Introduction to the principle, circuit, function and characteristics of voltage follower

The voltage follower should be familiar to many electronic enthusiasts, but for laymen, they may not have heard of the voltage follower, so let's talk about the function and characteristics of the voltage follower.

 

voltage follower

 

The voltage follower generally refers to the emitter follower, and the emitter follower is also a common collector amplifier circuit, which is a widely used circuit. Its main function is to amplify the AC current to improve the load capacity of the entire amplifying circuit. In actual circuits, it is generally used as an output stage or an isolation stage.

How a voltage follower works

In the circuit, the voltage follower is generally used as a buffer stage and an isolation stage. The output impedance of the voltage amplifier is generally relatively high, usually several thousand ohms to tens of thousand ohms. If the input impedance of the subsequent stage is relatively small, a considerable part of the signal will be lost in the output resistance of the previous stage. At this time, a voltage follower is needed to buffer it. Play a connecting role. Another benefit of applying a voltage follower is that the input impedance is increased, so that the capacity of the input capacitor can be greatly reduced, which provides a premise guarantee for the application of high-quality capacitors.

Another function of the voltage follower is isolation. In HI-FI circuits, the controversy about negative feedback has been around for a long time. In fact, if there is really no negative feedback function, I believe that most amplifier circuits will not work well. of. However, due to the introduction of a large loop negative feedback circuit, the back electromotive force of the speaker will pass through the feedback circuit and be superimposed on the input signal. The sound quality is blurred and the clarity is reduced. Therefore, the final stage of some power amplifiers uses a circuit without large-loop negative feedback, trying to eliminate the disadvantages of large-loop negative feedback by disconnecting the negative feedback loop. However, since the operating current of the final stage of the amplifier varies greatly, it is difficult to guarantee the degree of distortion.

 

voltage follower circuit

 

The voltage follower is a common collector circuit, the signal is input from the base, and the emitter is output, so it is also called the emitter follower. The base voltage is in phase with the collector voltage, that is, the input voltage is in phase with the output voltage. The main features of this circuit are: high input resistance, low output resistance, and a voltage gain of approximately 1, so it is called a voltage follower.

voltage follower

Function and characteristics of voltage follower:

So what is the role of voltage follower? In a nutshell, the voltage follower plays the role of buffering, isolation, and improving the load capacity.

The characteristics of high input impedance and low output impedance of the common collector circuit make it play the role of impedance matching in the circuit, which can make the amplifier circuit of the latter stage work better.

The output voltage of the voltage isolator is similar to the input voltage range, and it is in a high-impedance state for the front-stage circuit and a low-impedance state for the subsequent-stage circuit, thus playing an "isolation" role for the front-end and back-stage circuits.

A voltage follower is often used as an intermediate stage to "isolate" the influence between the front and rear stages, which is called a buffer stage in this case. The basic principle is to use its characteristics of high input impedance and low output impedance.

The characteristics of high input impedance and low output impedance of a voltage follower can be understood at an extreme point. When the input impedance is high, it is equivalent to an open circuit to the previous stage circuit; when the output impedance is very low, it is equivalent to an open circuit to the subsequent stage circuit. Constant voltage source, that is, the output voltage is not affected by the impedance of the subsequent stage circuit. A circuit that is equivalent to an open circuit to the front-stage circuit and whose output voltage is not affected by the impedance of the subsequent stage certainly has an isolation effect, even if the front-stage and subsequent-stage circuits do not affect each other.

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