Constant voltage circuit and constant current circuit composed of operational amplifier and triode

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Constant voltage circuit and constant current circuit composed of operational amplifier and triode [Copy link]

Constant voltage circuit and constant current circuit composed of operational amplifier and triode

Constant voltage circuit

The 15V voltage provides enough current to the output end of the circuit. R3 is a current limiting resistor (in order to prevent a momentary conduction current, also known as surge current, due to the junction capacitance between the c and e poles of the transistor when the 15V voltage is just loaded. This resistor is generated to limit this current). R4 also plays a current limiting role. The transistor realizes the function of small current controlling large current.

The working principle of the circuit is analyzed in detail: the circuit starts -> compares with the voltage at the input end, when the output voltage is too large (too small) -> the negative input end of the op amp is compared with the positive input end, because a follower is implemented here, the voltage at the output end of the op amp becomes smaller (larger), and the voltage at the output end is continuously feedback-controlled, thereby achieving the stability of the output voltage. (The resistance value of R4 here should be relatively small to make the extended follower close to the traditional follower)

Constant current circuit

The constant current circuit of the op amp mainly utilizes the "voltage following characteristic" of the op amp, that is, the circuit characteristic that the voltages of the two input pins Pin3 and Pin2 of the op amp are equal; when Vin is input to the resistor R4 to stabilize the power supply voltage, the voltage across the resistor R7 is also Vin, so no matter how the external circuit changes, the current flowing through the resistor R7 is constant; the same as the principle analysis of the transistor constant current circuit, the current of the R2 load is equal to the current of the R7 resistor, so even if the power supply of the R2 load is a variable voltage power supply, the current of the R2 load remains fixed, achieving the effect of constant current.

Problem: The constant voltage circuit and constant current circuit composed of the op amp and transistor. There is no doubt about the constant current circuit. The constant voltage circuit and the constant current circuit have the same structure, so how can it become a constant voltage circuit? I can't see any difference!

bull_wang

Find a book on analog circuits, turn to the chapter on op amps, and read it carefully and you will understand.

QWE4562009

Nobody knows?

tomas_li

QWE4562009 posted on 2022-10-11 16:35 Does anyone know?

The constant voltage circuit uses the voltage stabilizing diode D1 to provide a 5.6V reference source, while the constant current circuit does not have a voltage stabilizing diode. How can we say that the circuit structures are the same?

QWE4562009

tomas_li posted on 2023-1-12 19:43 The constant voltage circuit uses the voltage regulator diode D1 to provide a 5.6V reference source, and the constant current circuit does not have a voltage regulator diode. How can we say that the circuit structure is the same?

Is it 5 followed by 6 or 6 followed by 5?