Re: transimpeadance vs voltage amp for photodiode

From: John Larkin (jjlarkin_at_highlandSNIPtechTHISnologyPLEASE.com)
Date: 11/25/04 

Date: Thu, 25 Nov 2004 09:36:10 -0800

On 25 Nov 2004 06:02:03 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

>John Larkin wrote...
>>
>> On 24 Nov 2004 13:40:51 -0800, r.u@cox.net (Bill) wrote:
>>
>>> I am working on an amplifier circuit for a avalanche photodiode
>>> that is looking at very high speed 2 nsec rise 10nsec wide pulses.
>>> Previous work had been done on a circuit that uses 2 TI OPA695 X8
>>> amps looking at the photodiode signal across 50 ohms. I am
>>> pressuring a transimpeadance amplifier using the OPA695 with a 1k Rf.
>>> The photodiode current is ~300ua. On a general basis what are the
>>> pluses and minuses considering noise, bandwidth, etc. of using a
>>> voltage vs. a transimpeadance amp in this application.
>>
>>
>> Dumping your signal into 50 ohms is a waste, noise-wise. The place
>> for signal to go is into the silicon.
>>
>> I just did a PIN diode amp using two AD8014's, current-mode opamps.
>> The first stage was a TIA with (I recall) 680 ohms feedback || 1 pF,
>> and the second stage was an inverting gain-of-3 or so. Risetime (from
>> a vcsel laser pulse) was just under 2 ns overall with about a 1.5 pF
>> silicon pin diode. I had a similar amount of signal as you do, a few
>> hundred uA, so s/n was pretty good and edge jitter was around 5 ps
>> RMS, not bad. I think (data's at work, I'm not) I'm getting just
>> about 1 volt out per optical milliwatt in at 850 nm.
>>
>> Phil, avowed enemy of bad TIA designs, will probably rip me up for
>> this one.
>
> He won't be happy unless we're using a biased common-base input stage.

Well, that lets me off the hook. The inverting input of a current-mode
opamp *is* a biased common-base stage.

John