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    2021-10-21 17:42:25
    What is an Inverting Amplifier?


    I. Working Principle
    II. Troubleshooting
    III. Applications


    I. Working Principle of Inverting Amplifier


    The inverting amplifier circuit has the function of amplifying the input signal and inverting the output. "Inverted" means that the positive and negative signs are reversed. This amplifier uses negative feedback technology. The so-called negative feedback is to return a part of the output signal to the input. In the circuit shown in the figure, the connection method of connecting (returning) the output Vout to the inverting input terminal (-) via R2 is negative feedback.

    Operational amplifiers have the following characteristics. When the power supply voltage is not applied to the output terminal, the non-inverting input terminal (+) and the inverting input terminal (-) are considered to have the same voltage applied, that is to say, it can be considered as a virtual short circuit. Therefore, when the positive input terminal (+) is 0V, the voltage at point A is also 0V.

    The input impedance of the operational amplifier is extremely high, and there is basically no current in the inverting input terminal (-). Therefore, when flows to R2 via point A, the currents of I1 and I2 are basically equal. Based on the above conditions, using Ohm's law for R2, we get Vout=-I1xR2. I1 is negative because I2 flows from point A where the voltage is 0V. From another point of view, when the input voltage of the inverting input terminal (-) rises, the output will be inverted and amplified greatly in the negative direction. Since the output voltage in this negative direction is connected to the inverting input terminal via R2, the voltage rise of the inverting input terminal (-) will be blocked. Both the inverting input terminal and the positive input terminal voltage become 0V, and the output voltage is stable.

    Calculate the gain through the relationship between the input and output in this amplifier circuit. The gain is the ratio of Vout and Vin, that is, Vout/Vin = (-I1xR2) / (I1xR1) =-R2/R1. The resulting gain is -, which means the waveform is inverted.

    Special attention should be paid to this formula, the gain is only determined by the resistance ratio of R1 and R2. In other words. We can easily change the gain by changing the resistance. Apply negative feedback to an operational amplifier with high gain, and by adjusting the resistance value, the desired gain circuit can be obtained.

    Due to the different ratios of the input resistance to the feedback resistance, the inverting amplifier can be divided into three types of circuits. which is:

    (1) Inverter. The proportional relationship between the feedback resistance and the input resistance is R2=R1.


    (2) Inverting amplifier. The proportional relationship between feedback resistance and input resistance is R2>R1.


    (3) Inverting attenuator. The proportional relationship between the feedback resistance and the input resistance is R2


    Among them, the inverter is also called the inverter circuit, which is named because the output and output change trends are opposite and their absolute values are equal; the inverting amplifier has the dual function of signal inversion and amplification; the inverting attenuator has the signal inversion and The dual role of attenuation. All three types of circuits have universal applications.

    The starting point of analyzing its circuit principle is still playing the leading role in the series resistor divider, and the op-amp device is a supporting role. Or it can be analyzed from the perspective of input current. As shown in the figure above, the bias circuit is drawn separately to better explain the problem.

    In the circuit a in the above figure, R1 and R2 form a series voltage divider circuit. The signal polarity determines the direction of the signal current, from bottom to bottom, that is, from the signal end to the output end. Since the same current flows into R1 and R2, R1=R2, and the voltage divider point is 0V, it can be inferred that the OUT terminal must be -1V. The -1V of the output terminal (0V of the voltage division point) is determined by the conduction degree of the output stage Q2, and is adjusted by itself according to the input signal.

    It can also be analyzed from another angle. Since the signal current flowing into R1 is +1mA, the current flowing through R2 must be -1mA to make the voltage divider point 0V. The task of the op-amp device is to automatically control the voltage across R2 (or control the reverse current that flows through R2 in the same amount as R1) so that its inverting input becomes 0V (the control target of the inverting amplifier).

    It can be deduced from this that when R2>R1, in order to obtain the reverse current flowing through R2, the current is still equal to R1, and the OUT terminal must be adjusted to -3V; when R2

    The entire op-amp circuit is a game of series resistor divider. As long as you master the analysis ability of the resistor series voltage divider circuit, you have found the golden key to analyze the principle of the operational amplifier circuit.

    The inverting amplifier is a circuit structure in which the non-inverting terminal is grounded (or grounded via a bias resistor), and the input signal enters from the inverting input terminal. From the "virtual short" relationship between the two input terminals (due to the grounding of the non-inverting terminal), the concept of "virtual ground" can be derived, and this concept only refers to the inverting amplifier. Due to the different considerations of the designer, the input of the non-inverting terminal is directly grounded, and there are also those connected to the ground by adding a bias resistor R3, and there is no difference in the analysis of the circuit principle and the troubleshooting.

    Obviously, because the non-inverting input terminal is grounded, the final control purpose of the amplifier is to always make the inverting input terminal become 0V ground level under the dynamic adjustment of the output stage in the amplification area, no matter what the input signal is. In the above figure, R1 is the input resistance, R2 is the feedback resistance, and the voltage amplification factor of the circuit = R2/R3.

    In this regard, the basic characteristics of the normal working state of the inverting amplifier can be drawn:

    (1) Both input terminals are 0V to ground.

    (2) The input and input signal voltages show a reverse trend, and the magnitude depends on the proportional relationship between R1 and R2.

    If you need to analyze or get a detection judgment, you must first determine the circuit in the figure. What point is the signal input terminal?

    Due to the "virtual ground" characteristic of the inverting amplifier, the two input terminals of the op-amp device itself are at 0V ground level and do not change with the input signal voltage change (or only transient changes, which are extremely difficult to capture in the measurement). Obviously, the right end of the input resistance R1 is not the signal input end, but the left end is the signal input end, so R2/R1=-VOUT/VIN- (the input voltage at the left end of R1).

    Or you can think of it like this: In the circuit shown above, both ends of R1 are the input voltage, and both ends of R2 are the reverse output voltage. The voltage of the circuit is amplified and whether it is good or bad. Just use two test leads to clear the two. That's it-the voltage across VR1 is the input voltage; the voltage across VR2 is the reverse output voltage.

    II. Inverting Amplifier Troubleshooting

    (1) The normal working state of the inverting amplifier.

    That is to meet two fundamental characteristics:

    a. Both input terminals are 0V to ground.

    b. The input and input signal voltages show a reverse trend, and the magnitude depends on the proportional relationship between R1 and R2.

    Not for this, that is, a fault state.

    (2) The principle of repairing non-inverting amplifiers still applies to inverting amplifiers.

    a. After the "virtual ground" principle is not met, first check according to the comparator principle. If it meets it, the op-amp device is good, and the peripheral components are faulty.

    b. If the principle of the comparator is not met, the op-amp device is broken.

    III. Applications of Inverting Amplifier

    (1) Integrator: Replace the resistance of the original inverting amplifier R2 with a capacitor C2. At this time, the relationship between the input signal Vi and the output signal Vo forms an integral relationship.

    (2) Differentiator: Replace the resistance of the original inverting amplifier R1 with an electric capacitor C. At this time, the relationship between the input signal Vi and the output signal Vo is transformed into a differential relationship.

    (3) Adder: If the inverting amplifier is slightly changed, the relationship between the input signal and the output signal Vo at this time, if R1 = R2 = R3 =...= Rn = Rf, it can be simplified to Vo =-(V1 +V2+V3+...+Vn), forming an additive relationship.

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