How to calculate gain in transistor circuit

[ItZaLLgOOd]

Is there a simple calculation to determine the gain of a transistor circuit?  I understand the op-amp calculation but can't seem to grasp the gain of transistors.  Thanks for any help

[R.G.]

There is a simpl-ish way. In circuits where there is enough gain from the transistor, the gain approaches the collector resistor divided by the unbypassed emitter resistor.

Essentially the same current flows in each one. The emitter resistor MUST be the same signal as the base minus a gnat's eyelash, as it's a follower. Same current, bigger resistor, voltage gain must be in the ratio of the resistors. If you bypass all of the emitter resistor, there is still an unbypassed emitter resistor inside the transistor, the Shockley resistance of about 26mV/Ie. The gain increases to Rc/Rsh.

Notice I said transistors, not  FETs. MOSFETs do almost exactly as I said. JFETs have some issues. JFETs have much lower gain than bipolars, so they have lower gain to start with and lower follower gain. JFETs will show much less gain than bipolars.

FETs really should be approached from the standpoint of transconductance. The transconductance of a JFET is how much the current in the channel changes with a certain change in gate-source voltage. JFET transconductance is quite low, so with a source resistor and a drain resistor, the gain will be much lower than Rd/Rs in general. MOSFETs have such high transconductance that the approximation of Rd/Rs works fine for them.

But that's probably more complicated than you were looking for.

[Brett Clark]

RG is, as usual, correct. I would add, though, that the effect of low transconductance in FET's is pretty easy to estimate. Add the reciprocal (that's  1/X on your calculator) of the transconductance to the value of Rs.

The transconductance of an MF102 is about 5000 uS (that's microSiemens, where a Siemen, also called a mho, is 1/(1Ohm) ). SO put 5000E-6 in your calculator, press 1/X, and get: 200 Ohms. That "ghost resistor" is effectively in series with the source terminal of your JFET.

Therefore, the gain without the bypass cap is about: 1800/(560+200) = 2.4

At frequencies where the bypass cap is in full effect, the gain is about  1800/200 = 9

[ItZaLLgOOd]

Seems simple enough 

So if I have this right and we look at Arons beginner project we have a 10k collector resistor and a 5k pot with a bypass cap.  So we can have gain anywhere from 2 - 10 depending where the pot is.

Is transconductance on the FET's datasheet or is it something you can measure on the FET themselves. Thanks for helping me understand this.

[R.G.]

Transconductance is always there; it's called yfs or Gfs. Some times its spelled out as trasconductance. The units are Siemens, a Seimens being one amp per volt, which used to be called a "mho". Sometimes you'll see micromhos or umhos.

[ItZaLLgOOd]

Thanks for the help