Tee networks in TIA feedback loops

Nemo · 2011-09-17T08:02:15+00:00

I have a high capacitance photodiode (650pF with as much bias as I canput on it) which I want to measure at high gain and 100kHz bandwidth ormore. It works well enough to meet spec, but I'm trying to make itbetter simply to learn more about analog design. Simply using a TIA toamplify it by about 1M - I'm trying to get as much gain in this firstatge as possible to reduce Johnson noise in the feedback network - I getminor oscillations at around 100 - 150kHz unless I increase the feedbackC to a point where my bandwidth is adversely affected. I was wonderingabout using a T network to get round this, but my experiments didn'tseem to improve the instability, and I wondered if there was somethingobvious I was missing.I should say first that I rapidly found that there was a lot of DC driftfrom using a T, so if doing this on a PCB I would use one where theelements are on the same chip and had tight tempco matching (Vishay dosome with 2ppm matching, in a SOT23 package. Expensive, but price is nota concern here.)The circuit I tried was along these lines: _____1 - 3pF_____ | | | ___100k_______| | |i --> --|__| | | ______10k---1k___ 0V__| / | |/ 0VI reasoned I would be able to use a higher C, maybe 10 - 20pF inparallel with the 100k to get stability, but maintain the samebandwidth. But I found that increasing the feedback C to the point wherethe 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 afew weeks ago: JFET front end feeding low voltage noise op amp, withauto nulling circuit servo-ing the input to 0V. I've tried several opamps 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 feedbackresistance, but have no bandwidth advantage. But I...

Sep 17, 2011 40 Replies

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My practical experience is at the extreme end of transimpedance amps (gains of 100Tohm) so not much help, but some useful references for more normal applications are: (1) Graeme J (1995) Photodiode Amplifiers: Op Amp Solutions ISBN

007024247X. (2) Dostal J (1993) Operational Amplifiers ISBN0750693177. (3) Texas Instruments (previously Burr-Brown App Note AB076) Application Note sboa060 : Noise analysis of FET transimpedance amplifiers. (4) Texas Instruments (previously Burr-Brown App Note AB075) Application Note sboa035 : Photodiode monitoring with op amps. Prof78

Tee networks in TIA feedback loops

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