In electronic design, operational amplifier is one of the most common components used by engineers in various signal conditioning circuits. However, when there is not enough margin in the design to support the OPAMP to carry the load, the amplifier is prone to oscillations due to instability.

Lead to operational stability is the most common cause of the output capacitor, common capacitive load circuit including: capacitance, MOSFET, cable, high-speed optical coupling, etc., the capacitive load can't see specific capacitance values, the intuitive so in the design must check the op-amp output for connection to the above several kinds of capacitive load.

This article discusses why capacitive loads cause stability problems and presents a solution that uses isolation resistors to compensate.

1. The method of judging the stability of the circuit by the size of the phase margin

① Concept of phase margin and gain margin

Phase margin: The absolute value of the difference between the 180° phase and the open-loop gain in an amplifier's open-loop gain and frequency curve and the phase at which the gain decreases to a unit gain of benefit.

Gain margin: The absolute value of the difference between the gain at 180° phase and the gain at which the amplifier's open-loop gain decreases to gain per unit gain benefit in the amplifier's open-loop gain and frequency curve.

(2) Judgment of the stability of op-amp circuit

Stability can be judged by the criteria in the table. When designing the amplifier circuit, a phase margin of more than 45° should be ensured, preferably a phase margin of more than 60°. The gain margin should be above 10dB. When the phase margin decreases below 30°, convex peaks appear in the graph of frequency characteristics, and the very small load capacitance at the output will cause the circuit to oscillate.

It should be noted that different types of op-amps have very different capacity with capacitive load. The test circuit for testing op-amps with capacitive load capacity is often given in the data manual, which needs to be distinguished from the actual application. Not all opamps have a very capacitive output band.

Runshi Technology provides a wide range of operational amplifiers, which can better meet the different application needs of customers. Mainly includes the precise operational amplifier, high-speed operational amplifier, low noise operational amplifier and low disturbance voltage operational amplifier, micro power operational amplifier, high-pressure general amplifier, high pressure and high precision operational amplifiers, operational amplifiers, instrumentation amplifier, etc. Series of products, high quality, high performance, abundant selection, etc, are widely used in medical electronics, automotive electronics, Consumer electronics, industrial control and other fields!