[sdiy] Mind blown -- TL074 saturated outputs different

Steve Lenham steve at bendentech.co.uk
Sun Dec 9 15:58:02 CET 2018


>     If, however, it is only bipolar symmetry that you are after rather than
>     an exact level, consider putting a single Zener inside a diode bridge.
>     That makes the positive and negative clip levels identical.
> 
> 
> 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?

It is the same bridge configuration of four diodes that you would find 
in a power supply. Your signal goes to the two input pins (which would 
usually be marked "AC" in a power supply scenario) and your Zener (or 
TL431, or whatever) connects across the two output pins (usually marked 
+ and -, and where you would find the big smoothing capacitor in a power 
supply).

That's all there is to it! The resulting configuration will clamp in 
both directions at the Zener (etc) voltage plus two diode forward 
voltages (you can use Schottky diodes for the bridge to minimise this if 
you want). Don't forget that you still need a current limiting resistor 
as you would with a normal Zener circuit.

HTH,

Steve L.



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