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Electronic Circuits – EE359A Bruce McNair B206 [email protected] 201-216-5549 Lecture 5 0 Bipolar Junction Transistors (BJT) Small Signal Analysis Graphical Analysis / Biasing Amplifier, Switch and Logic Appl. 1 NPN Transistor Amplifier Example • NPN •Quiescent point VC VCC 2.3 x3 3.1V VBB VBE 3 0.7 IB 0.023mA RBB 100 3 0.7 I C I B 100 2.3mA 100 VCC 10V 3.0kohm R1 V1 100kohm R2 Q1 1DEAL_BJT_NPN 3V 2 BJT as Amplifier BJT as a voltage-controlled current source ( a transconductance amplifier) BJT as a currentcontrolled current source (a current amplifier). 3 PNP Transistor Amplifier Example • Voltage Gain • Signal Waveforms • Capacitor couples input signal vi to emitter • DC bias with V+ & V- 4 DC Analysis • Find operating pt. Q IE 10 VE 10 0.7 0.93mA RE 10 • Let =100 and =0.99 I C 0.99 I E 0.92mA VC 10 I C RC 5.4V • The transistor is active • Max. signal swing depends on bias voltage 5 Small Signal Analysis • Replace BJT with T equivalent ckt. • Why? Base is gnded. More convenient than hybrid = 0.99 re=25mV/0.93mA= 27 6 Small Signal Equiv Ckt • VO/Vi =0.99x5k/27=183 • Allowable signal magnitude? • But veb = vi For small signal limit to 10mV. Then, vc=1.833V 7 Graphical Analysis • Find DC bias point • Set vi=0 and draw load line to determine dc bias point IB (similar to diode ckts) 8 Graphical Construction • Load line has a slope of –1/RB • iB vs vBE from forward biased diode eqns Graphical construction for the determination of the dc base current 9 Collector Current Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage 10 Small Signal Graphical Analysis • Signal is superimposed on DC voltage VBB • Corresponding to each instantaneous value of VBB + vi(t) draw a load line • Intersection of the iB vBE curve with the load lines • Amplitude vi(t) small so ib linear 11 Collector Currrent • Corresponding to each instantaneous value of VCE + vce(t) operating point will be on the load line • Amplitude vi(t) small so ic linear 12 Bias Point vs Signal Swing • Bias-point location limits allowable signal swing • Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. • At the other extreme, loadline B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE. 13 Basic Single Stage Amplifiers Common-emitter amplifier with a resistance Re in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with ro eliminated. 14 Common Base Amp The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 15 Common Collector The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. 16 (c) The circuit redrawn to show that ro is in parallel with RL. (d) Circuit for determining Ro. 17 BJT Digital Logic Basic BJT digital logic inverter. 18 •voltage transfer characteristic of the inverter circuit •RB = 10 k, RC = 1 k, = 50, and VCC = 5V. 19 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. 20 The ic-vcb or common-base characteristics of an npn transistor. Note that in the active region there is a slight dependence of iC on the value of vCB. The result is a finite output resistance that decreases as the current level in the device is increased. 21 Common Base Characteristic The hybrid- model, including the resistance r, which models the effect of vc on ib. 22 Common-emitter characteristics. 23 Common Emitter in Saturation Region 24 Single Stage BJT Amplifier •Quiescent point, NPN •VCC=VEE=10V, I=1mA, RB=100 kW RC=8 kW, b=100 25 26 Characteristic Parameters of Amplifiers 27 Common Emitter Amplifier CE grounds emitter, also called “bypass” capacitor 28 Small Signal Analysis • R =V R • Replace BJT with hybrid equivalent ckt. • Rout=RC||rO in i/ i 29 Emitter Resistance 30 Common Base 31 Small Signal 32 Simplified Small Signal • Redraw with r_o in parallel with R_L. 33 Simplified (continued) 34 Thevenin Equivalent 35 Thevenin Equivalent This circuit can be used to find gain for R_L 36 High Frequency Model 37 Transfer Function (s domain) 38 39 40 41 42 Pspice of CE with Emitter Resistance 43 Frequency Response 44