Hi all,

I would like to construct a non-inverting amplifier with variable gain (from 1 to 10)
The input of the amplifier will have an offset of Vcc/2, hence why R1 is connected to Vcc/2.
Is the circuit below theoretically and practically correct for obtaining this (is it good practice as it is)?
Is it OK to have 0ohms at the feedback loop when the gain is set to 1 ?

 

Yes and yes.

 

Hi all,

I would like to construct a non-inverting amplifier with variable gain (from 1 to 10)
The input of the amplifier will have an offset of Vcc/2, hence why R1 is connected to Vcc/2.
Is the circuit below theoretically and practically correct for obtaining this (is it good practice as it is)?
Is it OK to have 0ohms at the feedback loop when the gain is set to 1 ?

View attachment 155488

Varying the amount of feedback to alter the amplifier gain is not the conventional method.

Normally the amplifier gain is fixed (at the maximum required), fed into a wiper pot to attenuate the output signal to the desired level.

The disadvantages of varying the feedback include the fact that the feedback signal is normally at a low signal level – running this to an accessible pot invariably adds noise; plus the performance of the amplifier may vary with gain.

 

Is it OK to have 0ohms at the feedback loop when the gain is set to 1 ?

If the OpAmp has compensation, either internal or external, for G = 1.

Otherwise its an oscillator.

Some ref material - https://www.maximintegrated.com/en/app-notes/index.mvp/id/864


Regards, Dana.

 

Thanks for the feedbacks (no pun intended ).

@Hymie , Do you mean connecting the output of the amplifier (with full gain) to a pot serving as a doltage divider? If so, I cannot do that as the signal is offsetted at Vcc/2.
@danadak , the op-amp datasheet stated that it is unity gain stable. Is this what you are referring to?

 

Thanks for the feedbacks (no pun intended ).

@Hymie , Do you mean connecting the output of the amplifier (with full gain) to a pot serving as a doltage divider? If so, I cannot do that as the signal is offsetted at Vcc/2.
@danadak , the op-amp datasheet stated that it is unity gain stable. Is this what you are referring to?

Your output pot would be connected between the op-amp out and Vcc/2

 

@danadak , the op-amp datasheet stated that it is unity gain stable. Is this what you are referring to?

Yes, thats correct. You are good to go. As long as your Cload is insignificant.

http://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html

http://www.analog.com/en/analog-dia...-to-avoid-instability-capacitive-loading.html


Regards, Dana.

 

Normally the amplifier gain is fixed (at the maximum required), fed into a wiper pot to attenuate the output signal to the desired level.

That's okay as long as the maximum input signal doesn't saturate the op amp at the desired maximum gain.