Electronic Circuits EE359A

Transcription

1 Electronic Circuits EE359A Bruce McNair B Lecture 4 0

2 Bipolar Junction Transistors (BJT) Small Signal Analysis Graphical Analysis / Biasing Amplifier, Switch and Logic Appl. 1

3 NPN Transistor Amplifier Example Quiescent point VBB VBE I B mA RBB IC IB mA 100 NPN VC VCC 2.3x3 3. 1V 3.0kohm VCC R2 10V V1 3V R1 100kohm Q1 1DEAL_BJT_NPN 2

4 3 Small Signal Analysis ) (2.3/ 25 ma mv I V r E T e V ma mv ma V I g T C m / k g r m

5 BJT as Amplifier BJT as a voltage-controlled current source ( a transconductance amplifier) BJT as a currentcontrolled current source (a current amplifier). 4

6 Small Signal 5

7 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

8 Signal Waveforms 7

9 PNP Transistor Amplifier Example Voltage Gain Signal Waveforms Capacitor couples input signal v i to emitter DC bias with V + &V - 8

10 DC Analysis Find operating pt. Q 10 VE 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

11 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

12 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

13 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

14 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

15 Collector Current Graphical construction for determining the dc collector current I C and the collector-to-emmiter voltage 14

16 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

17 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

18 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

19 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

20 Common Base Amp The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 19

21 Common Collector The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 20

22 (c) The circuit redrawn to show that r o is in parallel with R L. (d) Circuit for determining R o. 21

23 General Large Signal Model An npn resistor and its Ebers-Moll (EM) model. 22

24 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

25 BJT Digital Logic Basic BJT digital logic inverter. 24

26 voltage transfer characteristic of the inverter circuit R B = 10 k, R C = 1 k, = 50, and V CC = 5V. 25

27 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

28 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

29 Common Base Characteristic The hybrid- model, including the resistance r, which models the effect of v c on i b. 28

30 Common-emitter characteristics. 29

31 Common Emitter in Saturation Region 30