# Designing a two stage differential amplifier with MOSFETS

As a part of a "Test you understanding quiz" i had yesterday at school were we tested our understanding of differential amplifiers and mosfets i came across a question that asked us to design a circuit that matches the description. I have never really designed a circuit myself rather just did problems with them so i wanted to check whether the schematic i drawn is any good.

Here is the the problem statement:

Design a two stage differential amplifier with differential input (1st stage) and differential output (2nd stage) with the first stage consisting of NMOS transistors and active loads in form of a current mirror and the second stage consisting of PMOS transistors and active loads in form of a current mirror. You have one current source available for the design. For the given design, calculate it's differential input and output resistance as well as differential gain.

Here is the circuit i have drawn (Note that transistor next to M12 should be M11 not M1):

simulate this circuit – Schematic created using CircuitLab

Supposing the symmetry exist and using the bisection theorem and noting that: $$V_d=V_1-V_2$$ i calculate that my circuit has: $$R_{in}=\infty$$ $$R_{out}=2*r_{ds11}$$ and $$A_{d}=g_{m1}g_{m9}r_{ds3}r_{ds11}$$

I didn't write any lengths and widths because we were told not to do that as drawing the main idea is more important. Does the schematic makes sense and does it fit the description of the problem? I don't really have the experience to find the problems immediately myself so i could use suggestions as to what i could have done better and different.

• Looks good, now simulate it in Falstad in any browser and use default Vt=1.5 and gm=beta=20m or change it and add show V on each node wire when add a series R and measure Vin/In=Zin or from the voltage drop ratio multiply x Rin. If you have time create a mismatch in the most critical pair for Vt or beta(gm) and see the bias error. Then add external negative feedback to limit the gain and reduce the offset error. – Tony Stewart Sunnyskyguy EE75 Jun 20 at 19:29