cascade amplifiers using opa656

Hi,
I think you need to supply your schematic and details before we can help.

Regards,
Hooman

Hi
the first amplifier is transimpedance amplifier, and my transimpedance design is the same with the transimpedance example shown on the front page of the data sheet.
the second amplifier is voltage amplifier and OPA656 is used as an inverting amplifier, the configuration is RG=210Ω,and RF=2.1kΩ.
the output from the first amplifier as the input of the second amplifier, but this is no output from the second amplifier, why and how to solve this issue?

Hello,

Couple of further quesions:

1. Have you tried isolating the 1st stage and driving the 2nd stage directly with a function generator?

2. What are the power supplies set at and how much quiescent current is being drawn by the 2 amplifiers?

3. What is the power supply on the photo-diode? Are you pulsing/modulating the photodiode or are you just looking at a dc dark current setup for now?

4. What is the output of the 1st amp reading? What is the output of the 2nd amp reading?

As Hooman mentioned a drawn schematic always helps and makes things go quicker. Also, any scopeshots of the data will help better understand the issue.

-Samir

This link shows the hand-drawn schematic (the two threads are related to each other): 

Regards,

Hooman

Hi,Samir,
1,I have isolated the 1th stage and driven the 2th stage using function generator ,and the 2th stage worked very well.
2,the power supply set at+/- 4.7V,
3. photo diode information :
detecor Silicon PIN
Shunt Resistance Rsh 1 GΩ
Diode Capacitance CJ 40 pF
Rise Time tr 14 ns
Bias Voltage VR 10 V
Dark Current (with 1 MΩLoad) Id 0.35 nA
4. I modulated the IR laser(830nm） at the frequency of 5kHz sine waveform, the power on the detector is about 1mW, and the output signal from the 1th stage is 200mvp-p voltage at the frequency of 5kHz, the second amplifier output a signal is a sine signal about few Vp-p ,but the frequency at few hundred kHz and it exist without the laser on but will disappear when the power supply is off.

Hi Samir and Yu,
I'm jumping in here. :-)

I recommend that the 200ohm resistor of the 2nd stage (RG) be physically moved right to the output pin of the 1st stage (if not already). In other words, move the resistor physically close to the 1st stage in order to isolate any parasitics that the 1st stage might have to drive.

If that does not help, I'd then suggest changing the 200ohm resistor to two 100ohm resistors in series, with each 100ohm resistor placed physically close to the 1st and the 2nd stage respectively.

Regards,
Hooman

Hi， the following is my schematic:

a:(1)when I isolated the first stage, when the power supply is on and no input ,the output signal from the 2th stage is like is:

(2) when 100mVp-p 12kHz sine waveform input the 2th stage,the output signal is like:

so I add a feedback capacitor in the 2th stage, the capacitor value is 1pF, and I test the output signal from 2th when isolating the 1th stage without/with signal ,the output signal is like:

b, when connect the 1th stage with 2th stage together, and the laser is on, the output signal from the 1th stage and the input signal in the 2th stage look like  the following picture:

and the output signal from the 2th stage is like : 

the output signal from the 2th stage I tested at different time and I get different pictures, I don't know why cause these?

I cant seem to find any issue in your setup..things look like they should work. Is the scope AC Coupled or dc coupled in your attached scope shots? The only thing I can think of is that the "dark current" from your photo-diode is presenting a dc offset to the 2nd stage which after being gained up is slamming into the rail, thus you have little to no room to swing.

Hi Zhenfang,
What kind of oscilloscope do you use? The OPA656 is a pretty high bandwidth amplifier, and it is possible that it can ring at multi-MHz frequency (if PCB has parasitics, as it usually does). Could it be that the second stage is ringing as hell, and your scope does not see this? Your second picture is really abnormal.