Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DE69C97.firstname.lastname@example.org> <3DE76E53.email@example.com> <3DE90887.A854256A@rica.net>
Subject: Re: photodetector circuit, high speed with good ambient light rejection
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Date: Sat, 30 Nov 2002 10:47:55 -0000
NNTP-Posting-Date: Sat, 30 Nov 2002 10:48:00 GMT
Kevin Aylward wrote:
> John Popelish wrote:
>> With, say, R1 and R2 1 meg each, And C2 very much smaller than C1
>> (and, possibly adjustable) to produce a net follower gain of slightly
>> more than 1 at the high frequency end of the response, to not only
>> reduce the diode capacitance effect, but to neutralize the diode
>> capacitance at the transimpedance amplifier input. Wouldn't this do
>> away with the need for the cascode transistor and current adding and
>> subtracting resistors?
> I'll have a think on what you are showing here. In the meantime, its
> worth pointing out that this two transistor circuit is just an obtuse
> way of achieving the more common npn/pnp compound pair that looks like
> one npn transistor. Topologically, they are *identical*. That is, the
> re of the top emitter pnp transistor is reduced by the hfe of the
> lower npn, so that the npn can be ran at a lower current, thereby
> reducing its base current shot noise.
Actually, I have this description a bit mixed up (e.g. the lower npn
re...), I wish I could draw ASCII, anyway, I'm sure you'll figure out
what I mean:-)
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