--- In Electronics_101@..., "ktauyeung" <ktauyeung@y...>
wrote:
I am working on a high-gain current-to-voltage amplifier. I need to
convert a very low pulsed current signal (100 microsec, <nanoamp)
into a voltage signal (>10 mV) that I can read from the scope. I
tried the amplifier using a low-gain (0.2 V/microamp) setting, and it
works ok. When I tried it with a higher gain (2 V/microamp),
oscillations (~10 microsec, regular amplitude) showed up on the scope
display. I'd like to fix that, but don't know how. I need some help,
if you know how to fix it and what is the cause of that, please let
me know. Thanks!
Is the current source a photodiode? If so, that is probably the cause
of the oscillations. If you have access to a Spice simulator you
could quickly check it. What opamp are you using? 2M ohms is NOT a
large resistor for such an amplifier. They can easily be 10M or 100M.
If you use two stages you could be dogged by noise problems. The
dominant noise component is usually the feedback resistor. Reducing
this by a factor if 10 will only reduce the noise by a factor of
sqrt(10) so you are roughly 3 times worse off for signal to noise
ratio. Reduce it by 100 and you are 10 times worse off. There is
also opamp current and voltage noise to consider.
Keith.
