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Trans-impedance Amplifier using OTA.

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firasgany7

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Hi guys,
I'm trying to design a transimpedance amplifier for optical receiver, based on this acrticle:
https://www.ti.com/lit/an/snoa515a/snoa515a.pdf

the requirements:
-VDDA=1.8V
-Cload = 200fF (the output of the amp loaded by 200fF)
-the total gain At=Vout/Ipd = 3000[Ohm], (Ipd is the current source on the left on the schematic)
-BW>20Mhz



this is the schematic AFTER I closed the loop:
sse.JPG
*there is also a reference current source ICP (100u) inside the OTA.
*the value of Rf is 3k[ohm]
*miller capacitor (in the OTA schematic) was put for stability purposes.

first I reached a good operation point where all transistors are in saturation. the open loop gain was (81 dB) with a BW 30kHz. (Open Loop without the feedback loading which means without the resistor Rf and the capacitor Cf, and without the input resistance r0,r1).

the I measured Open Loop with the feedback loading I get: 47dB Gain and BW of 230kHz.

Lastly, I closed the Loop I got the following: 73dB Gain and BW of 20kHz.

how do I achieve 20 Mhz bandwidth in Closed Loop? what are the changes that I need to do in the circuit to make that happen? I tried to play with W,L,gm of transistors at the output of the OTA.. I didn't recognize a certain pattern/effect.

here are some details about the sizes of transistors and some parameters:
ota.JPG
closed Loop.JPG
EDA.JPG
op.JPG
 

Lastly, I closed the Loop I got the following: 73dB Gain and BW of 20kHz.
73dB is extreme high value for closed loop gain. It is not a surprise you reached 20kHz bandwidth with some hundred uA consumption. And with 73dB closed loop gain your output offset caused by mismatch will be also huge if you want to make it real. Even in your attached article the closed loop gain is 0dB (unity)when the bandwidth is 20MHz. Decrease your feedback resistor value.
 

The issue is lack of understanding the testbench.
First of all use ac amplitude set to 1 not 1m. In closed loop simulation, you should achieve transimpedance - relation between output amplitude vs input current. This function should in first order follows the feedback. So, 3kOhms of gain (70dBOhms) in dc with pole at RfCf. The impulse response will show then exponential behavior with rise time of 1/ugf_of_opamp and decay with RfCf time constant. And show us all characteristics which you describing, please.
 

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