Author Topic: Local voltage divider op amp bias  (Read 289 times)

jcknowles89

Local voltage divider op amp bias
« on: December 12, 2019, 08:49:49 AM »
Hello All,

So obviously an op amp in a non inverting configuration needs to be biased. Most of the time from a central Voltage divider network that feeds VCC voltage to the rest of the circuit. But I noticed in the Wampler Euphoria that there are local voltage dividers to provide Bias voltage instead of a central point.

Any Ideas on what this might do sound-wise to the circuit? I tried it on a breadboard and the overdrive had maybe a bit more Sparkle or shimmer or something like that.

Anyone else have any ideas or experiences?

Schematic referenced:
http://3.bp.blogspot.com/-FQJAmOvot_Y/UA0osWj9OtI/AAAAAAAAByQ/UOFTMmGWINc/s1600/wamp_Ecstasy.jpg

R.G.

Re: Local voltage divider op amp bias
« Reply #1 on: December 12, 2019, 09:39:51 AM »
Using multiple local bias references will do two things - prevent interactions of stages through loading of the central bias point, and cause minor differences in the bias of stages. It's very, very unlikely to cause audio changes on its own. Maybe through some other side effect, but not by there simply being multiple biases.

Taking the second item first, multiple bias points will vary from one another by at least the tolerances of the resistors that create them. They will also vary a tiny bit by the leakage of the caps bypassing them (if any) and any bias current drawn. That means each biased amp will have a slightly different DC level, even more different than the varying DC input offset of the individual opamps. No biggie - you just have to take into account that the stages will likely need DC blocking caps between them to prevent bias offset buildup.

Preventing interactions between stages can be a valid reason for multiple bias nets. If some stage(s) load or drive a bias point with a lot of current, it can inject that signal onto the bias point for other opamps and cause signal interactions between stages. Isolated bias networks prevent this at the cost of more parts, but with the benefit of (maybe) easier layout. In any case, each bias network needs to be thought through well enough to keep it noise free.

A particular issue is "grounding" clipping diodes to a bias point. This injects the diode conduction currents into the bias point, which may inject this as a voltage into other inputs. It's better to "ground" clipping diodes like this to a local ground point through a cap for DC blocking than to pollute the general reference voltage. Unless you like whatever side effects that causes.
R.G.

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PRR

Re: Local voltage divider op amp bias
« Reply #2 on: December 12, 2019, 11:46:45 PM »
It may be simpler to place several dividers than to route one Vref all over the board to multiple places.

Or it may just be Designer's Style (or mood).