inverting amplifier bias question

[bufferz]

Here is a simple inverting amplifier, my question is this:
I often see an additional resistor placed betwen the Bias voiltage and the non-invering input......what value does that add?




Cheers


***bonus question...... also I have seen non-inverting follower circuits placed at the very end of a circuit that do not have a resistor tied to to the bias voltage  - why would this bias volatge need to be introduced on buffer circuits at the begining of a circuit but not at the end?

[mth5044]

I can't see the picture, but I'm assuming you are talking about the Vref through a 1M resistor to bias? This is used for 'noiseless' bias or so it's called (or something)? Do a search for the noisless biasing on this forum and you will find a good explaination.

Signal is biased to work within the rails of the circuit, which is why it's done at the beginning. I'd assume you don't do it at the end because the signal is about to go into another pedal/amp that does its own biasing.

[induction]

The bias voltage is for setting the input signal to somewhere near the half-rail voltage so that it is in the op-amp's usable range. The output of the op-amp retains this bias, which must be removed before the output of the circuit by a coupling cap. This is called AC coupling.

But if the signal is going to be fed into another op-amp, you can retain the bias from the first stage. Then you won't need a connection to the bias divider on the input of the second op-amp because it would be redundant. This is called DC coupling.

The final op-amp has to be biased one way or the other, or it won't pass signal. The resulting bias of the output signal is usually more of an incidental consequence than a goal.

[bufferz]

Thanks guys, very helpful info.   

[bufferz]

Here is the thread regarding "noiseless bis" in case anyopne else comes looking : http://www.diystompboxes.com/smfforum/index.php?topic=93830.0

[bufferz]

so far everything I have found regarding the "noiseless bias" is referring to non inverting stages, it seems on the non-inverting follower:  the resistor from input to bias creates a high pass filter with the input cap....I still don't understand the application of the resistor tied from bias volatge to the the non-inverting input of an inverting amplifier stage

[merlinb]

Here is a simple inverting amplifier, my question is this:
I often see an additional resistor placed betwen the Bias voiltage and the non-invering input......what value does that add?

I cancels out input offset voltage, which is important in precision DC-coupled applications. In *some* situations it will also reduce common-mode distortion. However, it also adds noise, so it is of no use in guitar circuits.

also I have seen non-inverting follower circuits placed at the very end of a circuit that do not have a resistor tied to to the bias voltage  - why would this bias volatge need to be introduced on buffer circuits at the begining of a circuit but not at the end?

It gets the bias voltage from the output of the previous opamp, which has already been biased appropriately, i.e. it's output already rests at 4.5V.

(Noiseless biasing is a such a ridiculous name. Misleading and plain wrong.)