B2 · Diodes

Clipping and Clamping: Shaping Waveforms with a Diode

A single one-way valve can change the shape of a signal. In this unit you adjust clipping, which cuts off the peaks, yourself, and then move on to clamping, where the same diode shifts the baseline.

Lower the threshold and cut the peaks

The faint curve is the input, the bold curve is the output, and the gold dashed line is the threshold at which the diode turns on. Lower the threshold and the part above that line is cut off, flattening the output's peaks. Use the slider to find how far the threshold must come down before the peaks are cut off completely.

Clipping threshold VclipVclip = 0.95

Output peak

Vout = Vin

The threshold is above the peak. The diode never turns on, so the output is identical to the input.

Nothing clipped

A parallel diode pins the peaks

Pass the signal through a series resistor and connect a diode across the output, with its other end tied to a reference voltage. When the input exceeds the reference voltage plus the diode's threshold, the diode turns on and conducts, the excess voltage falls across the series resistor, and the output is held at that value. Above it, however high the input rises, the output cannot follow and is cut off. This is clipping.

Add a series capacitor and it clamps

In clamping, a capacitor is placed in series with the signal and the diode is connected across the output. When the input reaches its negative peak the diode turns on and the capacitor charges to the size of that peak; once the diode turns off, the capacitor holds that charge and adds a constant DC voltage to the signal. So the shape of the waveform stays the same while the whole thing shifts upward, and reversing the diode shifts it downward instead. Changing only the baseline without cutting anything is clamping.

The faint curve is the input and the bold curve the clamped output. The shape is the same and only the baseline has risen, so the output's lower peaks touch the horizontal axis.

One valve, two uses

Clipping and clamping are, in the end, the same action: a diode holding a voltage at its threshold. Placed across the output together with a series resistor it cuts off everything above the threshold and sets a limit; used together with a series capacitor it adds the voltage it once caught as DC and shifts the baseline. What gets cut or shifted is decided by the reference voltage and the direction of the diode.

ObserveVout = min(Vin, Vclip)

An ideal clipper cuts the output at the threshold.

ChooseVclip = Vref + ?

The real threshold is the reference plus the diode drop.

Fill inVout = Vin + ?

Clamping adds a constant DC to the input.

On your ownVdc ≈ ?

The added DC is the peak the capacitor charged to.

Back to the first screen

The lower you set the threshold, the more the peaks were cut, flattening the top of the output. That flat line is the trace of the diode turning on and holding the output at one value. Switch to a series capacitor and the same grip lifts the whole waveform instead of cutting it. In the end, clipping and clamping are two faces of one fact, that a diode fixes the voltage at its threshold, and this is how the one-way valve of B1 becomes a tool for shaping signals.

Clipping uses a diode connected across the output after a series resistor to cut off the part above the threshold and set a limit (Vout = min(Vin, Vclip)). Clamping uses a series capacitor and a diode across the output to add a constant DC and shift the baseline while keeping the waveform's shape (Vout = Vin + Vdc). Both are uses of the same principle: a one-way valve fixing the voltage at its threshold.

What comes next

So far the diode has worked in the forward direction. In the next unit you deliberately push it deep into reverse and meet the Zener diode, which at a certain voltage suddenly gives way and conducts while holding that voltage firmly. If clamping shifted the baseline, the Zener is a constant-voltage tool that fixes the output voltage itself.