[sdiy] Analog envelope generator offset

[rsdio at audiobanshee.com]

Hi Tom,

I'll take a shot at explaining this.

First of all, I always start with the ideal rules for op-amps. An op-amp tries to do whatever it can with it's output until the + and - inputs have the same voltage, then it settles down. Also, there is basically no current in or out of the input terminals, at least for the steady state.

Looking at the circuit (I'm not on MuffWiggler, so I'm looking at the whole circuit on electro-music but using the labels from redfish.net), this means U1B will do what it must to make its '-' terminal match its '+' terminal. When the ADRS Gate turns off, the 2k2 and 47k resistors pull '+' to ground (since no current should be flowing through U1B+ and the diodes after U1A are not conducting, then there is no voltage drop across those resistors to ground). So U1B will try to make its '-' terminal reach ground, too. Following that terminal around, you'll see that it connects to Env Out, so it drives the Envelope Output to 0 V (ground), too. This again assumes almost no current through R8 at the steady state.

Note that this only works if the op-amp has a power supply that extends below ground. When C1 is discharging through D7, the op-amp is sinking current by driving the voltage low. That means D4 will be conducting and the op-amp output voltage will have to be -0.7 V. I point this out, even though most modular circuits always have about ±15 V supplies, because today it's becoming more popular to run analog circuits from a 5 V unipolar supply so that things can run from USB voltage as a computer peripheral. In that case, and op-amp powered from +5 V and 0 V will not be able to produce -0.7 V at its output. What's worse, even rail-to-rail op-amps will have trouble going below +0.7 V, so the circuit won't work. Anyway, just make sure you have a bipolar supply for the op-amp (or use a virtual ground that's high enough above the most negative supply).

I've ignored gain for the description above, but that doesn't mean gain isn't part of the overall circuit function.

Brian Willoughby
Sound Consulting


On Nov 1, 2014, at 6:08 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> Can you explain a bit more how it works please? I sort-of get it in general (I think) but I'd like to properly understand it, if I'm to adapt it for my ADSR situation.
> 
> What I think is going on is something like this:
> Imagine a gate signal going into the non-inv input of U1B. U1B provides a Gate-signal-with-gain at its output. The cap then charges towards this boosted voltage level. Once the cap voltage gets within one diode drop of the charging level, the diodes around U1B start to conduct, and the gain on the Gate drops towards unity.
> 
> I don't really get how the gain around U1B is limited to the-gate-plus-one-diode-drop, although I can see that it's clearly because of the back-to-back diodes in the feedback loop. I just don't see how. This is where a lack of formal training in any of this stuff starts to be a pain in the neck - again!. Still, I'm always learning.
>