ESE 211 / Spring 2011 / Lecture 10 Bipolar Junction Transistor Let us first consider general transconductance amplifier loaded with short circuit Transconductance Obviously, power supplies are needed for circuit to operate. Often circuit requires bias and transconductance are defined for small variations of the input voltage near the bias point (input voltage -point). This small input voltage variation produces vartiation of the put current near its bias point (put current -point). Hence, defined in this manner the transconductance is small signal transconductance. Once biased properly, the general transconductance amplifier can be replaced with its low frequency equivalent circuit for low frequency A (signal) analysis. R in and R are small signal input and put impedances of the transconductance amplifier at the given bias Voltage controlled current ideal source 1
Net Transconductance from signal to load Signal source has internal impedance and load is almost never can be considered short circuit. Amplifier input voltage, i.e. voltage across R in is smaller than signal voltage Load current is smaller than internally generated current because of amplifier finite put impedance Net transconductance from signal to load is smaller then amplifier short circuit transconductance. 2
Transconductance amplifier put V When R in v i in in, R v i then G m g m 2 1 R V v i V urrent independent on voltage can be obtained from pn-junction diode under reverse bias. Reverse bias Trying to move electron from p to n and holes from n to p, but there is very little number of electrons in p and holes in n! 3
dea of BJT transistor Reverse biased pn-junction can not supply ppy large current. This current is fixed rather than controllable. onsider this: Forward biased pn-junction Reverse biased pn-junction an inject a lot of excess electrons into p-section urrent is small but independent on V RB Small number of electron is available Output current remains independent on put voltage but becomes determined by input voltage/current 4
npn BJT in common base configuration Useful current is current of electrons. ommon base current gain ommon emitter current gain Useful current is current of holes n properly designed transistor α 1 and β 100. This is true only in forward active regime, i.e. when base-emitter junction is forward biased and basecollector junction in not forward biased. 5
npn BJT in common emitter configuration nput V Output V 0.7 V 0.3 V Operation regimes: V BE 0.7 V and V E > 0.3 V - FORWARD ATVE V BE 0.7 V and V E < 0.3 V - SATURATON < V BE < 0.7 V - UTOFF 0 * base-emitter not forward biased and base-collector forward biased - REVERSE ATVE 6
Small signal parameters of BJT in forward active regime nput V Output V Observe finite slope of put V in real devices due to base width modulation (Early effect) real Early voltage ideal ~ 0.7 V Small signal transconductance 0.3 V Small signal put t impedance Small signal input impedance Small signal parameters relate linearly the variations of voltage and currents near their respective bias () points 7
Small signal equivalent circuit of BJT in forward active regime 8
BJT ommon Emitter Amplifier Bias circuit it to define:, VE, gm, r V B E, r B V V BB 0.7V R B R Make sure that O V E 0. 3 V oupling cap short for signal, open for D. Open circuit signal voltage gain. Than calculate small signal parameters g m V th,r π β g m,r O And use small signal equivalent circuit for signal analysis V A * The bias scheme shown in this circuit has serious problems. t is used here for simplicity. Realistic stable bias scheme will be considered later. 9
BJT ommon Emitter Amplifier Assume base bias current of 10 ua, ex. R B = 100k and V BB = 1.7 V. ommon emitter current gain of ab 100 and Early voltage ab 100 V Often can be neglected 10