The first test of a transistor

Transcription

1 Bipolar Transistors I Pag 1 Bipolar Transistors I Th first tst of a transistor This la uss an NPN transistor, th. Lik any NPN transistor, it onsists of two PN juntions, as shown in Fig. 1 () a) ) ) Figur 1: Viws of NPN Transistor: a) shmati, ) funtional, ) physial You an mak a rud tst of a transistor y using th diod tst option on a multimtr. This option sts th trminals of th multimtr so as to forward ias th juntion and thn to rad th voltag aross it. For a silion transistor lik th you xpt to find a forward voltag of 0.6 or 0.7 volts. Tst oth th asolltor and th as-mittr juntions of your transistor. (Not: With th multimtr in diod-tst mod, you an dtrmin whih mtr lad is positiv y onnting th lads to th osillosop.) 2. Common mittr haratristis Th goal of this stion is to mak a plot showing th olltor urrnt I C vs. th olltor-mittr voltag V CE for svral diffrnt valus of th as urrnt I B. To liminat som of th rptitious work w will do this using th LaViw program Tran_param.vi. Figur 2 low shows th iruit that is usd to do th masurmnt. Th voltag on th olltor is swpt through a rang of valus hosn y th studnt for svral valus of th as voltag. Th olltor urrnt is masurd y masuring th voltag drop aross th (nominally) 1k rsistor and th as urrnt is otaind similarly y masuring th voltag drop aross th 1M rsistor. Th rsistors should masurd with th Multimtr and thir xat valus usd in th program.

2 Bipolar Transistors I Pag 2 1M 1k 0-10V 0-10V Figur 2: Ciruit for. Follow ths stps: (1) Entr th valus of th rsistors. (2) Choos a rang of valus for th as voltags and th numr of points to masur. (3) Similarly slt a rang of olltor voltags and numr of points. Th program will rturn plots similar to th on low with th as urrnts orrsponding to ah lin givn in a rgistr. I C (ma) For I B =7.2µA For I B =4.4µA V CE (Volts) Figur 3: Colltor Currnt vs. Colltor-Emittr Voltag. Calulating β: Th urrnt gain of th transistor, β, is th hang in olltor urrnt dividd y th hang in as urrnt. From th data aov, hoosing V CE =6 volts as typial, on an alulat β as follows: (1.36 " 0.84) # 10"3! = = 186. (7.2 " 4.4) #10 "6

3 Bipolar Transistors I Pag 3 Calulat β for your transistor for a rang of as urrnts and mak a plot. Th transistor as a swith Rpla th 1K olltor rsistor in th iruit aov y a 10K rsistor. Try to mak th masurmnts of th olltor haratristis now with this largr olltor rsistor. Proaly you find that whn thr is any as urrnt at all, th voltag drop on th olltor rsistor is so larg that you an t gt V CE at all los to th supply voltag. You ar looking at th swithing haratristi of a transistor. Th transistor tnds to ithr on (larg olltor urrnt) or off (no olltor urrnt). Th ommon-mittr amplifir St up th iruit low. Th apaitor AC oupls th signal gnrator to th transistor amplifir so that th gnrator dos not upst th DC oprating point (quisnt point) adjustmnt. A ttr way to put that is to say that th apaitor DC- doupls th signal gnrator from th amplifir. 5 V pp =12k! R C Vout =1.0k! Figur 4: Common-mittr amplifir. (a) Explain why th gain of this amplifir is aout 10. () Explain why th ias rsistors and mak a snsil voltag dividr for iasing th as givn th goal of making th quisnt valu of v out 5 volts. You may assum that th transistor has β 100 and V BE 0.6v

4 Bipolar Transistors I Pag 4 5 V pp =12k! =1.0k! R C Vout Figur 5: Bypass apaitor addd. () Masur th voltag gain of th amplifir y omparing th input and output AC signals on a sop. Is th amplifir an invrting amplifir? An mittr ypass Add a apaitor, osrving th orrt polarity, to ypass th mittr rsistor as shown low and r-masur th gain of th amplifir. It should hav inrasd sustantially. 7. Th mittr followr A fw simpl hangs to th ommon mittr iruit aov ompltly hang th haratr of th amplifir. 5 V pp Vout Figur 6: Emittr Followr.

5 Bipolar Transistors I Pag 5 (a) Construt th mittr followr iruit in Figur 6 low: () Explain why rsistors and ar approximatly th sam for an mittr followr, givn that w would lik th quisnt valu of V out to aout 5 volts. () Masur th gain and show that it is los to unity. (d) Show that th amplifir is non-invrting.