In op amp circuits you will commonly see several capacitors, for example:
- Power supply VCC to ground connection
- Between the feedback network and the input/output pins
- Between the positive and negative inputs
Power supply decoupling capacitor
This capacitor is mainly used for filtering, keeping the power network cleaner and suppressing interference signals.
Typical markings are 103, 104, or 105, corresponding to about 10 nF, 100 nF, or 1 μF. Choose smaller capacitance for higher-frequency operation and larger capacitance for lower-frequency needs.
Capacitor in the feedback path (between input and output)
This capacitor has two main functions:
- Frequency compensation: it compensates for the input capacitance that slows the response, helping to prevent the op amp from oscillating.
- Filtering: it suppresses high-frequency interference to protect subsequent circuitry.
Typical values are on the order of tens of pF.
Capacitor between the positive and negative inputs
This capacitor is used for interference rejection. For example, if a strong interference spike appears at the non-inverting input, that input voltage momentarily exceeds the inverting input and the output produces a disturbance. With a capacitor between the inputs, the output disturbance is quickly fed back to the inverting input through the capacitor, because the voltage across a capacitor cannot change instantaneously. The disturbance therefore appears on both inputs and is largely cancelled.
Typical values for this capacitor range from a few pF to several tens of pF.
Summary
Most capacitors around an op amp are there for interference suppression and filtering. A circuit may operate without them, but adding these capacitors improves stability and reduces susceptibility to noise, so designers typically include them in op amp circuits.
