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Basic circuits of operational amplifier

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Op amp circuit diagram:

Op amp circuit diagram


  • Vp: non-inverting input

  • Vn: inverting input

  • Vo: output



1. The magnification of the Operational Amplifier is infinite (∞).


2. The voltage of the inverting input end of the operational amplifier is always equal to the voltage of the non-inverting input end.


∵ Vin = Vp - Vn and Vout = A × Vin (A: is the magnification)

Again ∵ A = ∞, Vin = Vout / A

∴ Vin => 0


Conclusion: Vp = Vn


☆Example 1:

the inverting input voltage is always equal to the non-inverting voltage

Explanation: Because the inverting input voltage is always equal to the non-inverting voltage, Vout is also 0.1V equal to the input (voltage follower), mainly because the input impedance of the op amp is infinite, and the output impedance is almost zero.

☆Example 2:


反向端电压永the inverting input voltage is always equal to the non-inverting voltage-1远等于同相端电压-1

Explanation: According to the basic principle that the inverting input is always equal to the non-inverting end, the inverting end voltage is 0.1V at this time, and because the voltage divider on the 1K resistor is 0.1V, then the voltage divider on the 100K resistor will be 10V, so Vo The output voltage is the sum of 100K and 1K resistor divider, which is 10.1V.

☆Example 3:


反向端电压永远等于同相端电压-2


Explanation: The circuit input is 24V DC power supply, which is filtered by C1, stabilized by R1 and D2 voltage regulators, and stabilized at 2.5V. At the same time, a 10K adjustable rheostat is connected in parallel at both ends of the voltage regulator tube, and the adjustable range is : 0V - 2.5V, and as shown in the figure, the non-inverting end of the operational amplifier is connected to the tap of the sliding varistor, so the voltage variation range of the non-inverting end is also: 0V - 2.5V, and the inverting end of the operational amplifier is connected to R2 and R3 In the middle of the voltage division, the voltage variation range on the 1K resistor of R3 is: 0V - 2.5V, then the voltage variation range on R2 is: 0V - 12.5V, and the output voltage is the sum of the voltages on R2+R3, so it is 0V - 15V variation range.

3. The input impedance of the operation Amplifier is infinite, which means that the input current of the op amp is zero.


运放同相端接地


Explanation: The non-inverting end of the op amp is grounded, then the inverting end voltage will be 0V, and the 100K resistor is 1V on the left and 0V on the right. If there is a potential difference, there is a current flowing through the 100K resistor, but because the input impedance of the op amp is infinite, there is almost no current flow into the op amp, so the current will flow through the 1000K resistor, then the voltage on the 1000K resistor is 10V. Since the output end is lower than the GND potential, the output is -10V (negative voltage).


Vout ⇒ A ⇒ B ⇒ GND


Explanation: The non-inverting end of the op amp is connected to a 3V voltage, then the inverting end voltage is also 3V, the 2K resistor is 5V on the left and 3V on the right, and there is a potential difference, there is a current flowing through the 2K resistor, but because the input impedance of the op amp is infinite, there is almost no The current flows through the op amp, so the current will flow through the 10K resistor, then the voltage on the 10K resistor is 10V, the voltage of Vout to ground is Vout ⇒ A ⇒ B ⇒ GND, so, Vout = (-10V) + 3V = - 7V.


★ Summary as follows:


1. The meaning of the non-inverting input and the inverting input:


(1) When the inverting end voltage is constant, the output waveform has the same phase as the non-inverting end.

(2) When the voltage of the non-inverting end is constant, the output waveform is opposite to the phase of the inverting end.

2. The magnification of the operation amplifier is infinite (∞).

3. The voltage of the inverting input end of the operational amplifier is always equal to the voltage of the non-inverting input end.

4. The input impedance of the operation amplifier is infinite, which means that the input current of the op amp is zero.


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