Op Amp Variable Gain

[Raven Luni]

Greetings,

I was reading a bit about amplifier design using a single supply and came across an article that mentioned balancing the feedback resistor with the input offset resistors to avoid unwanted bias current (or something like that anyway). This got me thinking, if I wanted an amplifier (specifically a pre-amp stage) with variable gain, the obvious thing to me would have been to make the feedback resistor variable, but after reading that, it doesnt seem like the best idea.

I had a good search and nothing really useful came up except a few circuits for varying the sign (gain from -1 to +1) which is no use. I'm not sure taking that concept and ''offsetting' it would be a great idea either.

Would I be right in assuming a variable input signal (grounded pot) and a fixed gain would have exactly the same results as a variable gain?

 

[Arouse1973]

Yep, PD before the amp, like how a volume control works but of course they use log pots for audio.

 

[Harald Kapp]

balancing the feedback resistor with the input offset resistors to avoid unwanted bias current

This is right, but may be overengineering for a simple pre-amp. You can easily remove any offset from the circuit using AC coupling (putting a capacitor in the signal path). Google finds lots of opamp circuits using variable feedback for gain control.
The drawback of this technique is that changing gain may also change the frequency response of the amplifier because the R*C time constants within the amplifier change. An opamp configuration with high feedback resistances will be more susceptible to this effect than one using comparatively low feedback resistance (due to overal smaller R*C constants).

Otherwise your assumption is correct and as Arouse says it is often done this way.

The fixed gain method has one drawback, too: By first attenuating a strong signal then restoring the amplitude with a fixed gain amplifier you reduce the signal to noise ratio and amplify any noise from the circuit.
Have a look at the gain requirements (min-max amplitude of input signal vs. min-max amplitude of output signal. Maybe a 2-stage amplifier configuration could do better than a single stage.
Stage 1 has fixed gain and creates a signal that is within the full range even for the max. input amplitude. Follow this stage by a potentiometer and a second stage that has gain sufficicent to create a full scale signal from the min. input sgnal amplitude.

If you feel like t, you can add tone control between the two stages similar to this transistor design.

 

[Raven Luni]

The pre-amp is intended to drive the power amp + surface transducer discussed in this thread. (I'm pretty happy with that thing now and its pretty stable thanks to all the advice and some of the above mentioned refinements to the biasing / feedback).

Since you mention over engineering it does occur that input volume control isnt really needed. The amplifier is intended for an acoustic guitar (with a coil pickup) so the volume control should do - no sense in attenuating the signal twice.

I was also planning on having a baxendall tone control like the one you linked to.