[sdiy] Current source for LM13700

Tom Wiltshire tom at electricdruid.net
Tue Apr 7 17:09:52 CEST 2020

Hi All,

There are various ways of driving the Iabc input of the LM13700, from a resistor upwards.

I’ve been looking at op-amp current sources, and there’s a couple of things I don’t understand and would like to check with you all, if I may.

Firstly, there’s this example:

http://www.bernacomp.com/elec/lockbox/lockbox_vca2.gif <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)

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?
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?

Secondly, there’s this example by Olivier Gillet:

https://mutable-instruments.net/archive/schematics/Shruthi-Analog-SMR4-mkII-v02.pdf <https://mutable-instruments.net/archive/schematics/Shruthi-Analog-SMR4-mkII-v02.pdf>

The two current sources are identical and both in the bottom left corner. If we take the lowest "gain control” one…
The current source takes a 0-5V PWM signal from the uP. With 18K that implies 277uA max current. The PWM doesn’t go completely to zero, so R9 / 1M is included to make sure the VCA turns off properly (notice this isn’t necessary on the Resonance control source above).

What is the purpose of R7 / 2K2? Its value doesn’t affect the output current, so what does it do?
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?

Thanks for any help. I’d like to understand these useful circuits better.


       Electric Druid
Synth & Stompbox DIY

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