Lecture 9 OUTLINE BJT Amplifiers cont d Common base topology CB core CB stage with source resistance Impact of base resistance Reading Chapter 5 3 2 EE105 Fall 2007 Lecture 9 Slide 1 Prof Liu UC Berkeley Common Base CB Amplifier The base terminal is biased at a fixed voltage the input signal is applied to the emitter and the output signal sensed at the collector EE105 Fall 2007 Lecture 9 Slide 2 Prof Liu UC Berkeley Small Signal Analysis of CB Core The voltage gain of a CB stage is gmRC which is identical to that of a CE stage in magnitude and opposite in phase Av g m RC EE105 Fall 2007 Lecture 9 Slide 3 Prof Liu UC Berkeley Tradeoff between Gain and Headroom To ensure that the BJT operates in active mode the voltage drop across RC cannot exceed VCC VBE IC VCC VBE Av RC VT VT EE105 Fall 2007 Lecture 9 Slide 4 Prof Liu UC Berkeley Simple CB Stage Example VCC 1 8V IC 0 2mA IS 5x10 17 A 100 Vb 1 354V R2 VCC if I1 I B R1 R2 Choose I1 10 I B 20 A VCC R1 R2 1 Av g m RC 2230 17 2 R1 22 3k R2 67 7 k 130 EE105 Fall 2007 Lecture 9 Slide 5 Prof Liu UC Berkeley Input Impedance of a CB Stage The input impedance of a CB stage is much smaller than that of a CE stage 1 Rin if VA gm EE105 Fall 2007 Lecture 9 Slide 6 Prof Liu UC Berkeley CB Stage with Source Resistance With the inclusion of a source resistance the input signal is attenuated before it reaches the emitter of the amplifier therefore the voltage gain is lowered This effect is similar to CE stage emitter degeneration Av EE105 Fall 2007 Lecture 9 Slide 7 RC 1 RS gm Prof Liu UC Berkeley Practical Example of a CB Stage An antenna usually has low output impedance therefore a correspondingly low input impedance is required for the following stage EE105 Fall 2007 Lecture 9 Slide 8 Prof Liu UC Berkeley Output Impedance of a CB Stage The output impedance of a CB stage is equal to RC in parallel with the impedance looking into the collector Rout1 1 g m RE r rO RE r Rout 2 RC Rout1 EE105 Fall 2007 Lecture 9 Slide 9 Prof Liu UC Berkeley Output Impedance CE vs CB Stages The output impedances of emitter degenerated CE and CB stages are the same This is because the circuits for small signal analysis are the same when the input port is grounded EE105 Fall 2007 Lecture 9 Slide 10 Prof Liu UC Berkeley Av of CB Stage with Base Resistance VA With base resistance the voltage gain degrades vout vout g m v RC v g m RC v vout r RB vP r RB r r g m RC vout r RB vP RC v vP vin 1 KCL at node P g m v g m r RE r EE105 Fall 2007 vout r RB vin vout R C g m RC RE vout RC RC 1 RB vin r 1 RE RB RE gm 1 Lecture 9 Slide 11 Prof Liu UC Berkeley Voltage Gain CE vs CB Stages The magnitude of the voltage gain of a CB stage with source and base resistances is the same as that of a CE stage with base resistance and emitter degeneration EE105 Fall 2007 Lecture 9 Slide 12 Prof Liu UC Berkeley Rin of CB Stage with Base Resistance VA The input impedance of a CB stage with base resistance is equal to 1 gm plus RB divided by 1 This is in contrast to a degenerated CE stage in which the resistance in series with the emitter is multiplied by 1 when seen from the base v KCL g m v ix r r v vx r RB EE105 Fall 2007 Lecture 9 Slide 13 1 r g m v x ix r r RB vx r RB 1 RB Rin ix 1 g m 1 Prof Liu UC Berkeley Input Impedance Seen at Emitter vs Base Common Base Stage EE105 Fall 2007 Common Emitter Stage Lecture 9 Slide 14 Prof Liu UC Berkeley Input Impedance Example To find RX we have to first find Req treat it as the base resistance of Q2 and divide it by 1 1 1 1 RB Rx g m 2 1 g m1 1 1 RB Req g m1 1 EE105 Fall 2007 Lecture 9 Slide 15 Prof Liu UC Berkeley
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