How to modify this circuit for variable clipping without affecting gain?

Question

Below is a basic implementation of a circuit used to clip an incoming signal using the diodes in the feedback loop of the op-amp. Since the diodes are different they have different characteristics, specifically Vf which causes each one to start conducting at slightly different voltage. The diodes are also placed in opposite to each other so one may conduct on the positive half of the wave and the other on the negative (given the input signal and gain product is high enough).

Each one of the diodes will start conducting when the voltage between the inverting input and output of the op-amp exceeds its Vf.

I would like to modify the circuit so it would behave in the same manner but I will be able to control the DC offset at one end of the diode.

For example, you can see that the input is bias with a 5V DC voltage, so a diode starts conducting when the voltage reaches 5V + Vf. I would like to be able to adjust the bias voltage over a small range so that a smaller signal and gain might cause clipping or the other way around.

I would like to do this without moving the diodes outside the feedback loop. I tried inserting a simple series resistor after the didoes and it seems to me that by altering the value of the resistor I can obtain the effect I want but in practice this does not work well enough and interfere with the gain of the circuit which I don't want to change because of the diode adjustments.

I'll appreciate suggestions if this is possible.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

OK so here is the circuit suggested in an run-able and edit-able version :)

schematic

^{simulate this circuit}

• user16222: What are you after? A clipping cct without varying gain? You could buffer the output with a push-pull stage powered from different rails and close the feedback loop around the push-pull
• user34920: @JonRB - I'm after a clipping circuit where I can change the threshold voltage without affecting the gain by doing so.
• Tut: Analog Devices AN-106 "A Collection of Amp Applications" by James Wong: Figure 37 (last page)

Answer 1

How to provide a variable clamp while keeping OPAMP gain the same.

An output push-pull stage powered from separate rails/references is able to provide clamping capability independent of OPAMP gain (within the range of the clamp)

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• user34920: Please see my updated original post. The idea is to change the voltage rails of the transistor so they will be lower than the op-amp's output thus clipping the signal? I see the cap on the output to allow a low impedance path for the AC signal I assume? What about the resistors? Why this configuration?
• user16222: The cap is a speedup cap. If you are always after clipping the rails to the output clamp could be provided by opamps with a less than unity gain. But if as you describe you always want it to clip and equally reduce... Then the output will tend towards zero. What exactly are you trying to achieve

Answer 2

You can also use front to back Zener diodes in the feed back path. Described in detail by Bob Pease here.

• Olin Lathrop: Link-only answers are not appreciated here. At least put the basics of the answer here, then you can refer to a link for more background information.
• user16222: not only that but the clamping can't easily be changed and is limited to the decimation of zener voltages - not knocking zener clamps, use them all the time for OPAMP integrators BUT it would not satisfy what the original question asks for
• George Herold: Hi @JonRB, (Have you read the article?) Figure 6. shows an "IC" zener in a bridge that I think could be used for variable clamping. (Maybe a TL431) And he has some active clamp circuits, with diode's and opamps in the feedback path.