Resistors in the feedback of a buffer: Ask why!

RE: Resistors in the feedback of a buffer: Ask why!

John Caldwell — Thu, 17 Jul 2014 00:43:59 GMT

Deepak, this effect is also mentioned in the OPA134 datasheet in the section on source impedance and distortion.

RE: Resistors in the feedback of a buffer: Ask why!

John Caldwell — Thu, 17 Jul 2014 00:36:20 GMT

Deepak,

When used in a non-inverting configuration, all op amps produce some level of "common-mode distortion". This effect is worst with high source impedances because in some op amps the input common-mode capacitance varies with common-mode voltage. Capacitors with a voltage coefficient always produce distortion when driven from a non-zero source impedance. By matching the impedances presented to each input, the distortion produced by the varying input capacitance can be cancelled. Douglas Self demonstrated this in his article series on op amp common-mode distortion: www.eetimes.com/document.asp (see the measurements on the TL072 op amp). This effect is reduced in modern JFET input op amps but not completely eliminated. I plan to write an article on this topic later in the year so stay tuned!

RE: Resistors in the feedback of a buffer: Ask why!

Soufiane Bendaoud — Thu, 17 Jul 2014 00:01:16 GMT

Hi John:

Allow me to say that sometimes not having a feedback resistor can hurt even with bipolar devices. In the presence of large transients the protection diodes can short the input to the output if the feedback resistor isn't large enough. Of course if it's too large than one should consider filtering the additional noise generated by Rf and compensate adequately for the phase shift and you've done an excellent job at explaining the phase lead compensation. Thank you

RE: Resistors in the feedback of a buffer: Ask why!

Deepak%20murali — Wed, 16 Jul 2014 13:53:47 GMT

Nice blog. Can you give me more information on this line from your blog:

"Also, some op amps, especially JFET input types, may produce lower distortion if both inputs have matched source impedances."