2/19/2008EE105 Fall 2007 1Lecture 9OUTLINE• BJT Amplifiers (2)– Common-base topologyEE105 Spring 2008 Lecture 9, Slide 1Prof. Wu, UC Berkeley–CB core– CB stage with source resistance– Impact of base resistanceReading: Chapter 5.3.2Common Base (CB) AmplifierEE105 Spring 2008 Lecture 9, Slide 2Prof. Wu, UC Berkeley• In common base topology, where the base terminal is biased with a fixed voltage, emitter is fed with a signal, and collector is the output.CB CoreEE105 Spring 2008 Lecture 9, Slide 3Prof. Wu, UC Berkeley• The voltage gain of CB stage is gmRC, which is identical to that of CE stage in magnitude and opposite in phase. CmvRgA=Tradeoff between Gain and Headroom .CvCTIARVVV=EE105 Spring 2008 Lecture 9, Slide 4Prof. Wu, UC Berkeley• To maintain the transistor out of saturation, the maximum voltage drop across RCcannot exceed VCC‐VBE.CC BETVVV−=2/19/2008EE105 Fall 2007 2Simple CB Stage ExampleVCC= 1.8VIC= 0.2mAIS= 5x10-17Aβ= 100EE105 Spring 2008 Lecture 9, Slide 5Prof. Wu, UC Berkeley12230 17.21300.6 ln 1.354VvmCCbBE TSAgRIVV VI==⋅=⎛⎞=+= =⎜⎟⎝⎠Ω=Ω=⇒+≅=≈>>+≅=k7.67 ,k3.22 2010 Choose if 354.1212111212RRRRVAIIIIVRRRVVCCBBCCbμInput Impedance of CBEE105 Spring 2008 Lecture 9, Slide 6Prof. Wu, UC Berkeley• The input impedance of CB stage is much smaller than that of the CE stage.1inmRg=Practical Application of CB StageEE105 Spring 2008 Lecture 9, Slide 7Prof. Wu, UC Berkeley• To avoid “reflections”, need impedance matching. • CB stage’s low input impedance can be used to create a match with 50 Ω.Output Impedance of CB StageEE105 Spring 2008 Lecture 9, Slide 8Prof. Wu, UC Berkeley• The output impedance of CB stage is similar to that of CE stage.||out O LRrR=2/19/2008EE105 Fall 2007 3CB Stage with Source Resistance1CvSmRARg=+EE105 Spring 2008 Lecture 9, Slide 9Prof. Wu, UC Berkeley• With an inclusion of a source resistor, the input signal is attenuated before it reaches the emitter of the amplifier; therefore, we see a lower voltage gain. • This is similar to CE stage emitter degeneration; only the phase is reversed.Practical Example of CB StageEE105 Spring 2008 Lecture 9, Slide 10 Prof. Wu, UC Berkeley• An antenna usually has low output impedance; therefore, a correspondingly low input impedance is required for the following stage.Realistic Output Impedance of CB Stage EE105 Spring 2008 Lecture 9, Slide 11 Prof. Wu, UC Berkeley• The output impedance of CB stage is equal to RCin parallel with the impedance looking down into the collector.[]()111(||) ||||out m E O Eout C outRgRrrRrRRRππ=+ +=Output Impedance of CE and CB StagesEE105 Spring 2008 Lecture 9, Slide 12 Prof. Wu, UC Berkeley• The output impedances of CE, CB stages are the same if both circuits are under the same condition. This is because when calculating output impedance, the input port is grounded, which renders the same circuit for both CE and CB stages.2/19/2008EE105 Fall 2007 4CB with Base ResistanceEE105 Spring 2008 Lecture 9, Slide 13 Prof. Wu, UC Berkeley• With an addition of base resistance, the voltage gain degrades.11out CBinEmvRRvRgβ≈+++Comparison of CE and CB Stages with Base ResistanceEE105 Spring 2008 Lecture 9, Slide 14 Prof. Wu, UC Berkeley• The voltage gain of CB amplifier with base resistance is exactly the same as that of CE stage with base resistance and emitter degeneration, except for a negative sign.Input Impedance of CB Stage with Base Resistance1rRvR+EE105 Spring 2008 Lecture 9, Slide 15 Prof. Wu, UC Berkeley• The input impedance of CB with base resistance is equal to 1/gmplus RBdivided by (β+1). This is in contrast to degenerated CE stage, in which the resistance in series with the emitter is multiplied by (β+1) when seen from the base.111BXBXmrRvRigπββ+=≈+++Input Impedance Seen at Emitter and BaseEE105 Spring 2008 Lecture 9, Slide 16 Prof. Wu, UC Berkeley2/19/2008EE105 Fall 2007 5Input Impedance ExampleEE105 Spring 2008 Lecture 9, Slide 17 Prof. Wu, UC Berkeley• To find the RX, we have to first find Req, treat it as the base resistance of Q2and divide it by (β+1).2111111BXmmRRggββ⎛⎞=+ +⎜⎟++⎝⎠Bad Bias Technique for CB StageEE105 Spring 2008 Lecture 9, Slide 18 Prof. Wu, UC Berkeley• Unfortunately, no emitter current can flow.Still No GoodEE105 Spring 2008 Lecture 9, Slide 19 Prof. Wu, UC Berkeley• The input signal is shorted to ground. The circuit still does not amplify.Proper Biasing for CB StageEE105 Spring 2008 Lecture 9, Slide 20 Prof. Wu, UC Berkeley()111out outXin X inmCSmEEvvvvvvgRRgRR==++()1||111(1 ) /Ein EmmEinXin in S m E S ERRRggRRvvRR gRRR==+==+++2/19/2008EE105 Fall 2007 6Reduction of Input Impedance Due to REEE105 Spring 2008 Lecture 9, Slide 21 Prof. Wu, UC Berkeley• The reduction of input impedance due to REis bad because it shunts part of the input current to ground instead of to Q1(and RC) . Creation of VbEE105 Spring 2008 Lecture 9, Slide 22 Prof. Wu, UC Berkeley• Resistive divider lowers the gain.• To remedy this problem, a capacitor is inserted from base to ground to short out the resistor divider at the frequency of interest.Example of CB Stage with Bias EE105 Spring 2008 Lecture 9, Slide 23 Prof. Wu, UC Berkeley• For the circuit shown above, RE>> 1/gm. • R1and R2are chosen so that Vbis at the appropriate value and the current that flows thru the divider is much larger than the base current.• Capacitors are chosen to be small compared to 1/gmat the required frequency.
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