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There is an amplifier circuit obtained between JFET and MOSFET. For the FET used in the circuit, \$I_{\text{DSS}} = 8\text{ mA}\$, \$V_{\text{GSoff}} = -3\text{ V}\$ and working point current is 1.42 mA.

Use MOSFET parameters as \$K_n = 1 \text{ mA/V}^ 2\$, \$V_{\text{TN}} = 1\text{ V}\$, \$g_m = 0.75\text{ 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.

enter image description here

I want to calculate the resistance values according to the parameters given in the question (in kΩ). I will use re model to solve this. However, the JFET is composed of a common drain. The resistance of R3 is confused. Books have common drain gain but no R3 resistance. What is the JFET's gain formula?

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  • 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. http://www.brainkart.com/article/Common-Drain-Amplifier_13260/ https://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
  • @G36 https://ibb.co/0t3LtmX Is the formula you provided is the same as in this picture? – Electronx May 25 '20 at 17:46
  • Yes it the same folrmula, and my formula ignores rd. – G36 May 25 '20 at 17:50
  • Your task is too fond resistor values to gest Av1 = 0.7V/V and 100V/V at given Id1 = 1.42mA and gm2 = 0.75mS? – G36 May 25 '20 at 18:04
  • the question gives the gain of the first and second stage: [ The gain of the first stage is 0.7 (JFET)] [the gain of the second stage -100. (MOSFET)] the question is given the resistance equations R7 = 101(R8) and R8 + R9 = 4.462 KΩ. Question: find all resistance values that provide the condition (all resistors are in kiloohms) – Electronx May 25 '20 at 18:25
  • In your class witch equation for MOSFET Id current, you are been using Id = K*(Vgs - Vt)^2 or Id = k/2 * (Vgs - Vt)^2 – G36 May 26 '20 at 14:33
  • thank you so much – Electronx Dec 12 '20 at 18:53

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