problem in photodiode preamplifier design....

[PARTHIBAN31288]

hello friends..

Actually we are using LMV794 Op-Amp in our design.By using this we designed one preamplifier, current to voltage converter.
while checking our circuit we had the problem in receiving the signal.what happened was,

with out emitter signal it shows the square wave signal, as i show in the image1 at the output.
when the signal comes it raises from that square wave signal only. not from zero to 3.3 or 1.65V to 3.3 (because i am using biasing for non-inverting input)
this is the problem. why this square wave comes..? what is the problem in our circuit.
and also i'm using 100n capacitor in front of the amplifier. whether it affect the speed of the amplifier..?

 

[keith1200rs]

I cannot see your circuit but it sounds like the amplifier is unstable. Show your circuit.

Keith

 

[PARTHIBAN31288]

Thanks for your reply..

here i attached my circuit.

 

[keith1200rs]

It should be stable with those values so I would suggest you look at decoupling the supply to the opamp. Also, the design using and unbiased photodiode and AC coupling it is a poor one. See this thread:https://www.edaboard.com/threads/247266/

Keith.

 

[PARTHIBAN31288]

oh OK OK .. for AC coupling photo diode application we need to give biasing to the photo diode and Non-inverting terminal of an op-amp or only the photo diode..?
and also the unbiased photo diode method was wrong..?
because while design the amplifier, i got lot of circuits in similar with the link you send .... that is no bias for photo diode and no bias for op-amp.... so only....

 

[keith1200rs]

The LMV794 datasheet shows a typical arrangement, also on their overview page: **broken link removed**

The Vb point would be a negative voltage to reverse bias the photodiode. That tends to be the way to get the fastest response. You could AC couple but you would still want to reverse bias the photodiode. First though, you need to fix your oscillations so look at decoupling.

Keith.