Diode clipping question on output of opamp

Question

I have been trying to implement a basic hard-clipping circuit for a distortion guitar pedal, based on two simple diodes, but somehow I can't get it to work as it is supposed to be. The basic design is the following:

However, my out signal is something like this, which doesn't seem very "hard clipped".

In this basic circuit the signal gets clipped, but i don't know what could be the difference between the two:

Am I doing something wrong? The signal in OUT_INPUT_STAGE is a sine wave with an amplitude of 2 V, so it should be enough to be clipped by the diodes.

Answer 1

There are diodes better than everyday diodes :)

This is a Q1 working "backwards" (emitter and collector flipped) as an improved diode that clips very nicely:

^{simulate this circuit – Schematic created using CircuitLab}

Compare the characteristic curve of the improved diode to an ideal diode and a regular Si diode:

We can imagine using these improved diodes in a clipper.

^{simulate this circuit}

A complete clipper will look as follows:

^{simulate this circuit}

The circuit is designed for a single supply, e.g. 9V. The buffer between CLIP and OUTPUT has not been shown.

OA1 provides gain to the input signal. The gain can be variable. What matters here is that the signal is buffered before it enters the clipper.

Here's how the clipper reacts to amplitude adjustment:

And here's what happens when the stiffness is increased - there's punch-through, which may be a desirable effect in itself:

Answer 2

You won't get hard-clipping with a basic diode because its forward conduction point is not on/off at 0.7 volts but, gradually increases from about 0.5 volts to heavy conduction at 0.8 volts (typically). No basic diode will do what you are trying to do.

Your scope image is precisely what I'd expect: -

Image from Diodes.

Answer 3

The diodes don't suddenly begin to conduct when some voltage across them (700mV) is exceeded. Rather they begin to conduct as the voltage nears 500mV, conducting better and better as the voltage continues to rise, until about 700mV where they have negligible resistance.

This gives rise to the somewhat sluggish curves you see, instead of the nice sharp corners you want.

You can obtain much sharper clipping using so-called "diode-connected transistors", which behave much more like "ideal" diodes, and you can further improve things by placing them in the op-amp's feedback path, instead of relying on clamping the output of a follower via those 470Ω resistances.

Here's the simplest implementation I can think of right now:

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

To get the clipping in the diagram you'll need to use ideal diodes in spice that have a better roll off.

If you want to build a more realistic circuit, try bringing down the resistor below 400Ω (lower the better until but still limit current from the opamp) and ditch the capacitor.

If you want hard clipping,try an opamp clipping circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

Determining noise performance of clipper circuit

Answer 5

Put a small current through the diodes to make them limit harder. The output is limited to 200mVpp in this example:

Another version with adjustable limit that needs four diodes. Supplied with 1.5V DC you get 600mVpp at the output:

Answer 6

The difference in your two circuits is the following:

The op-amp controls its output to be an exact replica of the input (amplified) until it reaches its output current limit.

In the first circuit you have another 470 Ω resistor inserted between the clipping diodes and the stiffly controlled op-amp output. So the point after that resistor fails to maintain the sharply clipped signal from the op-amp output because the rising diode current also creates an additional voltage drop across the resistor, which smooths the signal.

In practise you always do want some resistance in series with the diodes though, as otherwise you create a short circuit condition. That might not immediately fry your op-amp or diode but is not a graceful operation condition regardless. So if you still want an analog hard clipping, you have to invoke some of the suggestions from other answers.