2 19 2008 Lecture 9 Common Base CB Amplifier OUTLINE BJT Amplifiers 2 Common base topology CB core CB stage with source resistance Impact of base resistance Reading Chapter 5 3 2 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 EE105 Spring 2008 Lecture 9 Slide 1 Prof Wu UC Berkeley EE105 Spring 2008 Lecture 9 Slide 2 Prof Wu UC Berkeley Tradeoff between Gain and Headroom CB Core IC RC VT V VBE CC VT Av Av g m RC The voltage gain of CB stage is gmRC which is identical to that of CE stage in magnitude and opposite in phase To maintain the transistor out of saturation the maximum voltage drop across RC cannot exceed VCC VBE EE105 Spring 2008 EE105 Spring 2008 EE105 Fall 2007 Lecture 9 Slide 3 Prof Wu UC Berkeley Lecture 9 Slide 4 Prof Wu UC Berkeley 1 2 19 2008 Simple CB Stage Example Input Impedance of CB VCC 1 8V IC 0 2mA IS 5x10 17 A 100 1 Av g m RC 2230 17 2 130 I Vb VBE 0 6 VT ln C 1 354V IS EE105 Spring 2008 Vb 1 354V Choose I1 10 I B 20 A VCC R1 R2 R1 22 3k R2 67 7 k Lecture 9 Slide 5 Rin R2 VCC if I1 I B R1 R2 Prof Wu UC Berkeley Practical Application of CB Stage 1 gm The input impedance of CB stage is much smaller than that of the CE stage EE105 Spring 2008 Lecture 9 Slide 6 Prof Wu UC Berkeley Output Impedance of CB Stage Rout rO RL To avoid reflections need impedance matching CB stage s low input impedance can be used to create a match with 50 The output impedance of CB stage is similar to that of CE stage EE105 Spring 2008 EE105 Spring 2008 EE105 Fall 2007 Lecture 9 Slide 7 Prof Wu UC Berkeley Lecture 9 Slide 8 Prof Wu UC Berkeley 2 2 19 2008 CB Stage with Source Resistance Av RC 1 RS gm 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 EE105 Spring 2008 Lecture 9 Slide 9 Prof Wu UC Berkeley Realistic Output Impedance of CB Stage Rout1 1 g m RE r rO RE r Rout RC Rout1 The output impedance of CB stage is equal to RC in parallel with the impedance looking down into the collector EE105 Spring 2008 EE105 Fall 2007 Lecture 9 Slide 11 Practical Example of CB Stage Prof Wu UC Berkeley An antenna usually has low output impedance therefore a correspondingly low input impedance is required for the following stage EE105 Spring 2008 Lecture 9 Slide 10 Prof Wu UC Berkeley Output Impedance of CE and CB Stages 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 EE105 Spring 2008 Lecture 9 Slide 12 Prof Wu UC Berkeley 3 2 19 2008 CB with Base Resistance Comparison of CE and CB Stages with Base Resistance vout RC vin R RB 1 E 1 gm With an addition of base resistance the voltage gain degrades 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 EE105 Spring 2008 EE105 Spring 2008 Lecture 9 Slide 13 Prof Wu UC Berkeley Input Impedance of CB Stage with Base Resistance Lecture 9 Slide 14 Prof Wu UC Berkeley Input Impedance Seen at Emitter and Base v X r RB 1 R B iX 1 gm 1 The input impedance of CB with base resistance is equal to 1 gm plus RB divided by 1 This is in contrast to degenerated CE stage in which the resistance in series with the emitter is multiplied by 1 when seen from the base EE105 Spring 2008 EE105 Fall 2007 Lecture 9 Slide 15 Prof Wu UC Berkeley EE105 Spring 2008 Lecture 9 Slide 16 Prof Wu UC Berkeley 4 2 19 2008 Input Impedance Example RX Bad Bias Technique for CB Stage R 1 1 1 B g m 2 1 g m1 1 To find the RX we have to first find Req treat it as the base resistance of Q2 and divide it by 1 Unfortunately no emitter current can flow EE105 Spring 2008 EE105 Spring 2008 Lecture 9 Slide 17 Prof Wu UC Berkeley Still No Good Lecture 9 Slide 18 Proper Biasing for CB Stage 1 RE RE gm 1 g m RE Rin 1 vX vin Rin RS 1 1 g m RE RS RE Rin The input signal is shorted to ground The circuit still does not amplify EE105 Spring 2008 EE105 Fall 2007 Lecture 9 Slide 19 Prof Wu UC Berkeley Prof Wu UC Berkeley EE105 Spring 2008 Lecture 9 Slide 20 vout vout v X vin v X vin 1 g R R m C 1 1 g m RE S RE Prof Wu UC Berkeley 5 2 19 2008 Creation of Vb Reduction of Input Impedance Due to RE The reduction of input impedance due to RE is bad because it shunts part of the input current to ground instead of to Q1 and RC EE105 Spring 2008 Lecture 9 Slide 21 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 EE105 Spring 2008 Lecture 9 Slide 22 Prof Wu UC Berkeley Example of CB Stage with Bias For the circuit shown above RE 1 gm R1 and R2 are chosen so that Vb is 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 gm at the required frequency EE105 Spring 2008 EE105 Fall 2007 Lecture 9 Slide 23 Prof Wu UC Berkeley 6
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