question about op amp biasing (single supply)

[armstrom]

I was reading a TI document about single supply op amp circuits and found something interesting. Everything I've read up to this point indicates a biasing voltage of half the supply voltage should be applied to the non-inverting input to properly bias a single supply circuit.

This document, however, seems to indicate that this bias voltage can be applied to the inverting input instead. As a matter of fact, this seems to be touted as a preferred method.  Here's a link to the document: http://instruct1.cit.cornell.edu/courses/bionb440/datasheets/SingleSupply.pdf Pages 5-6 illustrate the type of circuit I'm talking about.

So, is there really anything to be gained by biasing in this way? Any drawbacks?

[teemuk]

I don't quite follow you. There is nothing peculiar or different in those circuits. If you reference one of the inputs to the DC bias it affects both of them (unless you do something to skew it).

Bear in mind that the portrayed circuits are simplified. The inverting one references the non-inverting input (to the DC bias) and the inverting input is capacitively coupled and hence left to float in the correct DC potential.

The non-inverting circuit lacks the important input resistor that would tie the non-inverting input to the correct DC bias point. Without this resistor/reference the input floats. (The circuit is simplified and in reality you often need this component to prevent problems caused by the floating input). Note the important part of the text: "Any dc offset in either stage is multiplied by the gain in both, and probably takes the circuit out of its normal operating range."

The input is naturally capacitively coupled and hence isolated from the DC potential present in the input source. The inverting input is once again referenced to the DC bias, if you capacitively couple it - like it is often wise (hi-pass filter=zero DC gain) - you can also reference it to ground or any other similar low-impedance reference. In this case having the DC bias reference in the non-inverting input naturally becomes mandatory.

[J M Fahey]

Dear Armstrom.
I agree fully with what Teemu says, only adding that I went through all example schematics (well, I got bored after Fig 20) and in all of them, bias or a resistive DC path are applied to the non inverting input, from now on the + input for short; *except*, as you accurately point out, in the inverting examples in Figs 3 and 13.
Please rest assured that a resistive path to Vcc/2 or some equivalent bias voltage is implied, even if the drawing is just a general example, not a working circuit.
I think that the "drawing man" was just not very consistent in his conventions.
I found your link *very* interesting, thanks.
It´s very opportune for me, since I started 2 or 3 nights ago to study different audio applications for the LM324 quad, very low voltage, low power, dirt cheap op amp.
Yea, I already know "it sucks", anyway your posting is great and a goldmine of useful information.
Thanks again.
JM

[Minion]

Generally how you bias an opamp depends on it"s configuration...For a Non-inverting configuration can you bias the Non-inverting input of the opamp or you can bias it at the end of the Feedback loop...For an inverting configuration you can bias it through the Non-inverting input through a resistor.....


Cheers

[JohnSS]

If you're looking for a heavy Marshall type sound in a SS package, Hughes & Kettner Warp 7 amps are very underrated.  Tons of gain but also plenty of headroom for clean sounds.

I myself own the following SS amps:

Peavey Bandit 65
Peavey Transtube Blazer 158
Behringer GX 110
Epiphone EP-800R
Yamaha G30-112
Crate BX-80

and  one hybrid: Music Man HD210