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From: email@example.com (Ken Smith)
Subject: Re: photodetector circuit, high speed with good ambient light rejection
Date: Sat, 30 Nov 2002 16:58:53 +0000 (UTC)
Organization: a2i network
NNTP-Posting-Date: Sat, 30 Nov 2002 16:58:53 +0000 (UTC)
X-Newsreader: trn 4.0-test76 (Apr 2, 2001)
Originator: firstname.lastname@example.org (Ken Smith)
I'm assuming your signal is small and broadband.
In article ,
Dr C.I. Swift wrote:
> Need to make such a circuit good for up to 90MHz. will have a considerable
>DC light component so want too reject this.
>Is it possible to reverse bias the photodiode, running it to ground via an
>inductor in series with the resistor? The idea being the ac component will
>seee a higher impedence and therefore produce a higher voltage.
Inductors are good in theory because they (unlike resistors) don't add
thermal noise to the system.
In practice, however, they can be real trouble in small signal circuits.
They pick up noise inductively. They work like little microphones. They
make noise if they have a core and that core is under mechanical stress.
>Or should I stick to the transimpedence amplifier with a feedback resistor
>small enough so that the system does not saturate and then just ac couple
>the signal to further amplification stages.
You can add a second op-amp to the amplifier just to servo out the DC
component. In the transimpedance case, this means you have two feedback
paths. The advantage of this circuit is that it lets you have a high gain
in the first stage so that you don't need to worry as much about the noise
in the later stages.
You should stay away from carbon resistors. Carbon really is a
semiconductor so there is a little extra noise from them.
You don't really need a flat bandwidth in the first stage of the
amplifier. If there is a one pole roll off, not to far below 90MHz, you
can cancel it with a zero in a following stage. I can hear you ask "why
allow the one pole?". The answer is: In the first stage of the circuit,
you may find a trade off between noise and bandwidth. When confronted
with this, I increased the resistance to lower the noise current at the
cost of some bandwidth and then fixed it in a later stage.
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