MOSFET Feedback Amplifier Gain: Why Not 10? | Analog IC Interview Question

Solve a three-MOSFET feedback amplifier gain problem from an analog IC/RFIC interview: build the small-signal model, apply KCL, and see why the resistor ratio alone is not enough. Try the free 10-question Analog IC/RFIC interview test: https://uplevelengineering.com Work through the moving shared-source node, effective transconductance, first-stage gain, output-divider loading, and the loaded common-source second stage. Then close the negative-feedback loop and explain why the voltage gain is about +5, not +10. The exact low-frequency result is Av = 1000/201, approximately +5 V/V. The familiar 1/beta feedback-gain approximation requires loop gain much greater than one; here the loop gain is only about one. A 1 mV example and a voltage-change sign check make the physical meaning clear. Assumptions: all three gm values are 2 mS, the MOSFETs operate in saturation, and output resistances are infinite. Ignore body effect and transistor capacitances in this low-frequency calculation. The closing RF discussion distinguishes gain accuracy from frequency-dependent stability. Chapters 0:00 MOSFET feedback gain: roadmap 0:17 Circuit, assumptions and interview question 1:00 Build the small-signal model 1:51 Shared-source KCL and effective transconductance 2:41 First-stage voltage gain 3:05 Feedback divider and output loading 3:52 Loaded common-source second stage 4:19 Close the loop and solve the exact gain 5:06 Why the gain is about 5, not 10 5:52 1 mV example and finite error voltage 6:17 Check the negative-feedback sign 6:50 Design lesson and RF stability caveat 7:24 Key takeaway and free interview test Topics: MOSFET small-signal analysis, negative feedback, closed-loop voltage gain, loop gain, KCL, and analog IC interview preparation. More worked Analog IC & RFIC interview questions: Analog IC & RFIC Interview Questions #AnalogIC #RFIC #MOSFET