Help me with calculations for transimpedance amplifier

[demodb]

transimpedance amplifier

Hello guys,

can any one verify my reasoning here. I have to build a transimpedance amp which is driven by a diode which generates currents ranging from 8u to 800u. The amp should switch between 0 and 5 [V]. So my reasoning is that the maximum feedback Rf shoudl be 5/8u=625k (see picture for clarification. Because the system has a bandwidth of 3M [Hz] and the parasitic capacitance of the diode is 5p [F] we can calculate the ratio between Aol and Rf. BW=1/RC with R=Rf/Aol and C is parasitic capacitance. This results in an open loop gain of 9.375, which seems low to me. What am i doing wrong here?

 

[pixel]

Re: transimpedance amplifier

I think that R is too high. Do you design amplifier or use op-amp? **broken link removed**

 

[demodb]

Re: transimpedance amplifier

i have to design the op amp, but what kind of value should i use for Rf?

 

[pixel]

Re: transimpedance amplifier

i have to design the op amp, but what kind of value should i use for Rf?

Design equations:
wa≥2*ao/(RfCd), where wa and ao are open loop amplifier bandwidth and gain
and transimpedance bandwidth (closed loop)
ftia≥0.7BitRate~2MHz

 

[demodb]

Re: transimpedance amplifier

so for closed loop you have Acl≈Rf, so Ftia=1/(2Pi*RfCd) which results in a Rf of about 16k [Ω]. Or am i still way off here?

 

[pixel]

Re: transimpedance amplifier

so for closed loop you have Acl≈Rf, so Ftia=1/(2Pi*RfCd) which results in a Rf of about 16k [Ω]. Or am i still way off here?

If you want to understand TIA design it is goot to read literature like Saekinger or Razavi's book for optical communication, which are available at edaboard.
By the way it is very simple if you understand just few equations, but also can be very difficult if you dont.
Optimal noise performance :
This amplifier design is not same as operational because it amplifiers pole is not dominant (dominant is 1/RfCd), and Ftia is much higher than 1/RfCd.
TIA signals can be small and noise should be taken in consider. For good noise performance Ftia should be low as possible. An optimum is at 2/3BitRate. If not it can still work for high signals, but not fo low.
Ftia dictates Rf, not reverse, and than you can see how big Rf can you have.
Ftia≥06-0.7*BitRate (for noise it is better if it is equal)