The Transistor as a Switch

To illustrate the application of a transistor as a switch, first consider a simple circuit comprising a resistor and a real switch (Figure 10.8).

Figure 10.8 Resistor-switch circuit (a) Circuit; (b) Waveform

The labels V_DD and V_SS are commonly used in circuits employing MOSFETs. At this point we have little interest in their actual values and, for the purpose of these examples, need only assume that V_DD is more positive than V_SS.

When the switch is OPEN (OFF), V_OUT is connected via the resistor to V_DD; when the switch is CLOSED (ON), V_OUT is connected via the switch directly to V_SS. In this latter case, V_OUT takes the value V_SS because, like people, electricity takes the path of least resistance, and the resistance to V_SS through the closed switch is far less than the resistance to V_DD through the resistor. The waveforms in the illustration above show a delay between the switch operating and V_OUT responding. Although this delay is extremely small, it is important to note that there will always be some element of delay in any physical system.

Now consider the case where the switch is replaced with an NMOS transistor whose control input can be switched between V_DD and V_SS (Figure 10.9).

Figure 10.9 Resistor-NMOS transistor circuit (a) Circuit; (b) Waveform

When the control input to an NMOS transistor is connected to V_SS, the transistor is turned OFF and acts like an OPEN switch; when the control input is connected to V_DD, the transistor is turned ON and acts like a closed switch. Thus, the transistor functions in a similar manner to the switch. However, a switch is controlled by hand and can only be operated a few times a second, but a transistor’s control input can be driven by other transistors, allowing it to be operated millions of times a second.