I was trying to understand a TIA (Trans-impedance Amplifier) design by Behzad Razavi which I found here.
But I don't get what the transistor marked below, which is connected to the Vout, does?
I even tried removing it from the circuit in spice simulations and it didn't affect the output (at least in terms of the trans impedance gain)
(Source: "The Design of a Transimpedance Amplifier" by Behzad Razavi in The Analog Mind from IEEE Solid-State Circuits Magazine, Winter 2023, pp 7 - 11, DOI: 10.1109/MSSC.2022.3219682)
I am trying to design my own discrete TIA by using his design as a reference, I have replaced RD2 with a 1 Mega-Ohm resistor since I want a transimpedance gain of 10^6 i.e. a 1mV output for a 1nA input current. I have used a CD4007 transistor array for my PMOS-es and a 2N7000 transistor for M1. Also, I read somewhere that the transistor is supposed to be a PMOS, so I am using a transistor from the array for instead of an NMOS.