Transistors as Switches

We will examine a switching transistorby reference to its output characteristics for the CE circuit configuration. Let the collector lead contain a load resistor R_C (Fig. 4.21).

Fig. 4.21. Transistor in pulse circuit CE

The applicable load line appears in Fig. 4.22. Before an input current or voltage pulse arrives at the input of this circuit, the transistor is turned off (it is said to be in the OFF state or at cutoff ). This condition corresponds to point Q₁. A small current I_CE0 is now flowing in the collector circuit and so this circuit may be assumed to be open. Nearly all of the supply voltage E_C is impressed on the transistor.

When we apply a current pulse I_Bmax to the input, the transistor is moved into the saturation regionand is operating at point Q₂. As a result, there appears a collector current pulse I_Cmax very close in magnitude to E_C /R_C. Sometimes, it is called the saturation current. At saturation, a transistor is in effect a closed switch and nearly all of E_C is dropped across R_C , with a very small remainder, called the saturation voltage U_CEsat, left across the transistor; its value is a few tenths of a volt.

Fig. 4.22. Finding transistor parameters in pulsed work from output characteristics

The maximum attainable value of collector current pulse is:

I_Cmax ≈ Е_C /R_C .

In addition to I_Cmax, I_Bmax and U_{CE sat}, the switching mode of operation is characterized by the force current gain В which, in contrast to the beta current gain, is defined in terms of the ratio between the currents at point Q₂:

B ≈ I_Cmax / I_Bmax <β.

Fig. 4.23 shows plots of an input current pulse which has a rectangular waveform and an output current pulse for a transistor with the CE connection.

Fig. 4.23. Pulse waveform by a transistor

Switching transistors must have small junction capacitances and a narrow base.

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