[sdiy] Current source for LM13700

Richie Burnett rburnett at richieburnett.co.uk
Tue Apr 7 18:32:06 CEST 2020

> Firstly, there’s this example:
> http://www.bernacomp.com/elec/lockbox/lockbox_vca2.gif
> As I understand it, the current would be:
> Iout = Vin / Rin  (so R88 / 33K in this example)

Yes, because no significant current flows into the inputs of an op-amp.

> With a 33K input resistor, you’d need 66V before you went over the 2mA 
> that’d damage the 13700. So what does R87/6K8 do? Is it just 
> belt-and-braces protection?

I guess it provides additional protection if the op-amp output went to the 
positive rail and risked reverse breakdown the B-C junction of the 

> Similarly with D3. Testing the circuit in a simulator suggests that you 
> don’t get any output current with negative input voltages (the transistor 
> is switched off). So what’s the need for D3? Is it there to protect the 
> transistor junction when it is reverse biased?

It's there to maintain feedback around the op-amp if the input was to go 
even the slightest bit negative.  Otherwise the output of the op-amp would 
go to the positive rail and risk breakdown of the BC and BE junctions.  It 
essentially just provides a current path to stop the op-amp going open-loop 
when the input goes negative.

> Secondly, there’s this example by Olivier Gillet:
> https://mutable-instruments.net/archive/schematics/Shruthi-Analog-SMR4-mkII-v02.pdf
> What is the purpose of R7 / 2K2? Its value doesn’t affect the output 
> current, so what does it do?

Again, for current limiting if something goes wrong.  He just put it in the 
emitter path instead of collector.  It doesn't matter since for high hfe 
collector current and emitter current are practically equal.

> C8/100n presumably provides LP smoothing (a -6dB rolloff) to the PWM 
> input, but which resistor does it work in conjunction with? Is it R7 or 
> R8?

Phase-lead compensation, I would say.  To make sure the op-amp is stable 
right up until wherever the loop gain eventually crosses unity, and it 
doesn't oscillate at some MHz frequency.

There is also a common-base (grounded base) single transistor method of 
providing Iabc that Roland seem to use a lot in their early analogue synths, 
that doesn't require an op-amp.


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