Transistor Gain

We know that the current flow between the emitter and base controls the current flow between the emitter and collector. Also, the amount of current flow between the emitter and base will affect the amount of emitter collector current. The ratio between these two currents is known as the "gain" of the transistor. This gain allows us to use a transistor to control a large current with a very small current similar to the way a relay operates. Example shown: if a transistor had a gain of 100 and the emitter-base current was increased by 10 milliamps or .01 amps, the emitter collector current would increase by 100 times or 1 amp. This type of increase will occur until the transistor reached saturation. This is the point where increasing the emitter-base current does not increase the emitter-collector current. Transistors used for switching usually operate at the saturation point when turned on, while transistors that are used for amplifiers operate in the range between off and saturation.

Another application for a transistor is amplification. This situation takes advantage of the relationship between the emitter base current and the emitter-collector current. Since a small change in current flowing through the transistor from the emitter to the base has a proportionally larger effect on the emitter-collector current, we can use transistors to increase the strength of a small signal in a radio or to provide a variable control for a motor.

On some Toyota models, transistors are being used to provide variable speed control such as the AC blower motor on the Cressida and the electric motor that runs the power steering pump on the 1991 MR2. By varying the emitter-base current of the transistor, the current flowing through the motor can be varied, thereby varying the motor speed.



ICs are classified by the number of parts included on one chip. The Small Scale Integration (SSI) IC has about 100 elements; the Medium Scale Integration (MSI) IC has 100 to 1,000 elements; the Large Scale Integration (LSI) IC has 10,000 to 100,000 elements; and the Very Large Scale Integration (VLSI) IC has more than 100,000 elements.

Taken with permission from the Toyota Advanced Electrical Course#672

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