Please take a look at this link about biasing input for single ended op-amp topology: http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/index.html

In figure 1, I notice they put a capacitor in SERIES with the non-inverting input. enter image description here

What exactly is the purpose of this capacitor? Is it input offset elimination? Does it affect overall op-amp performance and make the op-amp unstable by any chance?


1 Answer 1


It is just an AC coupling capacitor. It prevents any DC component of the input signal from being amplified. Ra and Rb serve to set the DC input voltage to halfway between the rails. It is common in any application where the DC component is unimportant, and especially in single supply circuits such as this one, where the "mid rail" voltage is +Vs/2 rather than 0 VDC. The DC level on the output is blocked by Cout, and the gain at DC is reduced to one by C1.

Because the capacitance is not inside the feedback loop, it does not affect the stability of the amplifier.

  • \$\begingroup\$ Thanks for your answer Evan. In figure 6 of the link, the coupling capacitor is placed at the inverting input and in series with the input feedback resistor, could it affect the op-amp performance in this topology? \$\endgroup\$
    – kingyo
    Jul 19, 2016 at 9:13
  • \$\begingroup\$ Yes. Capacitors connected to the inverting input affect the feedback path. In fact, C1 in that figure has the same effect as C1 in the non-inverting amplifier you posted above. It doesn't matter which side the signal comes in. However, generally capacitors from - to GND increase stability, while capacitors from output to - decrease stability, so both circuits are fine. \$\endgroup\$
    – Evan
    Jul 19, 2016 at 15:27

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