19-06-2017, 12:20 PM
Getting back to transistors, in common-emitter mode we have the internal 're' as approximately 26/Ie ohms (where Ie = emitter current in milliamps). Then gm = 1/re = 38 x Ie numerically in mA/V. Adding a collector load, you get a voltage gain of gm x Rl = 38 x Ie x Rl... and since Ic is approximately equal to Ie (alpha > 0.95 even for really crummy transistors!), you can see:
Voltage gain = 38 x volts dropped across collector load resistor.
So - transistor currents really aren't important, nor is the collector load, for working out gain. All you need is to calculate (or measure) the no-signal voltage across the load resistor, and the gain immediately follows!
Of course, this is a gross simplification - nobody is going to operate their transistor at an amp of current with a load of a few ohms, because battery life would be horrendous. And it also turns out that for low-noise applications, there is a broad optimum operating current. And the '26' and '38' numbers are temperature-dependent, too. But it does illustrate the relative insensitivity to details that exists.
Voltage gain = 38 x volts dropped across collector load resistor.
So - transistor currents really aren't important, nor is the collector load, for working out gain. All you need is to calculate (or measure) the no-signal voltage across the load resistor, and the gain immediately follows!
Of course, this is a gross simplification - nobody is going to operate their transistor at an amp of current with a load of a few ohms, because battery life would be horrendous. And it also turns out that for low-noise applications, there is a broad optimum operating current. And the '26' and '38' numbers are temperature-dependent, too. But it does illustrate the relative insensitivity to details that exists.
