Common Base CB Amplifier EE105 Spring 2007 Microelectronic Devices and Circuits Lecture 11 Bipolar Amplifiers Part 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 CB Core 2 Tradeoff between Gain and Headroom 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 3 To maintain the transistor out of saturation the maximum voltage drop across RC cannot exceed VCCVBE 4 Simple CB Example Input Impedance of CB Rin Av g m RC 17 2 R1 22 3K R2 67 7 K 1 gm The input impedance of CB stage is much smaller than that of the CE stage 5 Practical Application of CB Stage 6 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 7 8 CB Stage with Source Resistance Av Practical Example of CB Stage 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 An antenna usually has low output impedance therefore a correspondingly low input impedance is required for the following stage 9 Realistic Output Impedance of CB Stage Output Impedance of CE and CB Stages 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 10 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 11 12 Fallacy of the Old Wisdom CB with Base Resistance vout RC vin R RB 1 E 1 gm The statement CB output impedance is higher than CE output impedance is flawed With an addition of base resistance the voltage gain degrades 13 Comparison of CE and CB Stages with Base Resistance 14 Input Impedance of CB Stage with Base Resistance v X r RB R 1 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 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 15 16 Input Impedance Seen at Emitter and Base Input Impedance Example RX 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 17 Bad Bias Technique for CB Stage 18 Still No Good The input signal is shorted to ground The circuit still does not amplify Unfortunately no emitter current can flow 19 20 Proper Biasing for CB Stage Rin Reduction of Input Impedance Due to RE 1 RE gm vout 1 g m RC vin 1 1 g R RS m E RE 21 Creation of Vb 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 22 Example of CB Stage with Bias 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 23 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 24 Emitter Follower Core Emitter Follower Common Collector Amplifier When the input is increased by V output is also increased by an amount that is less than V due to the increase in collector current and hence the increase in potential drop across RE However the absolute values of input and output differ by a VBE 25 Small Signal Model of Emitter Follower 26 Unity Gain Emitter Follower VA VA vout RE 1 r 1 1 vin 1 RE gm 1 RE Av 1 The voltage gain is unity because a constant collector current I1 results in a constant VBE and hence Vout follows Vin exactly As shown above the voltage gain is less than unity and positive 27 28 Analysis of Emitter Follower as a Voltage Divider Emitter Follower with Source Resistance VA vout RE vin R RS 1 E 1 gm VA 29 Input Impedance of Emitter Follower 30 Emitter Follower as Buffer VA vX r 1 RE iX Since the emitter follower increases the load resistance to a much higher value it is suited as a buffer between a CE stage and a heavy load resistance to alleviate the problem of gain degradation The input impedance of emitter follower is exactly the same as that of CE stage with emitter degeneration This is not surprising because the input impedance of CE with emitter degeneration does not depend on the collector resistance 31 32 Output Impedance of Emitter Follower Emitter Follower with Early Effect Av R 1 Rout s RE 1 gm RE rO R 1 RE rO S 1 gm Rin r 1 RE rO R 1 Rout s RE rO 1 gm Emitter follower lowers the source impedance by a factor of 1 improved driving capability 33 Current Gain Since rO is in parallel with RE its effect can be easily incorporated into voltage gain and input and output impedance equations 34 Emitter Follower with Biasing A biasing technique similar to that of CE stage can be used for the emitter follower There is a current gain of 1 from base to emitter Effectively speaking the load resistance is multiplied by 1 as seen from the base Also Vb can be close to Vcc because the collector is also at Vcc 35 36 Supply Independent Biasing Summary of Amplifier Topologies The three amplifier topologies studied so far have different properties and are used on different occasions CE and CB have voltage gain with magnitude greater than one while follower s voltage gain is at most one By putting a constant current source at the emitter the bias current VBE and IBRB are fixed regardless of the supply value 37 Amplifier Example I 38 Amplifier Example II vout R2 RC R1 R1 RS 1 vin RE R1 RS 1 gm vout RC R1 RS R1 1 vin R2 R1 RS 1 gm …
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