Operational amplifier is a kind of amplifier that can realize different arithmetic operations by changing peripheral devices. Any operational amplifier integrates many transistors. These transistors not only constitute the basic working circuit, but also realize the protection functions such as input and output voltage clamping. However, due to the production process, parasitic transistors and diodes will inevitably be introduced in the process of manufacturing these transistors to ensure the normal operation of operational amplifiers. When the operational amplifier works within the working range specified in the specification, these parasitic transistors will not affect the operation of the chip. However, if the operational amplifier works beyond the scope of the specification, the output of the chip may be abnormal and enter the output clamping state, thus affecting the normal operation of the circuit. Taking lm358 as an example, this paper introduces the mechanism of its entering the output clamping state, and puts forward the solution to avoid the chip being clamped.1、 Principle of operational amplifier entering clamp state (taking lm358 as an example)Although the performance and specifications of operational amplifiers launched by various manufacturers are different from each other, generally speaking, standard operational amplifiers include the following three parts.1. Differential input stage: a differential amplifier is used as the input stage to provide high input impedance and low noise amplification.2. Gain stage: the main source of voltage gain of operational amplifier, which amplifies the input signal into single terminal output and sends it to the next stage.3. Output stage: the requirements of the output stage include low output impedance, high driving force, current limiting, short-circuit protection and other functions.80706ef6-4603-11eb-8b86-12bb97331649.pngIt can be seen that the basic components of all circuits are NPN tubes and PNP tubes, which are separated by a p substrate connected to the ground. As shown in Figure 2, this substrate separates all triodes. However, if the input terminal of lm358 (base pole of PNP) is too low and lower than a voltage of P substrate, this bias voltage will cause current to flow through the substrate and make the chip unable to work normally. Therefore, the specification of lm358 will specify the range of its input voltage, for example, the minimum voltage shall not exceed -0.3v.8092d162-4603-11eb-8b86-12bb97331649.png80b3ccf0-4603-11eb-8b86-12bb97331649.pngAs shown in Figure 4, the emitter of PNP tube, P substrate and collector of NPN tube constitute a parasitic NPN tube. When the input of op amp is lower than – 0.3V (such as – 1V), the emitter of parasitic NPN tube is – 1V lower than the base. This voltage is enough to turn on the parasitic NPN tube, so as to induce current from collector to base, There is an electrical connection between the two isolated transistors, and there is also an electrical connection with GND, so the chip will not work normally.80d6e406-4603-11eb-8b86-12bb97331649.pngFigure 5 shows all possible leakage current points of lm358 with red dots. Take point ① as an example. When it has leakage current to the ground, the PNP tube at the chip output will be turned on, so that the chip output is clamped to the low level.810059e4-4603-11eb-8b86-12bb97331649.pngThe leakage current at different locations will lead to different output states of the operational amplifier. Some may make the chip output high, and some may make the chip output low. For the same input, for example, the in input is – 1V, and the possible leakage current caused by it varies with the chip layout. Generally, the closer the transistors are, the more likely the leakage current is. For the same series of chips, such as lm358 and lm358b, due to the different layout of their bare chips, the same input exceeds the specification, The clamping state of the output is also different.2、 Possible problems caused by clamping state and their avoidanceIn the process of practical application, most engineers will avoid that the voltage of the input signal exceeds the range specified in the specification. However, due to the influence of power on sequence, the operational amplifier is prone to power on the measured signal earlier than the power signal, resulting in the chip exceeding the specification and entering the clamping state.Figure 6 shows a common voltage measurement circuit. Lm358b is powered by ± 5V. It is easy to calculate that Vout = VS1 * (- R3 / R1) = – 250V * (- 5.6 / 820) = 1.71v in normal operation.812eddaa-4603-11eb-8b86-12bb97331649.pngWe measured in the actual circuit and found that the output of the actual operational amplifier is related to the power on sequence of the chip, as shown in Table 1:816c4758-4603-11eb-8b86-12bb97331649.pngIt can be seen that when the power supply of the operational amplifier is powered on before the measured voltage, the output of the operational amplifier is normal, because the input voltage of the chip does not exceed the power supply voltage in the whole process, so it meets the application scope of the specification. When the power supply of the operational amplifier is later than the measured – 250V voltage, when the measured voltage is powered on and the operational amplifier is not powered on, the negative input terminal of the operational amplifier will have a negative voltage lower than – 0.3V. According to the analysis in Chapter 1, the output of the operational amplifier may be clamped to the positive power supply voltage or negative power supply voltage. For the lm358, its output is clamped to the positive power supply voltage. When the power supply is powered on, the + 5V power supply will pull the input terminal of the lm358 back to the positive voltage through the output terminal, so as to restore the output of the chip to normal. For lm358b, its output is clamped to the negative power supply voltage. When the power supply is powered on, the – 5V power supply pulls the input terminal of lm358b to a lower voltage (measured as -5.68v) through the output terminal. This voltage is 0.68v lower than the negative power supply. The output is always clamped near the negative power supply voltage and cannot return to normal.It can be seen from the above analysis that different clamping states may lead to different outputs. Since the clamping state is related to the layout of the chip, we cannot predict the clamping state of a chip. In order to avoid abnormal conditions, for the case that the input voltage may be powered on before the power supply voltage, we can:1. Manually control the power on sequence to ensure that the input voltage does not exceed the specification in the whole process.2. Add a ground clamp diode at the input of the operational amplifier to ensure that the input voltage of the operational amplifier will not exceed the specification in any power on sequence.In order to verify method 2, we added a reverse clamp diode to the negative power supply at the input of lm358 and lm358b, and the measured results are as follows:8197d6de-4603-11eb-8b86-12bb97331649.png3、 Conclusion1. If the input voltage of the operational amplifier exceeds the specification, the parasitic transistor inside the operational amplifier may generate leakage current, resulting in the output of the chip clamped to the positive power supply or negative power supply.2. Different clamping states may lead to different performances of the same circuit, some can work normally and some can’t.3. The clamping state of the operational amplifier is related to the layout of the die. Even the same series of operational amplifiers may have different layouts, resulting in different clamping states. Because the layout of the die is not public information, we cannot predict the clamping state of the operational amplifier. In order to avoid the abnormal operation of the operational amplifier due to the input exceeding the specification, we can add clamping diodes to the positive and negative power supplies at the input end to avoid the input exceeding the specification of the operational amplifier, so as to ensure the normal operation of the circuit.Author: Howard ZouSource: Ti


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