As the most general analog device, operational amplifier is widely used in signal conversion and conditioning, ADC sampling front end, power supply circuit and other occasions. Although the peripheral circuit of the operational amplifier is simple, there are still many points needing attention in the use process.
1. Pay attention to whether the input voltage exceeds the limit.
Part of the input electrical characteristics in OP07 data sheet of ADI, it can be seen that the range of input voltage is 13.5 V under the condition of 15 V power supply voltage. If the input voltage exceeds the range, then the operational amplifier will work abnormally and some unexpected situations will occur.
However, some operational amplifiers are marked with the common-mode input voltage range instead of the input voltage range, which is a part of TI’s TLC2272 data table. Under the condition of single power supply +5V, the common-mode input range is 0-3.5V. Actually, when the operational amplifier works normally, the input voltages of the non-inverting terminal and the inverting terminal are basically the same (virtual short and virtual broken), so “input voltage range” and “common-mode input voltage range” all have the same meaning.
2. Do not directly connect capacitors in parallel at the output of the operational amplifier.
In the DC signal amplification circuit, sometimes in order to reduce noise, the decoupling capacitor is directly connected to the output of the operational amplifier in parallel. Although the DC signal is amplified, it is unsafe to do so. When a step signal is input or turned on, the output current of the operational amplifier will be relatively large, and the capacitance will change the phase characteristics of the loop, resulting in self-excited oscillation of the circuit, which we do not want to see.
The correct decoupling capacitor should form an RC circuit, that is, a resistor is connected in series at the output of the operational amplifier, and then the decoupling capacitor is connected in parallel. This can greatly reduce the instantaneous output current of the operational amplifier, and will not affect the phase characteristics of the loop, thus avoiding oscillation.
3. Do not connect capacitors in parallel in the feedback loop of the amplifier circuit.
It is also a circuit for DC signal amplification. In order to decouple, the capacitor is accidentally connected to the feedback loop, and the phase of the feedback signal changes, so it is easy to oscillate. Therefore, in the amplifier circuit, the feedback loop cannot add any circuit that affects the signal phase. C3, which thus extends to the regulated power supply circuit and is connected to the feedback pin, is wrong. To reduce the ripple, C3 and R1 can be connected in parallel, and the negative feedback effect of ripple can be appropriately increased to suppress the output ripple.
