Transistors are one of the most commonly used components in modern electronic circuits and can be used in a variety of applications such as amplifiers, switches and voltage regulators, etc. It is a type of electronic circuit that uses the amplification of the transistor to amplify a small signal into a large signal to achieve signal processing and amplification functions. This article will explain in detail the three types of transistor amplifier circuit connection, first of all, to understand the basic knowledge of transistor amplifier circuit:

Transistor is a semiconductor device capable of controlling the flow of electrons, when the control terminal of the transistor (usually the base) receives an electrical signal, the flow of electrons inside the transistor begins to flow from the emitter to the collector, thus producing a current amplification effect. With proper circuit design, transistor amplification circuits can amplify small voltages to greater amplitudes to manipulate larger load currents.

There are two basic types of amplification circuits: common-emitter and common-collector. In a common-emitter circuit, the emitter of the transistor is connected to a common ground, the base is connected to the signal source through an input resistor, and the collector is connected to the load through an out resistor, so that when the amplitude of the input signal is increased, the transistor current is also increased, thus producing an amplification of the pay signal.

The advantage of common emitter is high gain but the disadvantage is that the output signal is opposite to the input signal (i.e., the phase difference is 180°).In a common collector circuit, the base of the transistor is connected to the signal source through an input resistor, the emitter is connected to the load, and the collector is connected to a common ground through a signal terminal. A common collector circuit provides a high resistance input, low resistance output circuit buffer that is characterised by low gain, but the output signal is in-phase (i.e. has a phase difference of 0°) with the input signal.

Transistor amplifier circuits can be used in a variety of household equipment such as televisions, stereos, radios, and computers, as well as in medical equipment, industrial control equipment, and communications equipment, and can be formed into circuits using either a single transistor or a number of transistors to obtain the desired output power and gain. In addition, amplifier circuits can be used for signal processing such as amplitude modulation, frequency modulation, amplification, filtering and distortion generation.

Three Connection Methods for Transistor Amplifier Circuits

Here focuses on three common connection methods of transistors: common emitter, common collector and common base:

1、Common emitter connection

Common emitter is one of the most common transistor amplifier circuit connections and the most basic. In a common emitter circuit, the emitter of the transistor is connected to a common ground, the base is connected to the signal source through an input resistor, and the collector is connected to the load through an out resistance.

When the amplitude of the input signal increases, the transistor current also increases, thus producing an amplification of the pay signal. The advantage of the common emitter is high gain, but the disadvantage is that the output signal is opposite to the input signal, i.e., the phase difference is 180°.

2, common collector connection

Common collector interface is a very popular transistor amplifier circuit connection, in a common collector circuit, the base of the transistor is connected to the signal source through an input resistor, the emitter is connected to the load, and the collector is connected to the common ground through a signal terminal.  

The common-collector circuit provides a circuit buffer with a high-resistance input and a low-resistance output, which is characterised by low gain, but the output signal is in-phase with the input signal, i.e., the phase difference is 0°. The common-collector circuit also has the unusual property that the output voltage remains almost constant when the load resistance varies, which makes it an ideal circuit for driving loads with constant signal strength.

3、Common-base connection

Common base connection is the least common of the three transistor amplifier circuit connection, in the common base circuit, the base of the transistor is connected to the signal source, while the emitter is connected to the common ground through a load resistor, the collector is also connected to the load through an output resistor.

Common base circuits are characterised by high input resistance and low output resistance and have the advantage of a wide pass band, high stability and linear amplification, and the disadvantage of lower gain than the other two circuits, but higher output voltage and power than the common emitter and common collector circuits.

These three transistor amplifier circuit connections have their own advantages and disadvantages and are suitable for different application scenarios, so when using transistors, you need to choose the appropriate connection method according to the characteristics of the desired amplifier circuit.