What are the characteristics of an emitter output device?

The characteristics of the emitter output device are: large input resistance, small output resistance, and voltage gain less than or close to 1. Because its voltage gain is less than 1 and close to 1, it has no voltage amplification effect, and the polarity of the output voltage is the same as the input, so this amplifier is also called a voltage follower, often referred to as "emitter follower".

The operating environment of this tutorial: Windows 7 system, Dell G3 computer.

The characteristics of the emitter output device are: large input resistance, small output resistance, and voltage gain less than or close to 1.

1. Because the circuit configuration of the emitter output device belongs to "voltage series negative feedback", the amplifier load resistance Re is not connected in the collector loop but in the emitter loop. Viewed from the input end The incoming input resistance Ri=rBE (1 β)Re≈βRe. It is equivalent to amplifying Re by β times, so the input resistance is very high. When the input end of the emitter output device is connected to the signal source, its power loss to the signal source is very small. This is an advantage of the emitter output device.

2. The output resistance of the emitter output device Ro=Ib(rBE Rg)/Ie=rBE Rg/1 β. It can be seen that the output resistance Ro of the emitter output device is β times smaller than the internal resistance Rg of the signal source connected to the input end. Even if a large load current is to be output, the impact on the output voltage is still small, which is also an advantage of the emitter output device.

3. Since the output voltage of the emitter output device is all fed back to the input circuit through Re, it is connected in series with the input signal and then added between the base and emitter of the transistor. The feedback voltage is the output voltage. Since the signal is input from the base and output from the emitter, Uo=Uo/Ui=Uo-Ube/Ui≤1. It can be seen that a characteristic of this circuit is that the voltage gain is less than 1.

4. Judging from the characteristics of the emitter output device’s large input resistance and small output resistance, it is equivalent to an impedance converter. Because its voltage gain is less than 1 and close to 1, it has no voltage amplification effect, and the polarity of the output voltage is the same as the input, so this amplifier is also called a voltage follower, often referred to as "emitter follower".

Extended information:

1. Purpose

In circuits, voltage followers are generally used as buffer stages and isolation stages. Because, judging from the characteristics of the emitter output device's large input resistance and small output resistance, it is equivalent to an impedance converter. Because its voltage gain is less than 1 and close to 1, it has no voltage amplification effect, and the polarity of the output voltage is the same as the input, so this amplifier is also called a voltage follower, often referred to as "emitter follower".

Serves as a link between the previous and the following. Another benefit of applying a voltage follower is that the input impedance is increased. In this way, the capacity of the input capacitor can be greatly reduced, which provides a prerequisite for the application of high-quality capacitors.

Another function of the voltage follower is isolation. In HI-FI circuits, there has been controversy about negative feedback for a long time. In fact, if there is really no role of negative feedback, I believe that most amplifier circuits It won't work very well. However, due to the introduction of a large loop negative feedback circuit, the speaker's counter electromotive force will pass through the feedback circuit and be superimposed on the input signal.

results in blurred sound quality and reduced clarity. Therefore, some power amplifiers use a circuit without large-loop negative feedback in the final stage, trying to eliminate the large-loop negative feedback by disconnecting the negative feedback loop. disadvantages. However, since the operating current of the final stage of the amplifier varies greatly, its distortion is difficult to guarantee.

The function of the voltage follower can be better applied. Placing the circuit between the preamplifier and the power amplifier can cut off the interference effect of the speaker's counter electromotive force on the preamplifier, greatly improving the clarity of the sound quality. .

2. Principle

Voltage follower, as the name suggests, means that the output voltage is the same as the input voltage. That is to say, the voltage amplification factor of the voltage follower is always less than and close to 1.

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