Problem about TIA for optical comm

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coxstreet

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Dear all, I am designing a TIA by using OPAMP whose bandwidth is 100MHz and gain is 100dB and shunt resistor feedback topology in TSMC 035 process.

I firstly design a OPAMP (one stage folded-cascode )with bandwidth and DC gain are 100MHz and 100dB respectively. But when I add a shunt resistor, eg 10K ohms, the TIA still has the gain of 80dB but the OPAMP has some transistors going to linear region.

I guess the explanation is even some are in linear but still the gain of OPAMP is relatively large such that the gain of TIA, Rf, will not be affected.

So my question is how to settle the linear transistors to saturation. Thanks!
 

Hello coxstreet,

Do your OPAMP has an output buffer stage after the folded-cascode stage?
 

Hi lamoun, Thank you for your answer. when I add buffer stage as you mentioned, all transistors are going right now.

But another problem came in. When the shunt resistor are 1k,10k,100k,1M, the TIA gain measured are 56dB, 98dB, 107dB, 109dB, respectively. The 56dB seems reasonable for Rf=1k ohms. But how could explain the other result? Thank you!

coxstreet
 

Seems your open loop gain now is ≧ 109dB, but your output impedance still is rather high (~10kΩ), so the increasing load of the feedback resistor more and more decreases the gain.
 

Hi erikl, yeah you are right. my open loop gain is about 103dB and my output Ro > 10k ohms. But when lower the Ro, the gain still cannot increase as the shunt resistor...... it is only right when Rf=10k ohms. ......So can you explain that?　Thank you!
 

Depends on your output buffer stage: probably its gain decreases with decreasing output impedance.
 

Thank you!

Does that mean I should change the output impedance for every shunt resistor? Can I have only one shunt resistor for all of that shunt resistor? If yes, what rules should I obey? Output impedance should smaller than that shunt resistor? Thankyou!
 

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