What is the actual function of transistor in a circuit?

[Izzy]

What is the actual function of transistor in a circuit?
Can anybody explain lil bit about it?

[Somicide]

I asked this question before, though worded differently.  I asked what the most basic amplification circuit was, utilizing only a single transistor. 

[guitar_199]

while a little simplistic, it serve to think of a transistor as a "current amplifier" or perhaps a "current multiplier".    It must be "biased" that is, given connections to power and ground and then given enough current to cause the transistor to "activate" and bring itself to "sort of" a middle "operating point".  Often capacitors (which block DC) are used to feed a signal to the base(usually the input) and to tap signal off of the collector or the emitter to serve as the output.  What happens is....when a signal is applied to the base it "wiggles" the value of base current act the frequency and amplitude that correspond to the input signal.  The transistor applies it's "multiplication factor" ( called hfe ) and allows that increased current to flow between the collector and emitter.  That "multiplied current" when it flows through collector and/or emitter resistors create voltage drops that are passed down the line.  That voltage is an amplified version of the original input.  Again...this is a dramatic oversimplification but I hope it helps you to visualize what is going on in terms of basics.

[petemoore]

  +/- Voltage Swings = Waveforms...we need links...
 

[Sir H C]

In general transistors can be viewed as voltage controlled current sources.  The voltage across the base-emitter or the gate-source will over a good range allow a set current to go through the collector-emitter or drain-source connection respectively.  The equation that controls this is:

Ic = Is *( e^(Vbe/Vt))-1 for bipolars
Id = Is * (Vgs - Vth) ^2 (I am being sloppy, Is is just a constant based on characteristics of the transistor)

So you can view it as a cool thing where you can control a lot of power with relatively little getting voltage and current gain (or one or the other in different configurations).

[R.G.]

What is the actual function of transistor in a circuit?

Sadly, it is not possible to say what the function of a transistor is in a cirucit without also stating the circuit, or at least some details about it.

The descriptions of transistor action you got are kind of correct - a small current in the base controls a larger current in the collector/emitter for bipolars, a voltage on the gate controls a current through the drain-source channel for FETs. But that does not tell you what the transistor is doing in the circuit. Rather, the rest of the circuit tells the transistor what to do.

A transistor can be an amplifier, which is what most people think of. It can amplify either current or voltage, depending on the circuit conditions, or both current AND voltage.
It can be a switch, being fully off with no current flow, then as fully on as it can be made to do. It can switch DC only, or AC depending on the circuit conditions. It can be a variable resistance in some circuits. And it can be a more-ideal diode than a simple junction diode if the circuit sets it up to be.

So it's not possible to give you a concise answer. You need to know the circuits, then you can say what the transistor's function is.

[Izzy]

ohh Ok, I got some Idea now.
So that mean the function of transistor depends upon the circuit.

So what is the puropse of transistor in this circuit?

http://img135.imageshack.us/img135/3623/creamerbv4.jpg

[GibsonGM]

Looks suspiciously like an input buffer, Izzy.  Now you have to look up "buffer" and "input impedance" and stuff, LOL    Have you built it yet? Try audio probing the output of the transistor; see what it sounds like, that will give you a hint as to the function. 

[amz-fx]

So what is the puropse of transistor in this circuit?

http://img135.imageshack.us/img135/3623/creamerbv4.jpg

Buffers.  See:  http://www.muzique.com/lab/buffers.htm

regards, Jack

[R.G.]

Better yet, since that's the generic circuit of the tube screamer, go to GEO (http://www.geofex.com) and read "The Technology of the Tube Screamer" for an in-depth view of what all of those pieces are doing.

[StephenGiles]

It's to stop folks wanting to upgrade an opamp

[The Tone God]

It's to stop folks wanting to upgrade an opamp

Those triangles are way too complex. I'd rather fiddle around with biasing a little part.

Andrew

[zpyder]

+1 for R.G.'s "Technology of the Tube Screamer"

Another thing to keep in mind is that a simplistic way to think of opamps in an IC is that a single opamp in an IC is (roughly) the same as an entire transistor biasing network + transistor.  That is, to use a transistor as a current amplifier, you need to bias it, as has been said before, between +v, grn, in, & out, using resistors.  the resistors specify how much current/voltage is going where in given transistor, and therefore, how much gain your transistor will yield to the signal put through it.  An Opamp IC is basically a more complex version of this entire network, transistors and all, inside a chip.  Many opamps are dual, in that there are two of these nets in one chip (I mentioned that in another thread of yours)

As for the schematic you provided, the two transistors are used for the input & output buffers, while a single IC containing 2 opamps is used for the gain/clipping stage and tone stage, respectively.  All of this and more is explained in R.G.'s article... w

TGIMFF
zpyder

[markphaser]

http://img135.imageshack.us/img135/3623/creamerbv4.jpg

The diodes in the feedback of the opamp#1 are to clip the waveform asymmetrically see how there is 2 diodes one way and one diode another way in the op-amp feedback stage this makes it clip more in the positive cycle and less in the negative cycle

The transistors in that circuit is a gain of 1 not a gain of zero big difference it does add some tone/timbre difference because of capacitor resistor network before it the signal is inphase on the output of the transitorin that circuit most buffers use FETS because of the very high impedance and low impedance output but a transitor is not really a good buffer

[markphaser]

the 2nd IC op amp is a differential stage it uses both (-) and (+) inputs the (-) input has a tone circuit so u can roll off frequency before the (-) input

The first stage op amp input is a clean sine wave the output of the First IC opamp is a squarewave with clipping so the 2nd IC op amp is going to see a squarewave input. The second IC op amp have a capacitor in the feedback this is a intergation circuit because it sees a square wave input so the output of the 2nd IC op amp is going to be a trianglewaveform with tone roll off curves

[markm]

These may help too;

http://ourworld.compuserve.com/homepages/G_knott/index7.htm
http://hyperphysics.phy-astr.gsu.edu/Hbase/solids/trans.html

[petemoore]

  Most of those web pages about 'transistors' googled are excellent views/reads of transistor innards and functions are great.
'how transistors work' might be a good entry for a search.