resistor and capacitor in series between op amp terminals

[Rainbow00]

Hi,

some op amp circuits have resistor (a few k) and capacitor (100pF or so) in series placed between inverting and non-inverting terminals. can someone advise what is the purpose?

another question, my understanding is that analog circuits draw continuous current from power supply as compared with digital circuits. however, this does not mean that analog circuits draw constant current from power supply. for example, op amp driving a large resistive load, the current for sure will be different when outputting HIGH or LOW. am i right? therefore, in some way, op amp is also switching and able to generate noise. op amp is quieter maybe because the output isn't switching that fast. am i correct?

thanks
br
Gordon

 

[LvW]

Re: resistor and capacitor in series between op amp terminal

Hi,

some op amp circuits have resistor (a few k) and capacitor (100pF or so) in series placed between inverting and non-inverting terminals. can someone advise what is the purpose?
...............

thanks
br
Gordon

Hi Gordon,

The RC series connection between both inputs is a kind of external frequency compensation. It is necessary only for non-universal compensated opamps.
The effect is as follows:
Above the corner frequency of the RC combination the feedback factor "beta" is reduced to a lower value which has to be chosen such that the stability criterion can be fulfilled with sufficient margin at the point where the loop gain is zero dB.
(That means: In the BODE plot the curve 1/beta goes up to a certain level and crosses the open-loop characteristic in a region with a slope of app. -20 dB/dec).

Interestingly, the overall closed loop gain is NOT influenced by this method since the input signal is damped by the same factor. As a result, even for an opamp which is not compensated you can choose the gain as you want and stabilize the circuit with sufficient margin.
That means: Gain formulas apply further: G=1+R2/R1 or G=-R2/R1 .
Disadvantages: Increase of the noise level and input resistance reduction (for non-inverting gains only.)

 

[Rainbow00]

Hi LvW,

thanks for you reply. appreciate it. i guess i need to brush up on feedback and compensation technique for op amps to understand you fully.

br
Gordon