Propeller Head Plaza

RE: Long-term DBT example

67.164.165.165

This Post Has Been Edited by the Author

In Reply to: RE: Long-term DBT example posted by rick_m on April 26, 2008 at 16:25:48

So the result is that there are two isolated signal paths, the main signal, and the half-wave rectified signal (the "distortion"). They are summed by the network feeding the non-inverting input to A4.

Yes, I agree, but I think we may be misunderstanding each other at some level. I'm familiar with the precision half wave rectifier circuit and how it works. For the moment, let's forget about the coupling caps at input and output, and look at the DC transfer characteristic of the circuit as a whole, from input to output, assuming ideal op-amps and shorted capacitors. The output of A3 will be zero when the input voltage is positive, and equal to the input when the input is negative. Then, when the output of A3 is scaled and summed with that of A1, the output of A4 (call it Vx) looks like this:

Vx = K1 * Vi for Vi >= 0
Vx = K2 * Vi for Vi < 0

This assumes Vi is the DC voltage at the non-inverting input of A1. As K1 approaches K2, the distortion approaches zero. This would be the situation if one could eliminate the contribution of the half-wave rectifier from the overall output. Distortion is increased by making K1 and K2 more different from each other. The distortion would be extremely high if the wiper of the pot were right at the A3 output, so the pot wiper must be positioned such that there is a lot of resistance between A3 output and A4 non-inverting input.

Let's call this DC relationship Vx = f(Vi). It has the interesting property that:

f(a * Vi) = a * f(Vi), where a is a constant (assuming no clipping).

In other words, it meets one of the requirements for a linear circuit, strangely enough. Scale the input and the output scales by the same amount. Of course, superposition does not apply because of the nonlinearity. Because of this relationship, the percent distortion (assuming ideal op-amps) will be independent of input level - just as the notation below the schematic says.

You are right, it ISN'T normal even order distortion, it's quite artificial but it does have the characteristic that at the origin there is no distortion energy.

I don't know what you mean by "at the origin there is no distortion energy". Assuming ideal components, the distortion percentage will be independent of signal level. Ultimately there will be some strange behavior for very small signals depending on the non-ideal nature of the real-world components.

It just increases with level in a different manner than normal higher order distorters do.

The distortion percentage should be constant with signal level, just as stated on the schematic.

Sorry for the verbosity of this post. I just wanted to clarify things as best I could. Hopefully I haven't confused matters more.

Follow Ups: