Possible factors affecting the instability of op amp output

Today's electronic products are inseparable from the support of circuits. In most applications of integrated operational amplifiers, the problem of unstable output of integrated operational amplifiers has always troubled many electronic engineers. In the application of integrated operational amplifiers, integrated operational amplifiers that have undergone phase compensation are used in most applications. It can meet the requirements. However, self-excitation sometimes occurs during application. In fact, it is mainly caused by the following six reasons.

1. The phase correction circuit and parameter values ​​recommended in the integrated operational amplifier instructions are not corrected.

The compensation methods and parameters recommended in the manual are obtained through product design and extensive experiments, and are effective for most applications. It takes into account changes in frequency response characteristics caused by factors such as temperature and power supply voltage changes, and ensures a certain stability margin.

2. Poor power decoupling

When the power supply is not decoupled well, the voltage drop on the internal resistance of the signal current of each amplifier stage will produce mutual coupling. If the coupled signal is in the same phase as the input signal of a certain stage, the circuit will produce parasitic oscillation. For this reason, power supply decoupling must be paid attention to. When decoupling, in addition to adding a large capacitor to the power supply end, a small ceramic capacitor should also be connected in parallel, because large capacitors such as electrolytic capacitors have large distributed inductance, which affects the decoupling effect.

3. Influence of distributed capacitance during circuit connection

Due to the existence of distributed capacitance in the circuit, sometimes the signal of the subsequent stage will be fed back to the previous stage through the distributed capacitance. When this feedback signal is in the same phase as the original input signal of the amplifier stage, parasitic positive feedback will also be formed, causing the circuit to self-oscillate. Therefore, when connecting the circuit, it is very important to minimize the distributed capacitance. In particular, attention should be paid to keeping the "+" input terminal of the integrated operational amplifier away from its output terminal.

4. The impact of excessive load capacitance of the integrated operational amplifier

When the load capacitance of the integrated operational amplifier is too large, the open-loop frequency response curve of the entire operational amplifier circuit will change, reducing the phase margin of the circuit and even causing self-excitation. If a small resistor (for example, within a few hundred ohms) is added between the output end of the op amp and the external load capacitance to isolate the op amp circuit from the load capacitance, the influence of the load capacitance can be reduced. But sometimes the effect of this improvement is limited. In order to eliminate self-oscillation, the load capacitance should be reduced, or an output stage circuit with greater output power and better high-frequency response should be added to the output end of the integrated operational amplifier.

5. The grounding resistance of the non-inverting input terminal of the integrated operational amplifier is too large

When a large resistance is connected between the non-inverting terminal and the ground, it forms a new pole with the capacitance at the differential mode input terminal of the op amp. Although the capacitance at the input terminal is not large, the external resistance between the non-inverting terminal and the ground is large, so the newly generated pole may Close to or lower than the handover frequency, which causes the self-excitation of the closed-loop circuit or the deterioration of the circuit dynamic characteristics. A simple solution is to connect a capacitor in parallel with the non-inverting end to ground resistor to form a high-frequency bypass.

6. There are parasitic capacitances between the output terminal of the integrated operational amplifier, the non-inverting terminal and the zero-adjusting terminal.

When designing a printed circuit board or doing circuit experiments, if the lead layout is inappropriate or too long or too close, it will bring parasitic capacitance and cause self-excitation. Usually in low-frequency circuits, self-excitation is unlikely to occur, while in broadband amplifiers, care should be taken to eliminate parasitic capacitive coupling. The above are the possible reasons why the op amp output is unstable, and designers need to pay attention to it during development.