collector are both a p-type semiconductor material and the base is n-type. A PNP junction transistor works on the same principle as an NPN transistor. But it differs in one respect. The main flow of current in a PNP transistor is controlled by altering the number of holes rather than the number of electrons in the base. Also, this type of transistor works properly only if the negative and positive connections to it are the reverse of those of the NPN transistor.

FIELD EFFECT TRANSISTORS

A field effect transistor has only two layers of semiconductor material, one on top of the other. Electricity flows through one of the layers, called the channel. A voltage connected to the other layer, called the gate, interferes with the current flowing in the channel. Thus, the voltage connected to the gate controls the strength of the current in the channel. There are two basic varieties of field effect transistors-the junction field effect transistor(JFET) and the metal oxide semiconductor field effect transistor (MOSFET). Most of the transistors contained in today's integrated circuits are MOSFETS's. "

Source - http://www.physlink.com/Education/AskExperts/ae430.cfm

[Zben3129]

as for using them in circuits...

http://www.williamson-labs.com/480_xtor.htm

[Dylfish]

thank you so much mate

[Dylfish]

ok im still having troubles

ive read alot of articles but its not sinking in

using the trotsky drive can someone please explain to me how its doing what its doing please? im really lost atm

i just want to know how the circut ampifies the signal and controls the gain.

thank you for your paitence

http://www.beavisaudio.com/Projects/TrotskyDrive/Schematic_1_14.gif

dylan

[asfastasdark]

I've been trying to figure stuff out forever now, and never have, because there simply isn't a good guide online to show you how a transistor is implemented and what every part of a transistor network does... so I think that next time when I'm buying parts, I'm also buying a couple of trim pots and various NPN's to just test how changing stuff affects the output.

[Dylfish]

im just no EE, but im keen to work out how its all working. its just every guide has a million formulas which make no deal of sense to me. if anyone out there can help it would be a great help

dylan

[sean k]

http://www.audioxpress.com/resource/audioclass/index.htm

Read through the Norman Crowhurst articles starting at part one on Voltage amplifiers and it's a very good starter for understanding the basic premise behind how transistors work. Okay it's tubes which is heaters and thermionic emission to get those electrons floating about in a vacuum but once you get into cathode (emitter and source), grid (base and gate) and plate (collector and drain) it'll start to make sense how those babies actually work.

[demonstar]

im just no EE, but I'm keen to work out how its all working. its just every guide has a million formulas which make no deal of sense to me. if anyone out there can help it would be a great help

What I seem to have found is that one can only follow the basic principle and simple theory of operation without formulae. Once someone wants to actually learn to calculate operating points and design properly you need to learn the equations.

I don't pretend to know those equations I've mentioned. I just know the above as I was asking the same questions a year or two ago and now and then when I have time available I read up more and more on the equations.

I'm pretty sure there are some great posts on this forum so keep searching. Look out for posts in particular by RG, alanan and Jdoyle. Those are a few of the names (well I hope I remembered them correct) that usually provide good answers to questions such as yours.

If you decide you want to learn the maths try getting a book. I've found the ones that unis use to teach are usually good (Can pick them up used a second hand book shops or ebay. I use oxfam). Lately I've been very impressed by the books by Newnes publishers (Borders seem to have quite a few). They're not that heavy though. I can't wait to go to uni next year to do EEE. 

[GibsonGM]

I'd recommend getting really good with Ohm's Law with all its finer implications...understanding how voltage, current and resistance are something like a see-saw.  "Getting" how resistors drop voltage and convert voltage to current is important to transistor theory (very important!).  Keep reading the tutorials and try to catch on to what's really happening with a simple BJT circuit (like on the williamson labs link).  Simulation software is great to see what is doing what, too!!

http://www.phy.davidson.edu/instrumentation/NEETS.htm
Try Module 7....

A transistor is really very simple on the surface, with some not-so-simple equations that govern its behavior.  In the basic common emitter amplifier config., a small current flowing from base to emitter causes a larger current to flow from collector to emitter. The output is taken from the collector, where a load resistor converts the current to a voltage, with a 180 degree phase shift ( when input is positive, output is negative - the signal is inverted).  Design with them is a little tougher, but essentially the current you want to flow from the collector is set, the emitter resistor(s) are chosen for stability (100 ohms+), Ohm's law is used to determine the voltage at the emitter for a given current....adding .7v to this shows you how to bias the base with a voltage divider (the transistor must operate in a region that will allow the input to swing without turning the damn thing off).  Then the collector resistor comes into play, governing what the output voltage will be when the current is converted to a voltage.  It's a little game, some mental juggling, ha ha. 

Keep at it, and you will eventually get it...I'm 40% there myself!  The concept of thinking in current is a little weird, but it comes in time.  Actually SEEING outputs for a given input with simulation is invaluable.     

[Dylfish]

cheers guys :0 starting to get it now