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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DE69C97.email@example.com> <3DE76E53.firstname.lastname@example.org> <3DE90887.A854256A@rica.net> <3DE9798E.22051DE1@SpamMeSenseless.us.ibm.com>
Subject: Re: photodetector circuit, high speed with good ambient light rejection
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Date: Sun, 1 Dec 2002 07:50:57 -0000
NNTP-Posting-Date: Sun, 01 Dec 2002 07:51:04 GMT
Phil Hobbs wrote:
> Kevin Aylward wrote:
>> 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.
> The Pauli principle is the physics that prevents shot noise in metal
I am just not happy with this terminology. A principle does not prevent
anything from happening. For a resistor to generate shot noise it must
be something other then a resistor.
>Shot noise occurs in a great many conduction processes, in
> fact any time that there is nothing forcing the electrons to be highly
> correlated--metals are the exceptions, but are of course common in
> circuits. People often try replacing metal resistors with, for
> example, current mirror sources, and are surprised at the poor
Shot noise is an effect that occurs when electrons are overcoming a
potential barrier in a statistical manner. ie. across semiconductor
junctions or from a heated cathode. It is not usually attributed as an
effect in bulk material. You would have to explain a mechanism of why
shot noise would appear in a homogeneous resistance.
>> 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.
> Um, no.
> That is, not unless you're going to call all circuits where
> one transistor drives another identical. Both transistors are NPN,
> for one thing. I don't think you've understood how the circuit works.
I understand exactly how the circuit works, apparently you dont. Do you
know what "topologically" means? Draw the small signal eqivelent circit
for the nmp/pnp compound pair and this dual npn circuit and it will be
>> 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!
> Jerry Graeme's book on transimpedance amps has a lot of circuits using
> discrete front ends on ordinary op amps. However, getting 1
> nV/sqrt(Hz) from a JFET requires a *big* JFET, for which read
> "high-capacitance". U309s are nowhere near 1 nV/sqrt(Hz)--the data
> sheet I'm looking at says 10 nV, typical, and that's at 10 mA drain
> current. At 1 MHz, that will contribute 6 pA/sqrt(Hz), which is 17
> dB above the shot noise of 2 microamps.
You have not looked at the actual graphs then. They get very low after
the 1/f region. There are a number of fets in the 5pf region with 1nv
type noise. The other option is a gasfet. I agree that you do need to
drive a bit of current through them though.
I prefer the jet source follower buffer, with its drain bootstrapped
(net Cin ~1p), driving a cascade of high pass amplifiers to produce an
overall flat response.
>> 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 lunch.
> Quite so--small diodes and high photocurrents make life very much
> easier, and should be used where possible. However, in the context of
> optical measurements, we commonly have to use large photodiodes to
> get a reasonable photocurrent--there's an invariant called the
> etendue, which is the product of the image size and the solid angle
> of illumination. For a given level of coherence, you can't focus an
> image below a certain size, which can easily be tens of thousands of
> wavelengths' diameter. Laser measurements or optical communications
> have a different set of tradeoffs, and I don't know a great deal
> about building 10 Gb/s photoreceivers. I picked a difficult but
> reasonably common sort of problem encountered in building optical
> You might be surprised at the number of people who build beautiful
> optical instruments and then go and lose 20 dB of SNR in a crummy
> front end amplifier because they don't know any better.
Not really surprised at all:-)
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