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# Understanding the working of the common source transimpedance amplifier

#### VisheshG24

##### Junior Member level 1
Hi
I am trying to understand how a common source tia works (see the attached image). Referencing the image, can I assume the nmos to be an opamp and apply the feedback concept? Like source is the non-inv terminal and according to the virtual short concept of the opamp, the gate will try to copy the voltage of the source, and the transimpedance gain will be Rf? So, if the source is grounded, then it means that the gate will also be grounded.
But what I am confused is, if what I said is correct, then shorting the gate and the source terminal will turn off the transistor?
What am I missing here. Is my analysis of the working of the tia as a resistive feedback to the voltage amplifier opamp is correct? I will really appreciate if someone can explain or guide me the working of the common source tia?

The reference of the image is from the article: A. Atef, M. Atef, E. E. M. Khaled and M. Abbas, "CMOS Transimpedance Amplifiers for Biomedical Applications: A Comparative Study," in IEEE Circuits and Systems Magazine, vol. 20, no. 1, pp. 12-31, Firstquarter 2020, doi: 10.1109/MCAS.2019.2961724. keywords: {Power demand;Medical devices;Biomedical monitoring;Pulse oximetry;Blood pressure measurement},

Like source is the non-inv terminal and according to the virtual short concept of the opamp, the gate will try to copy the voltage of the source, and the transimpedance gain will be Rf? So, if the source is grounded, then it means that the gate will also be grounded.
Nope.
Gate is a virtual ground, what means that impedance seen from source is low (relatively, as is it ca R_F/(gm·(R_F||R_D)).
And virtual ground is a small signal concept. The gate voltage is set by feedback to the value required to set given current flowed by FET. It is like diode connected devices biased by resistor (as input current source is an open circuit).

### VisheshG24

Points: 2
Helpful Answer Positive Rating
Hi Dominik,
Thanks a lot for the reply. What I understood is that the tia topology that I have shown only works when the Ipd signal varies by a small amount, i.e., a small signal input. I aim for input currents between 100nA and 10 uA and output voltages between 0 and 4V. I want to work in a low-frequency bandwidth like 100Hz. How do we deal with these large signals with this topology? Do you suggest any other topology, reference paper, etc.? I would appreciate any help.
Thanks and Regards,
Vishesh