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The book goes over several more examples that you should go over pp. 123 – 130. 3.3 BASIC TRANSISTOR APPLICATIONS Transistors can be used to switch currents, voltages, and power; perform digital logic functions; and amplify time-varying signals. In this section, we consider the switching properties of the bipolar transistor, analyze a simple transistor digital logic circuit, and then show how the bipolar transistor is used to amplify time-varying signals. 3.3.1 Switch Figure 3.42 shows a bipolar circuit called an inverter, in which the transistor in the circuit is switched between cutoff and saturation. The load, for example, could be a motor, a light-emitting diode, or some other electrical device. If vI < VBE(on), then iB = iC = 0 and the transistor is cut off. Since iC = 0, the voltage drop across RC is zero, so the output voltage is vO = VCC. Also, since the currents in the transistor are zero, the power dissipation in the transistor is also zero. If the load were a motor, the motor would be off with zero current. Likewise, if the load were a light-emitting diode, the light output would be zero with zero current. If we let vI = VCC , and if the ratio of RB to RC, where RC is the effective resistance of the load, is less than β, then the transistor is usually driven into saturation, which means that
In this case, a collector current is induced that would turn on the motor or the LED, depending on the type of load. Equation (3.30) assumes that the BE voltage can be approximated by the turn-on voltage. This approximation will be modified slightly when we discuss bipolar digital logic circuits. Design Pointer: Motors tend to be inductive, so that during start-up and shutdown a relatively large di/dt voltage could be induced in the circuit. This voltage, especially during shutdown, could cause the transistor to go into breakdown and be damaged. When a transistor is biased in saturation, the relationship between the collector and base currents is no longer linear. Consequently, this mode of operation cannot he used for linear amplifiers. On the other hand, switching a transistor between cutoff and saturation produces the greatest change in output voltage, which is especially useful in digital logic circuits, as we will see in the next section. 3.3.2 Digital Logic In the simple transistor inverter circuit shown in Figure 3.43(a), if the input is approximately zero volts, the transistor is in cutoff and the output is high and equal to VCC. If. on the other hand, the input is high and equal to VCC, the transistor is driven into saturation, and the output is low and equal to VCE(sat).
There is a single dc power supply, and the quiescent base current is established through the resistor RB. The coupling capacitor CC acts as an open circuit to DC, isolating the input signal source from the dc base current. If the frequency of the input signal is large enough and CC is large enough, the signal can be coupled through Cc to the base with little attenuation. Figure 3.50(b) is the dc equivalent circuit; the Q-point values are indicated by the additional subscript Q.
Figure 3.51(a) shows the transistor characteristics and load line for the previous circuit. Although we assumed a current gain of 100, a given transistor type may exhibit a range of values for this parameter as a result of slight variations in the manufacturing process. For example, a second circuit with the same configuration as Figure 3.50(a) could be fabricated using a transistor with a current gain of 50. The transistor characteristics of this new circuit are shown in Figure 3.51(b). We can see here that the Q-point has shifted substantially due to the difference in the current gain. In this circuit, then, the Q-point is not stabilized against variations in β, and this a real design problem. The Q-point is also influenced by variations in resistance values. Tolerances in discrete resistance and integrated circuit resistance values result from process variations and material property variations.
Another advantage of including an emitter resistor is that it stabilizes the Q-point with respect to temperature. Remember we noted in Ch. 1 that the current in a pn junction increases with increasing temperature, for a con- stant junction voltage. We then expect the transistor current to increase as the temperature increases. If the current in a junction increases, the junction temperature increases (because of I^2 R heating), which in turn causes the current to increase, thereby further increasing the junction temperature. This phenomenon can lead to thermal runaway and device destruction. However, from Figure 3.53(b), we see that as the current increases, the voltage drop across RE increases. The Thevenin equivalent voltage and resistance are assumed to be essentially independent of temperature, and the temperatuire-induced change in the voltage drop across RTH will be small. The net result is that the increased voltage drop across RE reduces the BE junction voltage, which then tends to stabilize the transistor current against increases in temperature.
We will study this circuit in more detail next semester in EE 429. 3.5 MULTISTAGE CIRCUITS Most transistor circuits contain more than one transistor. We can analyze and design these multistage circuits in much the same way as we studied single-transistor circuits. As an example Figure 3.59 contains an npn and a pnp in the same circuit.
3.6 SUMMARY
- In this chapter, we considered the basic characteristics and properties of the bipolar transistor, which is a three-terminal device that has three separately doped semiconductor regions and two pn junctions. The three terminals are called the base (B), emitter (E), and collector (C). Both npn and pnp complementary bipolar transistors can be formed. The defining transistor action is that the voltage across two terminals (base and emitter) controls the current in the third terminal (collector).
