4.3. METER CHECK OF A TRANSISTOR181needed to energize the lamp. Here we see the transistor functioning not only as a switch, butas a true ampliﬁer: using a relatively low-power signal to controla relatively large amountof power. Please note that the actual power for lighting up the lamp comes from the batteryto the right of the schematic. It is not as though the small signal current from the solar cell,thermocouple, or microphone is being magically transformed into a greater amount of power.Rather, those small power sources are simply controllingthe battery’s power to light up thelamp.• REVIEW:• Transistors may be used as switching elements to control DC power to a load. Theswitched (controlled) current goes between emitter and collector; the controlling currentgoes between emitter and base.• When a transistor has zero current through it, it is said to be in a state of cutoff(fullynonconducting).• When a transistor has maximum current through it, it is said to be in a state of saturation(fully conducting).4.3Meter check of a transistorBipolar transistors are constructed of a three-layer semiconductor sandwich, either PNP orNPN. As such, transistors register as two diodes connected back-to-back when tested with amultimeter’s resistance or diode check function as illustrated in Figure below. Low resistancereadings on the base with the black negative (-) leads correspond to an N-type material in thebase of a PNP transistor. On the symbol, the N-type material is ”pointed” to by the arrow ofthe base-emitter junction, which is the base for this example. The P-type emitter correspondsto the other end of the arrow of the base-emitter junction, the emitter. The collector is verysimilar to the emitter, and is also a P-type material of the PN junction.baseemittercollectorCOMAVVAAOFFCOMAVVAAOFFbaseCOMAVVAAOFFCOMAVVAAOFFemittercollectorFigure 4.8:PNP transistor meter check: (a) forward B-E, B-C, resistance is low; (b) reverseB-E, B-C, resistance is ∞.