Virtual ground w/ Differential input?

Meathands · Jan 30, 2012

Just cooking up a portable mic pre based on a 5532 and hoping to run it on a single voltage rail. I have seen many unbalanced designs that achieve this with a virtual ground--can the same be done with a differential input by simply biasing the + input to half of the power rail voltage?

JohnRoberts · Jan 30, 2012

Yes... V/2 is commonly used...

Back a few decades ago over enthusiastic IC makers even developed dedicated V/2 ICs... But there weren't enough suckers willing to pay a premium for such a simple circuit. So they disappeared into oblivion where they belonged.

JR

Meathands · Jan 30, 2012

Thanks. And a nice little historical anecdote to boot!

Rochey · Jan 30, 2012

a pair of 1% resistors, and maybe a nice cap to drain any noise?

Meathands · Jan 30, 2012

Rochey said:
a pair of 1% resistors, and maybe a nice cap to drain any noise?

Talkin' 'bout the voltage divider, right?

Rochey · Jan 30, 2012

yeah, just wondering if the cap would then go to the negative rail. (all high freq noise then get's "shorted" to negative rail)

Meathands · Jan 30, 2012

I usually see it going to ground. What do you mean "negative rail" in this case, the inverted input?

Rochey · Jan 30, 2012

well, your "ground" is now the negative rail

Meathands · Jan 30, 2012

Oh, I was thinking of it this way: in a virtual ground situation given a 9v battery the + rail is 9v the "ground" is 4.5v and the "- rail" is the earth/chassis reference.

gemini86 · Jan 30, 2012

Well to be technically accurate, ground is usually 0v, which is more negative than 9v. Or at least that's the point Rochey is making (I think...

)

But yes, it's pretty typical to add a cap from vbias to ground (negative rail). In a bipolar setup, you would have caps from both v+ and v- to ground, so you could probably add a series cap from vbias (4.5v) to v+ as well, but I don't know if there's any benefit.

alexc · Jan 30, 2012

the JLM meter buffer circuit is an example of a differential input amp using a single opamp with a positive supply rail and ground.

http://www.jlmaudio.com/AT51%20DIY%20VU.pdf

Cheers

PRR · Jan 30, 2012

> cap from vbias to ground (negative rail).

You bypass to external signal common.

While this might also be V- with NPN schemes, with PNP it was often V+, and with opamps neither of the two rails may be signal common (see Cmoy headphone amp).

Andy Peters · Jan 30, 2012

Meathands said:
Just cooking up a portable mic pre based on a 5532 and hoping to run it on a single voltage rail. I have seen many unbalanced designs that achieve this with a virtual ground--can the same be done with a differential input by simply biasing the + input to half of the power rail voltage?

What is your positive rail?

the others have answered about the VCC/2 bias, but you might want to think about a rail-to-rail input and output op-amp so your input range isn't for shit.

-a

Meathands · Jan 31, 2012

I have a +24vDC wall wart I was planning on using. I think that should be plenty?

Virtual ground w/ Differential input?

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