Common base amplifier, common base amplifier circuit

Common base amplifier, common base amplifier circuit

The AC signal voltage is superimposed on the DC voltage, causing the forward voltage between the base and emitter of the transistor to change. Through the control of the transistor, the collector current changes more, and its variation produces a large voltage variation on the collector resistor, thereby achieving voltage amplification. There are three configurations of amplifiers: common base, common emitter, and common collector.

The point on the characteristic curve corresponding to the base quiescent current and voltage or the collector quiescent current and voltage of the transistor is called the quiescent operating point of the amplifier.

The performance indicators of the amplifier include:

1. Amplification (gain)

① Voltage gain: Ku = Uo/Ui or Kus = Uo/Us

② Voltage gain: Ki = Io/Ii or Kis = Io/Is

③Power gain: Kp = Po/Pi = |UoIo/UiIi| = |KuKi|

2. Input and output resistance

The input resistance is the load effect that the amplifier presents to the signal source; or the equivalent resistance seen from the amplifier input terminal into the amplifier, Ri = Ui/Ii.

The output resistance is the equivalent resistance of the amplifier output terminal to a voltage source with internal resistance, in which case the internal resistance of the voltage source is the output resistance of the amplifier; or the equivalent resistance seen from the amplifier output terminal into the amplifier, Ro = Uo/Io.

3. Others

Passband, nonlinear distortion coefficient, maximum undistorted output voltage, maximum output power and efficiency.

The figure below is the circuit diagram of the common base amplifier circuit.

The common base amplifier circuit is a circuit with a current amplification factor slightly less than 1 and a large voltage amplification factor. Its input signal Vi is input from the E pole and the output signal Vo is output from the C pole. The input and output signals are the same as the common collector amplifier mode, and there will be no phase reversal. That is, the output is in phase with the input voltage, and at the same time, its input resistance is small and its output resistance is large. The particularly outstanding advantage of the common base amplifier circuit is that the cut-off frequency of the transistor is increased by (1+β) times compared with the cut-off frequency of the common emitter circuit transistor. Therefore, the common base circuit has a higher operating frequency and is suitable for wide-band amplifier circuits, high-frequency resonant amplifiers, etc. The common base circuit is widely used in high-frequency circuits.

The following table compares the performance of three basic transistor amplifiers.

In the common base amplifier circuit, the input signal is input from the emitter and base of the transistor, and the output signal is obtained from the collector and base of the transistor. Because the base is a common ground terminal, it is called a common base amplifier circuit. The
　　common base amplifier circuit has the following characteristics:
　　1. The input signal and the output signal are in phase;
　　2. High voltage gain;
　　3. Low current gain (≤1);
　　4. High power gain;
　　5. Suitable for high-frequency circuits.
　　The input impedance of the common base amplifier circuit is very small, which will cause the input signal to be severely attenuated, and it is not suitable as a voltage amplifier. But its bandwidth is very large, so it is usually used as a broadband or high-frequency amplifier. In some occasions, the common base amplifier circuit can also be used as a "current buffer".