JFET voltage gain.

Discussion in 'Homework Help' started by Petrucciowns, Jul 21, 2009.

1. Petrucciowns Thread Starter Active Member

Jun 14, 2009
62
0
Hi, does anyone know the equation to calculate voltage gain on a simple JFET circuit with 1 drain resistor ,1 gate resistor, and 1 source resistor?

Thanks.

Mar 6, 2009
5,448
790
3. scythe Active Member

Mar 23, 2009
49
6
do you have any specs of the JFET? that would be needed for any type of analysis, as well as information on how the JFET is hooked up to anything else in the circuit

Jun 14, 2009
62
0
5. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
In the absence of any other information a reasonable approximation for the gain of the amplifier is

Av=-Rd/Rs = -5000/500 = -10

[More accurately Av=-gm*Rd/(1+gm*Rs)]

So if gm*Rs >> 1 then the approximation is reasonable.

Note : For gain Av above the output is taken at the drain and the input at the gate.

The 1MΩ gate resistor to ground establishes the self bias condition - which you are told gives Id of 1 mA. It won't contribute to the gain other than when it might load the signal source, but that's unlikely at 1MΩ and we don't have any information regarding the nature of the signal source in this example.

6. Petrucciowns Thread Starter Active Member

Jun 14, 2009
62
0
Awesome, thanks TNK you are always so helpful, and detailed.