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We have a laser diode-PMT system. The output of PMT module is very low signal. We use an opamp-based(OPA657) transimpedance amplifier to amplify the output of PMT. Our frequency is 15-20 MHz. But we want to reach to 100MHz (at least 50MHz). My first observation is when we block the laser diode signal, there is oscillation signal at obvious frequencies(above 20MHz). When the laser block diode is on, there is signal but when we block the laser diode, this signal dont interrupt. I'm confused about OPA657 causes this oscillation and if yes, how can we prevent this? I'll be appreciate if you give some advices.
Thank you,
Ozge
Hello Ozge,
It would help to see the whole circuit but in any case you always have to compensate the Zt design with a feedback capacitor. That is all described in this high speed presentation. most importantly, we would need the source capacitance. There is an article about to publish that steps through this in a pretty simple fashion, but you might also take a look at the new OPA8187180.Transimpedance design flow using high speed op amps.pptx
Hello Ozge,
So this next article did post yesterday, this will help you on transimpedance design - towards the end of this article on decompensated high speed VFA.
https://www.planetanalog.com/author.asp?section_id=3404&doc_id=565165&
SOory you were not well Ozge,
This is not quite enough, this does not now look like transimpedance, looks like a voltage gain? where do R1 and R4 go? And R2, 1kohm seeems large for R2. Is the 1pF intentional or parasitic?
Sorry for quick answer and missings. R1 is input and it goes to function generator or photomultiplier tube output. R4 goes to ground, there was a parallel capacitor actually. But we do not use this resistor and capacitor, they are short-circuited. Supply capacitors are between +/- supplies and ground. R2 is output and it is 50ohm. Input and output 50ohm is for impedance matching as my supervisor says. 1pF is intentional, my supervisor says it reduces the noise and DC signals.
By the way, i'm not sure this circuit is effective. We tried different resistors(1.8k,2k,5k,10k) but these are not enough for low and high frequencies.
Thank you,
Ozge
wow, well lets be careful here,
So a signal generator and a PMT will have completely different source impedances, pick one,
your supervisor is handwaving through some subtle issues. feedback caps can work, less so with decompensated parts.
There is a lot of somewhat difficult material out there that deals with these things, here is one, tell me what you are trying to do -
https://www.planetanalog.com/author.asp?section_id=3404&doc_id=565165&