[sdiy] Clipping Amplifier questions...

[harrybissell]

Hi Bret (et al)

The circuit as shown cannot work.  It will have inverting gain of whatever
the resistor ratios are... but when the zeners conduct it will have a gain of
ONE.
For a clipper, you want the output level to be limited so that it does not
increase.

Someone might have thought this worked... if they chose a really high gain on
a small signal (guitar fuzzbox?)... and they saw the sudden reduction in gain as

being a sign of  "clipping"  In other words if the normal gain is 100, and the
limiting
gain is 1 ... it will act sort of like a clipper.  But if the normal gain is 2
its going to
really suck !

A better way would be to use the amplifier as an amplifer...and clip the output
with
a series resistor and the zener diodes directly...  or use two inverting
amplifiers and
make one a clipping amp...

H^) harry

Bret Truchan wrote:

> Hello!
>
> Does anyone happen to have the book "Practical Electronics for Inventors"?
> In it, there's a very nice looking circuit that they call the "Noninverting
> Clipper Amplifier" (page 245).  If you don't, I made a copy by hand
> (http://www.voxglitch.com/images/clip_amp.jpg)
>
> I can't, for the life of me, get this wee-little circuit to work.  The
> circuit is a normal op-amp based amplifier, but an addition of two zener
> diodes accross the feedback loop was supposed to limit the voltage (both
> positive and negative) to the cutoff voltages of the zener diodes.
>
> I intended to use this amplifier circuit for one of the inputs to a module.
> The module has an imbeded effects board, which has a 4V maximum rating on
> it's audio input.
>
> I had _thought_ that I had it working at one point and I created a PCB with
> the circuit on it.  However, it doesn't seem to be doing the job.  The zener
> diodes are 3.3V, but I can measure a full 12V on the output line of the
> op-amp.  I wired up the op-amp circuit again on a breadboard and got the
> same results.  Something is afoul.  I'm using TL082 op-amps.
>
> If anyone can take a moment to look at the circuit in question, I would be
> very happy.  Or, if I'm approaching this in the wrong way, let me know.  My
> intention was to have an input that I could control the level on (via
> potentiometer), but also have the input voltage clipped below 4 volts to
> protect the imbedded effects unit.
>
> Thanks!
> - Bret
>
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