A strange phenomenon was discovered during an experiment recently. Please help from experts to analyze:
Circuit description:
My circuit is framed in red on the left side of the figure. At this time, it uses 9V battery power supply. Zener tube Z1 is a 10V Zener diode (the power supply of the actual circuit is up to 30V). In order not to load the subsequent stage (it is an LDO, Not shown here, only a load resistor Rload is used to indicate that the input voltage is too high, so this emitter follower circuit is designed. The red frame on the right side of the circuit is another testing device made to test whether the voltage at point VF1 meets the requirements of the specification. LM358 is used for proportional follow-up. The non-inverting end of the op amp uses a 40M pull-down resistor. When the side circuit is not connected, prevent the op amp input from floating. As for why 40M is used, it is mainly because the resistance of R14 in the circuit on the left is too large to be 2.2M.
Phenomenon found:
1 When a 9V battery is used for power supply, the voltage at VF1 point measured by a multimeter is about 7.34V without connecting the subsequent detection circuit
2 Connect to the later stage operational amplifier detection circuit, and use a multimeter to measure the voltage at point VF1 to be about 6.73V. At this time, it is very strange that the input pin 1 voltage of the measured operational amplifier is about 6.73V, but the output pin 1 voltage 8.0V
3 Test the op amp follower circuit separately, use a 5V 9V power supply to add to pin 3, the follower output follows normal.
4 Modify R2 to 20M, then the measured VF1 voltage is 6.5V, and the follower output voltage is 7.3V.
5 Remove R2 and replace C1 with a 47pF capacitor. The measured VF1 is 7.02V and the follower output voltage is 8.3V
The R2 resistance of the later stage operational amplifier detection circuit and the R14 resistance in my product constitute a voltage divider, but due to the existence of the Z1 voltage regulator tube, the voltage divider relationship is not established. It can be determined that there is leakage current in Z1.
Question: Why does the output voltage of the LM358 follower output greater than the input? What are the factors?
Byron 게시 날짜 January 13, 2021
I suggest that you use an oscilloscope to measure, whether the op amp is oscillating, and whether the current flowing through the Zener diode is correct and whether it has a voltage stabilizing effect.
Adelaide 게시 날짜 January 13, 2021
It should be said whether it can stabilize the voltage when it is overvoltage.
Virginia 게시 날짜 January 13, 2021
I suspect that it is caused by the bias current of the op amp. The bias current of LM358 is tens of nA. Connecting an external resistance of tens of M will introduce an error of about 1V.
Marisol 게시 날짜 January 13, 2021
The actual input bias current is too small, that is, the bias is insufficient, but the offset takes the leading role; first, make the DC bias of the input differential circuit of the op amp right.
