Electronic Circuits EE359A Bruce McNair B206 bmcnair@stevens.edu 201-216-5549 Lecture 4 0
Bipolar Junction Transistors (BJT) Small Signal Analysis Graphical Analysis / Biasing Amplifier, Switch and Logic Appl. 1
NPN Transistor Amplifier Example Quiescent point VBB VBE 3 0.7 I B 0. 023mA RBB 100 3 0.7 IC IB 100 2.3mA 100 NPN VC VCC 2.3x3 3. 1V 3.0kohm VCC R2 10V V1 3V R1 100kohm Q1 1DEAL_BJT_NPN 2
3 Small Signal Analysis 8 10. 0.99) (2.3/ 25 ma mv I V r E T e V ma mv ma V I g T C m / 92 25 2.3 k g r m 1.09 92 100
BJT as Amplifier BJT as a voltage-controlled current source ( a transconductance amplifier) BJT as a currentcontrolled current source (a current amplifier). 4
Small Signal 5
Small Signal Analysis Employ either hybrid- model. Using the first model BJT as Amplifier R1 Dependent Current Source C 100kohm B V1 V BE R2 1.1kohm I1 92mMho R3 3.0kohm XMM1 1V 1Hz 0Deg E 6
Signal Waveforms 7
PNP Transistor Amplifier Example Voltage Gain Signal Waveforms Capacitor couples input signal v i to emitter DC bias with V + &V - 8
DC Analysis Find operating pt. Q 10 VE 10 0.7 I E 0. 93mA R 10 Let =100 and =0.99 I V C C The transistor is active Max. signal swing depends on bias voltage E 0.99I 10 E 0.92mA I R 5.4V C C 9
Small Signal Analysis Replace BJT with T equivalent ckt. Why? Base is gnded. More convenient than hybrid = 0.99 r e =25mV/0.93mA= 27 10
Small Signal Equiv Ckt V O /V i =0.99x5k/27=183 Allowable signal magnitude? But v eb = v i For small signal limit to 10mV. Then, v c =1.833V 11
Graphical Analysis Find DC bias point Set v i =0 and draw load line to determine dc bias point I B (similar to diode ckts) 12
Graphical Construction Load line has a slope of 1/R B i B vs v BE from forward biased diode eqns Graphical construction for the determination of the dc base current 13
Collector Current Graphical construction for determining the dc collector current I C and the collector-to-emmiter voltage 14
Small Signal Graphical Analysis Signal is superimposed on DC voltage V BB Corresponding to each instantaneous value of V BB + v i (t) draw a load line Intersection of the i B - v BE curve with the load lines Amplitude v i (t) small so i b linear 15
Collector Currrent Corresponding to each instantaneous value of V CE + v ce (t) operating point will be on the load line Amplitude v i (t) small so i c linear 16
Bias Point vs Signal Swing Bias-point location limits allowable signal swing Load-line A results in bias point Q A with a corresponding V CE which is too close to V CC and thus limits the positive swing of v CE. At the other extreme, loadline B results in an operating point too close to the saturation region, thus limiting the negative swing of v CE. 17
Basic Single Stage Amplifiers Common-emitter amplifier with a resistance R e in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with r o eliminated. 18
Common Base Amp The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 19
Common Collector The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 20
(c) The circuit redrawn to show that r o is in parallel with R L. (d) Circuit for determining R o. 21
General Large Signal Model An npn resistor and its Ebers-Moll (EM) model. 22
The transport model of the npn BJT. This model is exactly equivalent to the Ebers-Moll model Saturation currents of the diodes in parentheses 23
BJT Digital Logic Basic BJT digital logic inverter. 24
voltage transfer characteristic of the inverter circuit R B = 10 k, R C = 1 k, = 50, and V CC = 5V. 25
Saturation Region The minority-carrier concentration in the base of a saturated transistor is represented by line (c). (b) The minority-carrier charge stored in the base can de divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results in driving the transistor deeper into saturation. 26
The i c -v cb or common-base characteristics of an npn transistor. Note that in the active region there is a slight dependence of i C on the value of v CB. The result is a finite output resistance that decreases as the current level in the device is increased. 27
Common Base Characteristic The hybrid- model, including the resistance r, which models the effect of v c on i b. 28
Common-emitter characteristics. 29
Common Emitter in Saturation Region 30