5.3 Emitter follower

Build a light-activated motor speed control circuit.

Go back to chapter 3 and review problems 3.5 and 3.6 and lab exercise 3.6. here: https://openbooks.library.umass.edu/funee/chapter/3-7/

Those exercises dealt with a light-sensor circuit based on a voltage divider and a photo-sensitive resistor. When the output voltage was connected to a high resistance load, such as voltmeter, the circuit readily differentiated between light and dark conditions and the voltage reading on the multimeter gave a quantitative indication of light intensity.  Loading down the circuit with a low-resistance such as a DC motor pulls down the output voltage, rendering it inoperable. You should back and do this experiment if you haven’t previously done so. In this section, we will learn how to use a transistor as a buffer or current amplifier to solve this loading problem.

Active mode Emitter Follower.  Figure 5.16 depicts an emitter follower circuit that makes uses of the active mode of a transistor to produce a variable voltage across a motor. The motor, represented by resistor , is connected between the emitter of the transistor and ground while the base is connected to a control voltage, . Assume varies between and .  The base-emitter (BE) junction is forward-biased in this circuit while the BC junction is reverse biased, and the transistor is in the active mode with equivalent circuit shown in Fig. 5.16.

Applying KVL to the equivalent circuit, we have from which we have . The voltage drop across the motor, is below and follows changes in : when the input voltage increases, increases and likewise for decreasing input voltage.  This circuit is called a “voltage follower” since the motor’s supply  voltage follows changes in the input voltage, albeit with a decrease. The current supplied to the motor primarily from the collector and draws little from the base part of the circuit. This can be shown by applying KCL to the emitter node, :  .   is typically 100 or higher so we can write V_{m}, while sourcing the motor’s current from the battery through the collector.

Figure 5.17 shows a light-sensitive voltage divider ahead of the transistor implementing a motor speed control circuit depending on incident light.  This circuit is assigned as a lab exercise 3.3.