# Common drain small signal analysis and unresolved question for fet

question:

There is an amplifier circuit obtained between JFET and MOSFET. For the FET used in the circuit, IDSS = 8 mA, VGSoff = -3V and working point current is 1.42 mA.

Use MOSFET parameters as Kn = 1 (mA / V ^ 2), VTN = 1V, gm = 0.75 mA / V.

The voltage gain of the first floor in the circuit is calculated as 0.7, and the voltage gain of the second floor is calculated as -100.

R7 = 101(R8) and R8 + R9 = 4.462 KΩ.

According to this; Determine the resistance values in the circuit to be KΩ.

Hi It is desired to calculate the resistance values according to the parameters given in the question (in kilo ohms). I will use re model to solve this. However, jfet is composed of common drain. The resistance of R3 is confused. Books have common drain gain but no R3 resistance. can somebody help me with the jfet's gain formula and question? @G36

• Since it is your choice of the value why not put R3 to zero? It is not necessary for the circuit to function. There are also a number of other components that are not necessary and can be put to zero (eg R2, R6, R5) – Kevin White May 25 '20 at 15:51
• @KevinWhite Hi ; thanks for feedback. I am studying electronic engineering. This is Electronic 2 course. I'm taking an online course. this is one of the final exam questions that our teacher asked to force us. I am in the same opinion with you, but asked for the question in kilohm. – electronx May 25 '20 at 16:06
• Do you know the purpose of R3 resistor? – G36 May 25 '20 at 17:04
• @G36 Frankly i dont know. brainkart.com/article/Common-Drain-Amplifier_13260 bestengineeringprojects.com/common-drain-amplifier-using-fet I have reviewed these links and also the book. In the book, common drain only contains source resistance( Rs). – electronx May 25 '20 at 17:13
• If you don't know then pick R3 value so that it will not affect the DC operating point. CD (Q1) stage gain is $\Large A_{V1} = \frac{R_4}{R_4+ \frac{1}{g_{m1}}}$ – G36 May 25 '20 at 17:20