HP HP-Bench-Briefs-1974-11-12 free download

File name HP-Bench-Briefs-1974-11-12.pdf

ION FROM HEWLETT-PACKARD V NOVEMBER-DECEMBER 1974 conditions found in digital Expanding on the above idea leads circuits. to our second troubleshooting tip. TRANSISTORIZED In troubleshooting transistor cir- TIP #2: Modify the control sig- CIRCUITS FASTER cuits, the most important area to nals present and see if the circuit examine is the base-emitter junction responds accordingly. by George Stanley as this is the control point of the transistor. Remember that conven- tional PNP and NPN transistors are Many books and articles have been basically "off" devices and must written on transistors and tran- be biased "on" to their operating sistorized circuits but very few point. This is done by forward bias- have been written about trouble- ing the base-emitterjunction. There- shooting transistorized circuits. fore, the status of the base-emitter This article is aimed at providing diode tells exactly what the tran- practical troubleshooting tips for sistor should be doing. This diode is those of you repairing transis- made out of either silicon or germa- torized equipment. nium. If the transistor is silicon and has approximately 0.6V forward Before describing specific tips let's bias between base and emitter, the take a moment and review several transistor should be "on". The important transistor characteris- amount it should be on depends I ~ tics: upon the current gain ( B ) of the For example, Figure 1 shows a A conventional PNP or NPN tran- transistor, the resistors in series normally biased NPN silicon tran- sistor has three operating states: with the collector and emitter, and sistor with the bias resistors ad- the supply voltage. If the transis- justed to have the transistor A. Off, that isan open switch. tor were germanium and had turned on half way. Now remove approximately 0.2 volts forward the forward bias on the base- B. Part way on, bias voltage are bias between base and emitter, emitter diode junction by adding set so the transistor can am- it would behave in the same general th