Transistor Operation

Author: N.H. Crowhurst

Tubes are not the only devices used for amplification. Transistors use an entirely different principle. We could epitomize the operation of a tube by saying that the plate and grid voltages combine to control the plate-to-cathode current. Or alternatively, in a given arrangement, the grid voltage controls the voltage and current in the plate-to-cathode circuit. In a transistor, it is the current in the circuit between emitter and base that controls the voltage and current between collector and base.

Comparison between a vacuum tube and a transistor

To understand what all this means, we need to know something about a transistor. It is essentially a piece of special alloy, using germanium or silicon as a base, with very small amounts (a few parts in a million, very accurately controlled) of an "impurity" added. Two electrodes are connected to this carefully blended base. They may be point contact "whiskers" or a "grown junction," formed by a process called electrolytic deposition.

Transistor operation: current in emitter circuit (at low voltage) produces almost identical current flow in the collector circuit (at much higher voltage).

However it is made, the transistor works by having a potential applied to the electrode called the collector in the direction against the normal flow of current for this kind of device. (Either of the junctions between the electrodes and the base would act as a rectifier allowing current to flow one way but not the other).

Until the emitter circuit has current flowing in it, the current in the collector will be very small - almost zero. When current is drawn through the emitter in the direction of easy flow, however, it allows an almost equal flow of current between collector and base, so the total current flowing in the base circuit is quite small compared to the other currents in the circuit. This action can be regarded as the emitter supplying the base with surplus electrons, which the collector can then draw off. The collector current is dependent on the emitter current. It is, af course, also dependent on the collector voltage; if there were no voltage on the collector, no current would flow in the collector circuit.

Amplification is achieved, as in the tube circuit, by connecting a resistor in the collector circuit, so the changes in collector current (which follow the changes in emitter current) produce fluctuations in collector voltage. We can, therefore, make an amplifier by having the current in the emitter circuit control the current and voltage in the collector circuit.

Transistor amplification: current fluctuations in emitter and collector circuit are almost identical; emitter voltages are smaller, collector voltages are much higher.