About Voltage Follower

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About Voltage Follower [Copy link]

The voltage gain of the voltage follower is 1, so it is called a voltage follower. It is a common collector circuit composed of a triode. The signal is input from the base and output from the emitter. The base voltage and the collector voltage have the same phase, so it is also called an emitter follower. Its simple circuit diagram is as shown in Figure (b). So:

The input signal is added to the non-inverting input terminal of the integrated operational amplifier. The output voltage is connected to the inverting input terminal, forming the simplest voltage follower.

The function of voltage follower:

Many materials say that the functions of voltage followers include buffering, isolation, and improving load carrying capacity. However, the explanations of these functions in these materials are incomplete. Therefore, I will explain my own views as follows.

The notable feature of a voltage follower is that it has a high input impedance and a low output impedance. It is easy to achieve an input impedance of several megohms. The output impedance is low, usually a few ohms or even lower.

Buffering function: The output impedance of a voltage amplifier is generally high, usually ranging from several thousand ohms to several tens of kiloohms. If the input impedance of the subsequent stage is relatively small, a considerable portion 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. It plays a connecting role.

Isolation effect: The output voltage of the voltage isolator is close to the input voltage amplitude, and it is in a high-impedance state to the previous circuit and a low-impedance state to the subsequent circuit, thus playing an "isolation" role for the previous and subsequent circuits.

Improve load capacity: The characteristics of high input impedance and low output impedance of the common collector circuit make it possible to match impedance in the circuit (impedance is literally different from resistance, in which only one word is the same, what about the other word? Simply put, impedance is resistance plus reactance, so it is called impedance; to be more comprehensive, impedance is the sum of resistance, capacitive reactance and inductive reactance in vector. In the world of direct current, the effect of an object on the current is called resistance. All substances in the world have resistance, but the resistance value is different. Substances with low resistance are called good conductors, substances with large resistance are called non-conductors, and superconductors, which are called superconductors in the high-tech field recently, are a kind of material with a resistance value of several Something close to zero. However, in the field of alternating current, in addition to resistance, capacitance and inductance will also hinder the flow of current. This effect is called reactance, which means the effect of resisting current. The reactance of capacitance and inductance are called capacitive reactance and inductive reactance, respectively, or capacitive reactance and inductive reactance for short. Their measurement units are ohms, the same as resistance, and their values are related to the frequency of alternating current. The higher the frequency, the smaller the capacitive reactance and the larger the inductive reactance. The lower the frequency, the larger the capacitive reactance and the smaller the inductive reactance. In addition, capacitive reactance and inductive reactance also have the problem of phase angle, and have a vector relationship. Therefore, it is said that impedance is the sum of resistance and reactance on the vector. It can make the next stage of the amplifier circuit work better. Take a typical example of application: the signal output of an electric guitar is high impedance. When connected to a recording device or speaker, adding this voltage follower before the tone processing circuit will make the impedance match and the tone more perfect.

summary:

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