Why is it called a source follower? What are the functions and characteristics of a source follower?
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Why is it called a source follower? What are the functions and characteristics of a source follower?

Posted Date: 2024-01-31

Why is it called a source follower?

Source Follower is a common amplifier circuit, also known as voltage follower or common emitter follower. Its name comes from its property: the output follows the input voltage (that is, the source voltage).

The basic structure of a source follower consists of an NPN transistor or field effect transistor (BJT or FET) and a load resistor. The input signal acts on the base or gate of the transistor, while the output signal is obtained from the source (for BJT) or drain (for FET) of the transistor.

The name "source follower" is given for several reasons:

1. The source voltage follows the input voltage: The output voltage of the source follower is almost equal to its input voltage, so it can be said that the output voltage closely follows the change of the input voltage, and the follower plays a driving role in the input voltage.

2. The source current follows the input current: the output current of the source follower is almost equal to its input current, and the current change of the input signal will be reflected by the output current of the follower.

3. The source is a signal source: In some circuits, the input voltage of the source follower is directly provided to the downstream circuit as a signal source, so its function as a signal source can be inferred from its name.

The figure shows a common drain (source follower) amplifier circuit. Its working principle is similar to that of an emitter follower. It has the characteristics of extremely high input impedance, low output impedance, and voltage amplification factor of approximately 1. At the input end of measuring instruments, field effect transistor source followers are often used for impedance transformation.

Characteristics of source follower

Source Follower has the following main features:

1. Small gain: The voltage gain of the source follower is close to 1, that is, there is almost no voltage amplification effect. This is because the output voltage follows the input voltage, so when the input voltage changes, the output voltage changes accordingly, but with little amplification in amplitude.

2. Low noise: Since the input resistance of the source follower is relatively high and the output resistance is relatively low, the noise will be divided and damped. Therefore, source followers generally have lower noise levels.

3. Input-output impedance matching: Source followers have lower output impedance and higher input impedance. This impedance characteristic allows it to provide a good match between circuits, reducing signal distortion and reflections.

4. High current transfer capability: The source follower can provide a large output current because its output current is almost equal to the input current. This allows it to drive relatively large loads.

5. High stability: The source follower has good temperature stability and low drift. This is because its operating point is mainly controlled by the source voltage, which is relatively stable in many cases.

Source followers also have some limitations. For example, its voltage gain is small and it cannot achieve large voltage amplification. Furthermore, due to its operating principle, the output voltage of a source follower reduces the threshold voltage of one transistor.

The role of source follower

Source Follower has many applications and functions in circuits, mainly including the following aspects:

1. Buffer amplifier: When a source follower is used as a buffer amplifier, it can follow and amplify the voltage of the input signal to a lower output impedance to provide a stable signal source that can drive subsequent circuits.

2. Signal isolator: Through the source follower, the voltage of the input signal drives the source current, thereby generating a current related to the input signal at the output end to achieve voltage-to-current conversion. This current output feature can be used for signal isolation and driving circuits requiring higher currents.

3. Voltage step shifter: The source follower can move the voltage level of the input signal to a lower voltage range, such as converting a high-level signal to a low-level signal. This is very useful in certain circuit and system designs.

4. Impedance converter: The input impedance of the source follower is relatively high and the output impedance is relatively low, so it can be used as an impedance converter to convert a high-impedance input signal into a low-impedance signal output to match different circuits impedance between.

5. Level shifting of operational amplifiers: In some operational amplifier circuits, the source follower can be used as an input level shifter to move the level of the input signal to an input range more suitable for the operational amplifier.

As a common amplifier circuit, source follower has a wide range of applications in electronic equipment and circuit design. It can realize functions such as signal buffer amplification, level conversion, impedance matching and signal isolation.

Basic principles of source follower

The basic principle of the source follower is to use the working characteristics of the transistor or field effect transistor to achieve the following relationship between the input voltage and the output voltage.

Taking a transistor as an example, the basic principle of a source follower is as follows:

1. The input signal acts on the base of the transistor, and the base current drives the amplification of the transistor.

2. When the voltage of the input signal increases, the base current also increases accordingly. In an NPN transistor, the base current causes an increase in the collector current.

3. The increased collector current will cause the current of the PN junction (emitter junction) in the transistor to also increase through the amplification effect of the transistor. This current passes through the emitter of the transistor and is received by the load resistor.

4. The load resistor converts the current flowing through the emitter of the transistor into the corresponding output voltage. Since the load resistance is relatively low, the output voltage can be approximately equal to the collector voltage, that is, the source voltage.

The basic principle of the source follower is that the input signal is amplified by the transistor, driving the output current to flow through the load resistor, producing an output voltage similar to the input voltage. The output voltage is almost the same as the input voltage, so it can be said that the output signal follows the input signal. Compared with the input terminal, the output terminal has lower impedance and can provide larger output current.


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