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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 10:43:26 -0000
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John Popelish wrote:
> Winfield Hill wrote:
>> Hobbs points out that with moderate signal currents that create a
>> high r_e, an external bias current can be added to the BJT's
>> emitter and a like current taken from the collector. This approach
>> relies on the absence of shot noise in resistor-generated currents.
I think he is a bit of a smart arse for stating "because of the Pauli
exclusion principle...metal resistors..no noise.." etc. There is no shot
noise in resistors because there is no basic mechanism to generate shot
noise in resistors. Its not even worth mentioning, imo.
>> It would seem practical up to bias/signal ratios of up to say 25 or
>> 50, but at some point weak signals get one into trouble attempting
>> to balance the two bias currents and struggling with an unknown
>> lost-current factor from the BJT's beta. Hobbs suggests using
>> high-beta transistors like the MPSA18 (B>500), but watch out for
>> their input higher capacitance (e.g. 8pF), further complicating
>> issues at high frequencies. As is often the case, a panoply of
>> compromises must be examined.
> Win, I found Hobbs paper to be very interesting and thought
> provoking. I am weak on noise calculation problems and intend to
> study this at some length, to learn more. But the paper has given me
> a couple of other ideas I would like you and others to criticize.
> Again, referring to Philip Hobbs article:
> Figure 9 shows a combination DC voltage bias source and follower added
> to the diode to help reduce the effect of diode capacitance by
> bootstrapping. Is there any benefit to this approach without the
> cascode amplifier between the diode and the transimpedance amplifier?
> Isn't the main purpose of the cascode to allow the opamp to tolerate
> the diode capacitance better?
The cascode is to put a low resistance across the diode capacitance, but
by a method that does not involve a virtual earth.
> What if you replaced the follower transistor with a fast opamp
> follower like this:
> | |
> +-----------+----|-\ |
>> | | >-+--
>> ^ 0v-|+/
> +-|+\ |
> | >---+--+
> +-|-/ R1 |
>> | C1
> R2 |
> | C2
> | |
> +15 0v
> 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.
> One might even add a series resistor to C1 to extend the bandwidth of
> the follower.
> Even if this follower had the desired capacitance reducing effect,
> what other problems would such a circuit create?
The simplest improvement/simplification is simply to replace the 2
bipolar with a low noise jet. Your then left with basically 1nv/sqrthz
(e.g U309) voltage noise and essentially zero current noise. This
results in a much simpler circuit with equivalent or better performance,
and the analysis is also much simpler to boot!
In addition, this technique only works for the large diodes considered
here, e.g. 100p, in the sense of achieving the *best* S/N at a high BW.
The idea is that one is converting the 100p diode capacitance to the
input capacitance of the op-amp via the low re of the transistor. If the
diode was a high speed one in the 1pf range, and the circuit was
optimised as such, the capacitance at the opamp input node would kill
you. You would be back to square one, i.e. a current source with
capacitance, driving a virtual earth. There is no such thing as a free
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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