Common base amplifier circuit problems

shaorc

Common base amplifier circuit problems [Copy link]

The figure below is the circuit diagram of a common base amplifier circuit. This type of circuit inputs signals from the emitter and outputs signals from the collector. However, the direction of the be junction of the transistor in the figure is outward. How do we understand that the input signal is input from the emitter?

maychang

The direction of the arrow simply indicates the direction of the DC current in the transistor.

The signal is input from the emitter, and it should be understood that the AC signal Uin sometimes reduces the current flowing out of the emitter and sometimes increases the current flowing out of the emitter.

maychang

In fact, the same is true for the common emitter amplifier circuit. For NPN tubes, the base current always flows into the base from the outside. The input signal of the common emitter amplifier circuit also sometimes reduces the current flowing into the base, and sometimes increases the current flowing into the base.

maychang

Of course, "sometimes...sometimes..." refers to the Class A amplification of small signals. Both the common emitter circuit and the common base circuit can work in the Class B state or even the Class C state. When working in the Class B state, there is no such thing as "reduction".

guyanqiu

Personally, I think the common base circuit is more used in digital circuits. Early TTL inputs are generally this kind of common base circuit. When VE is high, the transistor is cut off and VOUT is high; when VE is low, the transistor is turned on and VOUT output is low.

In addition, there are many cases in RF circuits. The RF signal is relatively small, and the change of VE will cause the change of VC. In other words, to change VC, the current of IB must be controlled. The size of IB can be changed by controlling the VBE voltage. VBE can change VB (common emitter) and VE (common base).