<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Hi All,<div class=""><br class=""></div><div class="">There are various ways of driving the Iabc input of the LM13700, from a resistor upwards.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">Firstly, there’s this example:</div><div class=""><br class=""></div><div class=""><a href="http://www.bernacomp.com/elec/lockbox/lockbox_vca2.gif" class="">http://www.bernacomp.com/elec/lockbox/lockbox_vca2.gif</a></div><div class=""><br class=""></div><div class="">As I understand it, the current would be:</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>Iout = Vin / Rin <span class="Apple-tab-span" style="white-space:pre"> </span> (so R88 / 33K in this example)</div><div class=""><br class=""></div><div class="">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?</div><div class="">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?</div><div class=""><br class=""></div><div class="">Secondly, there’s this example by Olivier Gillet:</div><div class=""><br class=""></div><div class=""><a href="https://mutable-instruments.net/archive/schematics/Shruthi-Analog-SMR4-mkII-v02.pdf" class="">https://mutable-instruments.net/archive/schematics/Shruthi-Analog-SMR4-mkII-v02.pdf</a></div><div class=""><br class=""></div><div class="">The two current sources are identical and both in the bottom left corner. If we take the lowest "gain control” one…</div><div class=""> </div><div class="">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).</div><div class=""><br class=""></div><div class="">What is the purpose of R7 / 2K2? Its value doesn’t affect the output current, so what does it do?</div><div class="">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?</div><div class=""><br class=""></div><div class="">Thanks for any help. I’d like to understand these useful circuits better.</div><div class=""><br class=""></div><div class="">Tom</div><div class=""><br class=""><div class="">
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