I have a high capacitance photodiode (650pF with as much bias as I can put on it) which I want to measure at high gain and 100kHz bandwidth or more. It works well enough to meet spec, but I'm trying to make it better simply to learn more about analog design. Simply using a TIA to amplify it by about 1M - I'm trying to get as much gain in this first atge as possible to reduce Johnson noise in the feedback network - I get minor oscillations at around 100 - 150kHz unless I increase the feedback C to a point where my bandwidth is adversely affected. I was wondering about using a T network to get round this, but my experiments didn't seem to improve the instability, and I wondered if there was something obvious I was missing.
I should say first that I rapidly found that there was a lot of DC drift from using a T, so if doing this on a PCB I would use one where the elements are on the same chip and had tight tempco matching (Vishay do some with 2ppm matching, in a SOT23 package. Expensive, but price is not a concern here.)
The circuit I tried was along these lines:
_____1 - 3pF_____ | | | ___100k_______| | | i --> --|__|\ | | \______10k---1k___ 0V__| / | |/ 0V
I reasoned I would be able to use a higher C, maybe 10 - 20pF in parallel with the 100k to get stability, but maintain the same bandwidth. But I found that increasing the feedback C to the point where the instability is reduced, just decreases the bandwidth dramatically, as if the C is also being multiplied by a factor of 11.
The amplifier is a compound one along the lines Phil Hobbs suggested a few weeks ago: JFET front end feeding low voltage noise op amp, with auto nulling circuit servo-ing the input to 0V. I've tried several op amps of various GBW's and all display this slight ripple in the output.
I concluded that T's are good if you want an unfeasibly high feedback resistance, but have no bandwidth advantage. But I hope I'm missing a trick here and you guys will drop a hint 8)
I also wondered what is really meant by a "stable" amplifier. Students are told "stick more C in the feedback loop to make it stable." Is this one of those glib simplifications? I'm looking at the output at high gain and there is always _some_ ripple present if there is significant C on the input and you turn the gain on the 'scope up high enough. Surely there will always be SOME degree of chase-your-tail hunting round the feedback loop for any amplifier? Or is it possible to get a TIA which is actually truly stable with this kind of C on its input?
Nemo