[sdiy] [Potential SPAM] Re: Mind blown -- TL074 saturated outputs different

[David G Dixon]

Yeah, Neil, that's more or less what I was trying to say, but since I didn't
do my doctoral thesis on 1N4148 diodes, I couldn't find the words. 

> -----Original Message-----
> From: Neil Johnson [mailto:neil.johnson71 at gmail.com] 
> Sent: Monday, December 10, 2018 1:42 PM
> To: David G Dixon
> Cc: Mattias Rickardsson; Steve Lenham; SDIY List
> Subject: Re: [sdiy] [Potential SPAM] Re: Mind blown -- TL074 
> saturated outputs different
> 
> Hi,
> 
> Matthias wrote:
> > Sorry to be late on this ball, but I remember seeing (and 
> saving) a schematic example of this, a single zener 
> surrounded by 2 or 4 diodes in some bridge configuration - 
> but now I cannot find it. The main point of it as I remember 
> was to get a very symmetric output by avoiding having more 
> than one zeners (since they are rarely as well matched as 
> regular diodes), and instead use the same zener as the level 
> reference for both the positive and the negative side. I 
> guess a TL431 could give an even better result.
> >
> > Can anybody give a pointer to where this has been described?
> 
> One example from the literature is Figure 10.6 in Sergio 
> Franco's "Design With Operational Amplifiers And Analog 
> Integrated Circuits"...
> see page 234 of the PDF in the web archive:
> https://archive.org/details/SFrancoDesignWithOperationalAmplif
> iersAndAnalogIntegratedCircuits1Pdf
> 
> David G Dixon wrote:
> > The idea is that the zener bridge gives more symmetrical 
> results than two back-to-back zeners.  However, it relies 
> upon the assumption that the two pairs of regular diodes are 
> identical.  This is probably more true than assuming that any 
> two zeners of the same nominal voltage are identical, but it 
> is still an assumption.
> 
> Well, almost, except without the ASSumption.  We don't ASSume 
> the bridge diodes are identical, it works on the basis that 
> any two randomly-selected silicon diodes' forward voltage 
> drops are significantly closer to each other than the reverse 
> avalanche/zener voltages of two randomly-selected Zener 
> diodes.  For example, if we consider a Vishay BZX85C5V1 zener 
> diode its VZ is 5V1 +/- 300mV (the spread gets wider for 
> higher voltage diodes).  Compare this to, say,
> BAT85 schottky diode, or 1N4148 silicon switching diode, and 
> the VF  - while still having some spread, is much tighter (in 
> the region of a few mV).  So no, we don't ASSume the diodes 
> are identical, but for the cost of four extremely cheap 
> silicon diodes we get a much better match of the positive and 
> negative clamp limits without the cost of either 
> hand-selecting zener diodes or paying for tighter VZ binning, 
> and as others have suggested you can also use more esoteric 
> devices like
> TL431 and so on.
> 
> Neil
> --
> http://www.njohnson.co.uk