Voltage follower: the secret weapon to improve the quality of data transmission

Voltage follower is a kind of special electronic circuit, which can closely follow the input voltage change, and is widely used in the fields of signal buffering, matching resistance and level shifting. Its high input impedance and low output impedance characteristics effectively protect the signal transmission quality, and through the negative feedback mechanism and the special working characteristics of the transistor, the voltage and the input voltage are closely related. In practical applications, the voltage follower can significantly improve the efficiency and performance of electronic systems.

As a special electronic circuit, the main feature of the voltage follower is that its voltage is closely related to the change of the input voltage. This feature makes the voltage follower play an important role in electronic systems such as signal buffering, matching resistance and level shifting.

一、 The basic concept of voltage follower

A voltage follower, as the name suggests, is a circuit that makes the voltage closely follow the input voltage change. It usually consists of a transistor (such as the common collector of a bipolar transistor, or the source follower of a field-effect transistor) and its necessary bias and load elements. The main feature of the voltage follower is that the input impedance is high, the output impedance is low, and the voltage gain is close to 1(theoretically 1, there will be slight errors).

二、 The circuit structure of the voltage follower

Taking the common collector voltage follower of a bipolar transistor as an example, the basic circuit structure consists of an EPM1270F256C3 transistor, a base bias resistor, a collector load resistor, and an emitter resistor (sometimes omitted). The signal is input from the base of the transistor and amplified from the emitter according to the transistor. Due to the presence of emitter resistance (or the output characteristics of the transistor), the voltage will closely follow the change in the input voltage, but there is usually a certain current gain, and the voltage gain is close to 1.

三、The output voltage follows the input voltage system

1. Negative feedback effect

The voltage follower can closely follow the input voltage mainly because of its internal negative feedback system. In a common collector configuration, a portion of the voltage (depending on the emitter resistance or the output characteristics of the transistor) is fed back to the base and compared to the input voltage. This feedback will automatically adjust the operating state of the transistor, thereby reducing the difference between voltage and input voltage. When the input voltage changes, the negative feedback system will respond quickly, so that the voltage changes accordingly, thus keeping the two close.

2. The working characteristics of the crystal pipeline

The common collector of the transistor has a special output characteristic, that is, the voltage changes with the change of the input voltage, but the change amplitude is relatively small. This is because in the common collector configuration, the collector current of the transistor is mainly controlled by the base current, and the voltage is mainly determined by the collector-emitter voltage. Because the collector resistance is relatively large, the collector current change has little effect on the collector-emitter voltage, so the voltage can follow the input voltage change well.

3. With resistance and adjustment function

The high input impedance and low output impedance characteristics of the voltage follower also play an important role in its derived voltage following the input voltage. High input impedance means that the voltage follower absorbs very little current to the input signal, thus reducing the impact on the input signal source. The low output impedance means that the voltage follower can drive a larger load current while keeping the voltage stable. This matching resistance and regulation function enables the voltage follower to effectively protect the interaction between the front and back circuits during signal transmission and improve the quality of data transmission.

四、The relationship between voltage and input voltage

Ideally, the voltage of the voltage follower should be exactly equal to the input voltage. However, in practical applications, due to the nonlinear characteristics of the transistor, temperature drift, power supply fluctuations and other factors, there will always be some differences between the voltage and the input voltage. This difference is generally small, but may require special attention in some high-precision applications.

In order to reduce this difference, we can take a series of measures to improve the performance of the voltage follower. For example, select high-performance transistors, improve circuit layout to reduce the influence of parasitic parameters, and choose a stable power supply. In addition, by introducing an external feedback network, the performance of the voltage follower can be further improved, so that the voltage follows the change of the input voltage more accurately.

五、 Application examples of voltage follower

Voltage follower is widely used in electronic systems. Here are some typical application examples:

1. Signal buffering

In order to reduce signal attenuation and interference during signal transmission, a voltage follower can be added between the signal source and the load as a buffer stage. The characteristics of high input impedance and low output impedance of the voltage follower can effectively protect the interaction between the signal source and the load, and improve the quality of data transmission. For example, in audio amplifiers, in order to improve the driving ability and anti-interference of audio signals, voltage followers are often used as the derived buffer stage of the front circuit.

2. Match resistors

In circuit principle, matching resistance between different circuits is often necessary. The voltage follower can be used as a matching resistance circuit because it has the characteristics of low output impedance and high input impedance. By adjusting the parameter value and connection mode of the voltage follower, the excellent matching between different circuits can be achieved, and the efficiency and effect of signal transmission can be improved. For example, in RF circuits, voltage followers are often used in impedance converters to achieve a matched resistance between the antenna and the RF front end.

3. Level change

In some cases, it is necessary to adjust the output level of one circuit to the required level range of another circuit. The voltage follower can achieve the effect of level shifting by changing its bias voltage or introducing an external element. For example, in a digital circuit, a low-frequency signal can be converted to a high-level signal by a voltage follower to meet the solution of a subsequent circuit.