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From: firstname.lastname@example.org (WHF)
Subject: Re: Heart Rate Sensor Circuit - Circuit Explanation
Date: 5 Nov 2002 09:03:18 -0800
NNTP-Posting-Date: 5 Nov 2002 17:03:18 GMT
I have a few questions regarding this circuit, if anyonje has the time
or the inclination to look at them I'd really appreciate it,
a) Why is the diode drop provided by Q4 & Q5 needed, is it purely to
dominate the gain for small voltages? also, why not just use normal
b) What effect do C3 & C5 have at high frequencies?
email@example.com (Tom Bruhns) wrote in message news:<firstname.lastname@example.org>...
> email@example.com (Padraig Fitzgerald) wrote in message news:...
> > Hi,
> > I have found this circuit on the net but I'm looking for information
> > on it's operation.
> > It's located at http://www.elecdesign.com/1997/dec1597/ifd/1IFD.pdf .
> > Basically it's the Lowpass filter part of the circuit surrounding
> > OpAmp A1 and BJTs Q4 and Q5 that I am confused about. I have searched
> > a lot of electronic books and Filtering techniques but have never seen
> > anything like this. I was wondering if anyone out there could shed
> > light on its operation, perhaps equations/formulae etc or at least
> > point me in the direction of some literature which would explain this.
> Note that both Q4 and Q5 have their base and emitter tied together.
> That means they will simply behave as the base-collector junction,
> which generally is a low-leakage diode. In series circuits of diodes
> and resistors (R2), it's difficult to write an analytic expression for
> the current versus voltage, but it's often enough to know that the
> diode looks like a very high resistance for low voltage. In other
> words, the R2 plus diodes is essentially not there for low voltages.
> I presume the idea is to bias things: they are the only path for the
> input bias current for the op amp inputs they are connected to on the
> left. They also provide some feedback for larger signals.
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