I have a couple of “understanding” question about the Cadet II clipper circuit. I am trying to understand the exact behaviour of the diodes in the inverting op-amp feedback paths (page 1 of 2 of the schematic). I thought for a while that they might be biasing the op-amp virtual ground, but I’ve since decided that’s probably not right. Does anyone know what they do and how do that part of the circuit works?
The other thing I don’t quite understand is why the clip_ref is summed into both of the inverters. I thought maybe the clip output just needs to be biased, but if that’s the case then why do both of them need it and not just one?
There are a few ways to do a black/white level clipper circuit. This method, of using two sets of rectifier circuits in series was one we came up with early on.
First off, this type of rectifier circuit (wideband opamp with schottky diodes) is very accurate all the way down to 0V. The point of a precision rectifier circuit is to split the signal into two – negative portions and positive portions.
In the first opamp, the input signal is biased up by CLIPREF and inverted. What this does is set the input signal’s WHITE LEVEL (0.714V) to 0V. If white level were the top of its skull, it’s like the signal is now standing on its head!
Now with the headstand, if we only take the positive output from the first rectifier, we get the input signal clipped off at 0V (signals above 0V are currently below white). So this is our white level clipper, but the output is inverted and biased so that white is at 0V.
The second rectifier inverts the signal again and then biases up by CLIPVREF a second time. This places white back where it’s supposed to be (0.714V) and restores 0V = BLACK LEVEL. If we take the positive portion from the second rectifier, we now have a signal where all voltages below 0V (BLACK) have been clipped off (black level clipping.)
Schottky diodes and a high bandwidth opamp are important for this circuit or else you can get ugly distortions and poor recovery near the clipping thresholds.
Thanks Lars, that makes sense. So the (negative) clip_ref voltage is basically added by the first inverter, then (effectively) subtracted back out by the second one, all in order to facilitate a non-0V rectification in the first op-amp. The positive rectifications are achieved by splitting the feedback loop into two separate paths (via the diode orientation) in the feedback loop and then tapping only the positive one. That’s snazzy!