So this weekend marked the last screwing around (breadboarded, at least) with the VCO design for WTPA2. I knew that I could get the frequency range I needed out of a 4046 based VCO with a current sink in place of a frequency setting resistor but I was a little worried about temperature drift and very low frequency performance. Just for shits, I gave the circuit above a shot. It replaces the current sink with a standard two-transistor current mirror. The two devices above are crappily thermally coupled with a piece of heat shrink.
What I found with this is that temperature performance wasn’t too much better. It was still pretty easy to get this VCO to go crazy with the heat gun, although it was possible to get it to go crazy BI-DIRECTIONALLY based on the direction you were slathering on the sweat. Cute, but not useful.
One thing that WAS cool is that the current source compliance was great! The control voltage could get a lot higher than it could with the one-transistor sink before saturation, I assume because of the lack of emitter resistors in the circuit. Still, that was like a consolation prize.
So I caved and built this:
Here’s the schematic for it:
It’s what all the docs I saw suggested originally (a closed loop current sink where the op-amp compensated the temp drift of the transistor) and it totally works and does what it’s supposed to. The only annoying thing is that at VERY low currents the device is non-linear, but what are you gonna do. At a hundred Hz sampling rate all samples sound like farts anyway.
The CV has to be kept below about 2.2v to keep the transistor from saturating (the VCO goes nuts when this happens) but you can fix this with a divider on the input. The range with these components is about 0-20kHz with a CV from 0 to 2.1v or so.
Now I gotta figure out what to do with that other half of the 358. I sort of don’t want to pass it audio, because it’s a recipe for coupling noise into the circuit. We’ll see….
This week I’m working on the board layout for the first WTPA2 revision. I expect the next post here will be about that.