Basic question on "pull-down/up resistors" and op amps

[boogietone]

Looking over many, many schematics using op amps as input buffers, output buffers, gain stages, etc. I cannot figure out the rational for the selection of the resistor between the op amp input and the 1/2 voltage line. I believe that this is a pull down resistor that fixes the DC voltage of the op amp input it is attached to. The range seems to go from 10M to 10K. Does it matter? Does it matter whether the input is the + or the - pin of the op amp? Does it matter which gain stage (I do seem to notice larger values used in earlier stages but may be imagining it)? Does this resistor figure into the gain of the stage? Everybody describes the basic gain theory and calculations of the feedback loop. However, I can find little on the theory behind this resistor how to select an appropriate value.

Thanks.

[Hides-His-Eyes]

It's setting the pin to "virtual ground". It's not involved in gain calculations but in a non-inverting gain stage does contibute to input impedance. You need it there because if you just used a wire...

-the input signal would be grounded through the V-ref smoothing cap, or

-If you had multiple gainstages you'd be shorting their inputs together, 'skipping one out'

etc.

The buzzword you need to google is "Virtual Ground", anyway.

[merlinb]

The larger that resistor is, the greater the output offset of the opamp, especially with BJT opamps. However, output offset is almost never of any importance in stompbox design.

Therefore, the remaining consideration is input impedance. Usually there will be a pulldown resistor on both sides of the input coupling cap, to prevent popping. The input impedance is therefore equal tot he parallel combination of these two resistors.

The heaviest load you want to present to a humbucker before noticeable tone loss is around 500k. Less with single coils. Above 1Meg there is negligible improvement, so this is the most common value.

Therefore, if you want to get an input impedance of 1Meg then you could, for example, use 2.2Meg on each side of the capacitor, making 1.1Meg total. This might be handy if you wanted to minimise the number of different valued parts to keep in stock, since both resistors are the same.

Alternatively you could use 10Meg on the 'outside' of the cap, and 1Meg on the 'inside', making just over 900k total, but with half the opamp offset, if you cared about such a thing.

[alanlan]

It serves 2 functions:
1) Sets the DC operating point of the stage
2) Provides input bias current for the + input (for bipolar input op-amps)

If you take the maximum input bias current required by the given op-amp and multiply by the bias resistor then that is how many volts might potentially be dropped across it.  This sets the maximum value.  Large values will also produce more thermal noise but how significant this may be depends on how much subsequent gain there is and how much is shunted by the resistance seen looking back into the preceding circuitry.  The value can be as low as you like but bear in mind it will load the previous stage i.e. it will affect the input impedance of the stage.