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Both types of BJT function by letting a small current input to the base control an amplified output from the collector. The result is that the BJT makes a good switch that is controlled by its base input. The BJT also makes a good amplifier, since it can multiply a weak input signal to about 100 times its original strength.
At low frequencies and under small-signal conditions, the circuit in Figure 1 can be represented by that in Figure 2, where the hybrid-pi model for the BJT has been employed. The input signal is represented by a Thévenin voltage source v s with a series resistance R s and the load is a resistor R L. This circuit can be used to derive the ...
The hybrid-pi model is a linearized two-port network approximation to the BJT using the small-signal base-emitter voltage, , and collector-emitter voltage, , as independent variables, and the small-signal base current, , and collector current, , as dependent variables.
Fig. 4 Top: Small-signal BJT cascode using hybrid-pi model Bottom: Equivalent circuit for BJT cascode using amplifier low-frequency parameters. The g-parameters found in the above formulas can be used to construct a small-signal voltage amplifier with the same gain, input and output resistance as the original cascode (an equivalent circuit).
Conversely, a voltage follower inserted between a small load resistance and a driving stage presents a large load to the driving stage—an advantage in coupling a voltage signal to a small load. This configuration is commonly used in the output stages of class-B and class-AB amplifiers. The base circuit is modified to operate the transistor in ...
It is frequently used as a small-signal transistor, [4] [5] and it remains a small general purpose transistor [6] of enduring popularity. [7] [8] [9] The 2N2222 was part of a family of devices described by Motorola at a 1962 IRE convention. [10] Since then it has been made by many semiconductor companies, for example, Texas Instruments. [11]