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I referred to the Wideband Photodiode Transimpedance Amplifier circuit at page 1 of the OPA657 datasheet.What do you mean with " if the source (photodiode) capacitance already allows a gain > 10."
Cf=1pF won't achieve stable operation with OPA657, which OP did you use there?I have build another device with universal capacitor (Rf=2k2, Cf=1pF) and the same result was seen
I referred to the Wideband Photodiode Transimpedance Amplifier circuit at page 1 of the OPA657 datasheet.
Cf=1pF won't achieve stable operation with OPA657, which OP did you use there?
I understand, that you are referring to an apparent high frequency gain drop at high input levels, while the frequency response is correct with optical attenuation? Are you sure, that the bias circuit is keeping the voltage level at high input level?
It would be worthwhile looking at the output signal with an oscilloscope in both cases (with & without optical attenuation). Are you sure the modulation is 1/500 of the optical power? That seems very low. When I have worked with modulating lasers the modulation has usually been around 80%.
Keith.
So does the bandwidth improve as you reduce the modulation? My suggestion of looking at the output with an oscilloscope is to see what the peak to peak signal is, on top of the DC output level.
Keith.
I think the possibility of oscillation is still there. I would suggest trying a different (slower) opamp with a simple capacitor across the feedback. Something with 100MHz bandwidth should be more than adequate and reduce the likelihood of oscillation.
Keith.
Yes, it has to be expected with OPA657, because it's no unity gain stable. I don't know however, if the oscillations are also causing the observed frequency response, but at least, it's not normal operation.I saw a ~294MHz signal in the oscilloscope
When you add small decouplers on high speed opamps you still need to keep the large ones as well.
I notice you have a 1N4148 across the photodiode. I assume this is just for modeling, not in the actual circuit, but it will screw up your simulations.
My simulations show ringing around 125MHz with your first circuit, around 250MHz with your second which seems to be around half the frequency you are seeing.
I don't understand the "Vin Laser". I assume this is the modulating voltage you are applying to the laser.
The low apparent output voltage on the second circuit suggests strong oscillation which is what your results show.
You cannot make sense of the measured results while the opamps are oscillating. You need to stop the oscillations first.
You really need to stop using those opamps and use something with a more manageable bandwidth.
The fact that the first polse of the OPA637 is at 100kHz is not relevant.
keith1200rs said:Just a thought - do you even need a transimpedance amplifier? With just 1k or so of transimpedance you could just use a 1k resistor to ground then buffer it. While I have used and designed transimpedance amplifiers with just a few k ohms impedance it is usually when I am trying to get 1GHz bandwidth. For 10MHz I am not sure it is necessary.
Keith.
Personally, I use quite a few values. I cannot be certain that all of them are required in all cases, but I have often ended up with 4 values to remove oscillations and poor flatness on broadband amplifiers:
If you don't use a transimpedance amplifier you will still need a voltage amplifier (AC coupled) to boost your signal up to a reasonable size. I am not saying it is the "best" way, but with such a low bandwidth and gain it is worth considering.
Keith.
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