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Ch 3b 2006

# Ch 3b 2006 - The book goes over several more examples that...

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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 v I < V BE (on), then i B = i C = 0 and the transistor is cut off. Since i C = 0, the voltage drop across R C is zero, so the output voltage is v O = V CC . 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. EE 329 Introduction to Electronics 126

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If we let v I = V CC , and if the ratio of R B to R C , where R C 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 V CC . If. on the other hand, the input is high and equal to V CC , the transistor is driven into saturation, and the output is low and equal to V CE (sat). EE 329 Introduction to Electronics 127
Now consider the case when a second transistor is connected in parallel, as shown in Figure 3.43(b) above. When the two inputs are zero, both transistors are cutoff and V

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Ch 3b 2006 - The book goes over several more examples that...

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