Vs and Vs/2 on LM324

[Jdansti]

Does the Vs/2 shown on pin 3 below mean that I need to put a voltage divider on Vs and connect it to this pin?

[deadastronaut]

that's what i would do..

another way of writing Vb i guess...

i may be wrong of course..and often am..

[Cozybuilder]

Looks right to me

[Jdansti]

Thanks, guys. Now for the next part. I know how to make a voltage divider that will supply 1/2 Vs. I read R.G.'s article here http://geofex.com/circuits/Biasnet.htm and it said to look at the bias current of the opamp. The LM324 data sheet says that the bias current is typically 45nA and max 100nA. Does that mean I just need to choose resistors that will be able to deliver at least that current? The article also says to avoid moving Vs around too much.

If I use R.G.'s example of using two 100k resistors in the divider, they "would keep the current down to 4.5V/100K = 45uA per input". Since I'm only using one input and need no more than 100nA, does two 100k resistors seem like a good choice?

[Cozybuilder]

For the voltage divider, you want to draw no more than 10% of supply current to keep the voltage constant. In your case you are only drawing 0.1 - 0.2%, well within that consideration.

[Jdansti]

Thanks, Russ!

[anotherjim]

Note the 324 input & output range is not symmetrical like most op-amps. The circuit may not care about or possibly even invites asymmetry, but if you do want to avoid it to maximise clean headroom, make Vcc/2 a little lower - about 0.75 volts below half of Vcc.

[R.G.]

Yep. Plus the LM324 inputs are known to be PNPs, so you know the input bias current direction is out of the input pins.

But DC bias point accuracy is not a huge consideration for AC amplification on 9V. If you're doing something like complex LFO generation, yes; or if you're running from much smaller voltages, which the LM324 can do, yes again.

[Jdansti]

Thanks Jim and R.G. I'm making an Atari Punk Console (APC) for my nephew, and the LM324 will be in a tremolo circuit that will be enclosed with the APC. Here's a link to the tremolo circuit: http://www.simplecircuitdiagram.com/2011/01/27/tremolo-effect/

As shown, the LM324 modulates the resistance of two diodes to create the tremolo effect.

As a side note, I'll need to make sure the output of the APC is <0.6V p-p to avoid clipping (I'll play with this on a breadboard, varying the output of the APC, and seeing if the clipping is desirable). I plan to run the output of the tremolo through an onboard 386 amp to a small speaker mounted in the enclosure.

[anotherjim]

Indeed, bias point probably not critical in that circuit, but it might be relevant if the LFO wave isn't symmetrical.
I haven't seen many diode VCA's - and that one is single supply. Saved to memory.