Morning clampers!

[PerranOak]

I was intrigued to learn of the "clamper circuit" - see attached.

I fiddled about in LTSpice but could only get the lowest peak to -164mV; so it's still a bit negative. The Schottky gives the best (i.e. closest to zero or +ve) even though its forward voltage is supposed to be 450mV.

I saw another circuit that uses a voltage source (say, a battery) "under" the diode to boost the level but this seems like cheating(!)

My questions are:
Is a Schottky the best diode choice?
Why is the lowest point 164mV and not 450mV?
Is there a way to stop it going negative without a battery?

Cheers.

[Berni]

Yep schotkey is the best you can do with this setup.

The way to prevent it from going negative is to use 4 diodes in a specific forward biased configuration like it is used here:
https://www.maximintegrated.com/en/products/analog/analog-switches-multiplexers/MAX4936.html

Its a bit of an unusual circuit that can handle very large voltages, while instead having the limitation of the signal it is passing getting clamped before it even gets to 0 so it instead needs a negative supply if you want your output signal to actually reach 0 at all. A very popular use of this circuit is medical ultrasound machines to protect the milivolt RX amplifiers from the >100V TX pulses.

[TimFox]

Another name for this circuit, especially in analog video circuits, is "DC Restorer", which is a misnomer.  It is used in AC-coupled signal chains to get the "black" level in a waveform that is known to be unipolar (i.e., positive or negative-definite).  A better method for video purposes is to use a FET switch ("keyed clamp") that is turned on at a specific interval ("back porch") of the waveform, instead of the diode.