With a transistor β of 100, this represents a substantial increase in measurement sensitivity. Perhaps the most direct solution to this measurement problem is to use a transistor (Figure below) to amplify the solar cell’s current so that more meter deflection may be obtained for less incident light.Ĭell current must be amplified for low intensity light.Ĭurrent through the meter movement in this circuit will be β times the solar cell current. Supposing that our need here is to measure very low-level light intensities, we are pressed to find another solution. Because the solar cell has to supply the meter movement’s power needs, the system is necessarily limited in its sensitivity. High intensity light directly drives light meter.Īlthough this approach might work for moderate light intensity measurements, it would not work as well for low light intensity measurements. In fact, the simplest light-exposure meters for photography work are designed like this. It is possible to directly connect a meter movement to a solar cell (Figure below) for this purpose. We want to measure the intensity of incident light with the solar cell by using its output current to drive a meter movement. Suppose that we were interested in using the solar cell as a light intensity instrument. The lamp’s brightness will steadily increase as more light falls on the solar cell. When there is just a little light shone on the solar cell, the lamp will glow dimly. Knowing now that transistors are able to “throttle” their collector currents according to the amount of base current supplied by an input signal source, we should see that the brightness of the lamp in this circuit is controllable by the solar cell’s light exposure. This is not the only way in which a transistor may be used as an amplifier, as we will see in later sections of this chapter.Ĭommon-emitter amplifier: The input and output signals both share a connection to the emitter.īefore, a small solar cell current saturated a transistor, illuminating a lamp. It is called the common-emitter configuration because (ignoring the power supply battery) both the signal source and the load share the emitter lead as a common connection point shown in Figure below. One of the simpler transistor amplifier circuits to study previously illustrated the transistor’s switching ability.
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