But what determines the gain, or amplification factor, of an N-channel JFET?
Hate to expose my ignorance, but... Is it the biasing? Or what? What I
have doesn't specify such on the packaging, and I am wondering.
TNX
Dave
The datasheet should specify a typical transconductance at some drain
current, and probably has a graph of same. The common-source voltage
gain is nearly equal to the transconductance (gm, in Siemens)
multiplied by the load resistance (in ohms) in the drain circuit.
The operating current is a function of the gate-source voltage,
another spec or curve. Actually, gm is the derivative (slope) of the
Id versus Vg curve. In general, the higher the drain current, the
higher the transconductance.
A typical small-signal jfet might operate with the gate a half volt or
so negative relative to the source, and might have a gm of, say, 0.003
Siemens at 4 mA or so drain curent. If it dumps into a 2K drain
resistor, the voltage gain would only be about 6.
Jfets have low gains and, usually, terrible part-to-part
repeatability, with Idss sometimes specified over a 5:1 or even 10:1
range, which makes design tricky.
"Amplification factor" for a tube is transconductance times plate
resistance, which is the voltage gain you'd get with a
constant-current (very high impedance) plate load. The same idea
exists for a jfet, namely gm times the slope of the drain curve. I'm
guessing numbers like 25 or so for a typical jfet.
So, in general, jfets suck. They are handy in some niche applications,
like low-noise, high-impedance amplifiers.
John