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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 b = 100 Vb 1.354V R2 VCC if I1 I B R1 R2 Choose I1 10 I B 20A VCC R1 R2 1 Av g m RC 2230 17.2 R1 22.3k, R2 67.7k 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 bRC vout r RB vin v v v v bR 1 KCL at node P : g m v P in g m out C r RE RE r g m RC EE105 Fall 2007 vout bRC RC 1 RB vin r b 1RE RB RE gm b 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 (b+1). This is in contrast to a degenerated CE stage, in which the resistance in series with the emitter is multiplied by (b+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 vx ix r r RB vx r RB 1 RB Rin ix b 1 gm b 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 (b+1). 1 RB Req g m1 b 1 EE105 Fall 2007 1 1 1 RB Rx g m 2 b 1 g m1 b 1 Lecture 9, Slide 15 Prof. Liu, UC Berkeley