[sdiy] Mind blown -- TL074 saturated outputs different

Steve Lenham steve at bendentech.co.uk
Wed Nov 28 10:18:05 CET 2018

On 28/11/2018 08:50, David G Dixon wrote:
> Thanks, René.  I think I will add an output resistor and a couple of zeners
> to set the output voltages of the comparators to certain values, as in the
> "precision schmitt trigger" of the link you provided.  I need to rely on a
> fixed output voltage to generate the specific positive and negative
> threshold voltages for the comparator logic, and I can achieve this with the
> zeners.

A warning based on previous experience: Zeners do not make very good 
small-signal clamps. Because they do not have a "hard knee" to their 
characteristic curve, when you run them at low currents the voltage at 
which they start conducting can be very different to that marked on the 
packet. To get the advertised voltage, you need to drive them with a 
fair bit of current.

To get real precision and repeatability, I'd be looking at an active 
(opamp-based) clamp circuit.

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.


Steve L.
Benden Sound Technology

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