measuring ac gain

[gaussmarkov]

there's lots of info on measuring the gain (hfe) of a transistor.  but how do you measure the gain of a circuit, or a portion of a circuit, for an ac signal?

[BrianJ]

The HFE of a transistor and the AC voltage gain of a circuit of effectively unrelated.  The HFE (current gain) of a transistor has more to do with how good of a job it will do "transistoring."  That said, a low current gain transistor may reduce voltage gain by drawing too much base current and loading down a previous gain stage. 

As for detemining the gain of a circuit, the voltage gain of a single transistor stage is the calculated by dividing the collector resistor by the emmitter resistor.  If the emmitter resistor is bypassed by a cap the the emmitter "reactance" (at an arbitrary frequency) + something called "little r.e." can be substituted for the emmiter resistor in your calculations.  Little r.e. is the intrinsic emmitter resistance.  It is usually small, but it varies with current and ambient tempurature- it can be calculated using the Ebers-moll equation- Have fun with that.

If that is too much trouble you might try measuring the gain with an oscilloscope by inputting a small voltage and dividing the unclipped output by the input.  You could use Digital Multimeter but it is less acurated and you need a sine wave input signal as well as some way of making sure you aren't clipping the waveform.  If you want extra credit you could take the quotient of your gain calculation and use this equation to calculate a decibel figure:

20 x (Log10xgain)

A gain of 2 should be roughly 6 db; a gain of 10 would be 20db...

Believe it or not I find this stuff endlessly entertaining.  You should see me when I try talking to girls.

[seanm]

f that is too much trouble you might try measuring the gain with an oscilloscope by inputting a small voltage and dividing the unclipped output by the input.  You could use Digital Multimeter but it is less acurated and you need a sine wave input signal as well as some way of making sure you aren't clipping the waveform.

That is how I measure gain.

But if you don't have an oscilloscope, you can use a voltmeter. Input a signal. Measure the input and output. If you are suspicious that the output is clipped, lower the input signal. If the relative gain goes up, you are clipping.

[gaussmarkov]

thanks!     i was hoping there is a dmm method.  just to be sure:  i measure AC voltage, right?  i don't think that would work with my ratshack meter because it only has AC 200V and 500V settings.    but i have a newer meter that measures AC or DC with settings from 200m to 1000 or 750 (DC vs. AC).

re clipping.  i suppose that the audio probe would also help me catch the most obvious clipping?

again, thanks for your help.

[seanm]

Correct, AC volts. And an audio probe will catch the obvious clipping.