[sdiy] voltage controlled slew with transistors

[René Schmitz]

Hi Chris,

here is how you do that with a one-mirror discrete OTA.
Basically you need to turn the opamp into an integrator, for that you 
should use the output current rather than the voltage difference of the 
collectors. So the long tail works on a current mirror, and the current 
difference is fed to the integrator. Then you need to add some feedback 
and you get a current controlled one pole. See the attachments.

Best,
  René

On 14.10.2021 03:08, Chris McDowell via Synth-diy wrote:
> Howdy list,
>
> I'm working on an envelope design, and currently using an LM13700 for the "make ramps" part, but wondering if anyone knows of some resources or reasonable strategies for replacing that with a few transistors? I've been playing in LTSpice trying to see how a discrete OTA arrangement can do it (and I know it can) but I don't understand it enough to make it a 1 pole filter cell instead of a VCA. I've got a long tailed pair with tail current control, and I understand how we can take the difference of the collector voltages to get a VCA, but not how to keep it a current output for charging a cap. obviously have some kind of hole in my understanding! :) any pointers would be much appreciated.
>
> Cheers,
> Chris
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Version 4
SHEET 1 880 680
WIRE 96 160 16 160
WIRE 208 160 176 160
WIRE 208 240 208 224
WIRE 16 256 16 240
FLAG 208 240 0
FLAG 16 256 0
SYMBOL cap 192 160 R0
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL res 192 144 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 4k
SYMBOL voltage 16 144 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value PULSE(0 1 0.1 1u 1u 5)
TEXT -18 280 Left 2 !.tran 15