Use of Capacitors and Resistor Values in Differential Op Amp

[RoyAaron]

I am using the MC33171N as a differential op amp. (I’ve attached the schematic of that section.)
The general design is based in part on the IC’s product sheet (which I’ve attached) as well as similar designs on the net. One, in particular, includes a 4.7uF electrolytic cap in series with each input.
Would someone tell me what purpose those caps serve?

Also, from examples on the net I saw that the feedback resistor was equal to the resistor put to ground on the non-inverting side. Is this actually correct? My understanding is that for a gain (factor) of 10 I would use a feedback resistor ten times the value of the non-inverting side grounded resistor. (What is that resistor called, and what does it do?)

 

[rayma]

Those are DC blocking capacitors, and will prevent the pots from making noise and degrading.

 

[JonSnell Electronic]

Not sure who drew that schematic but it will be grossly unstable.
Overall gain is;
Rf + Rin / Rin.

Example; if
Rf = 22k
Rin = 1k
Gain = 22 + 1 / 1 = 13

 

[rayma]

For a bipolar op amp, the inputs should see equal DC resistances to minimize DC offset and drift.

 

[MarcelvdG]

It's strange that R12 and R13 have different values and that there are no fixed resistors in series with the capacitors. Now the op-amp's closed-loop gain becomes very large when you set the circuit to either zero or maximum gain.

 

[Mark Tillotson]

For a bipolar op amp, the inputs should see equal DC resistances to minimize DC offset and drift.

Except for audio or ac signal where offset and drift are irrelevant - its even got a fat electrolytic on the output in this case...

The main issue with the circuit is its not a differential amplifier since R12 and R13 are very different values, meaning the common mode gain is not zero.

High value feedback resistors can make stability more depedent on layout since stray capacitances will dominate the circuit up at MHz frequencies.

 

[Citizen124032]

It's a follow up of this.

 

[RoyAaron]

For a bipolar op amp, the inputs should see equal DC resistances to minimize DC offset and drift.

thank you for your reply . . . i will adjust

 

[RoyAaron]

Except for audio or ac signal where offset and drift are irrelevant - its even got a fat electrolytic on the output in this case...

The main issue with the circuit is its not a differential amplifier since R12 and R13 are very different values, meaning the common mode gain is not zero.

High value feedback resistors can make stability more depedent on layout since stray capacitances will dominate the circuit up at MHz frequencies.

thank you for your reply - understood