2/4 quadrant multiplier theory

[Martin Czech]

> Okay, just sitting here thinking about something.
> A 2 quadrant multiplier is essentially a VCA right?
> And a 4 quadrant works similarly except that if the
> control signal goes negative the polarity of the
> signal is reversed (with respect to + control signals).
> Would it be possible to get similar results by using
> a Voltage Controlled Mixer mixing 2 signals 180 degrees
> out of phase.  Assuming a certain bias added to the 
> "control" signal, at 0 input we'd have 0 output (equal
> mix of signal and inverted signal), if the control
> swings positive, we get more straight signal, if it 
> swings negative, we get more inverted signal.  Am I 
> missing anything?  Thanks.  dave moylan
> 
There's an app note about this for the CA3080 at the Harris web site.
They use only one OTA and a resistor. Problem: tempco of gm.
Maybe it is a better idea to use a dual OTA, like you proposed.
But what happens at the crossover point for very little
control current Iabc ~ 0 ?

m.c.