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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. Common Base (CB) Amplifier.

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lecture 9
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

common base cb amplifier
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.
small signal analysis of cb core
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.
tradeoff between gain and headroom
Tradeoff between Gain and Headroom
  • To ensure that the BJT operates in active mode, the voltage drop across RC cannot exceed VCC-VBE.
simple cb stage example
Simple CB Stage Example

VCC = 1.8V

IC = 0.2mA

IS = 5x10-17 A

b = 100

input impedance of a cb stage
Input Impedance of a CB Stage
  • The input impedance of a CB stage is much smaller than that of a CE stage.
cb stage with source resistance
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.
practical example of a cb stage
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.
output impedance of a cb stage
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.
output impedance ce vs cb stages
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.
a v of cb stage with base resistance v a
Av of CB Stage with Base Resistance (VA = ∞)
  • With base resistance, the voltage gain degrades.
voltage gain ce vs cb stages
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.
r in of cb stage with base resistance v a
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.
input impedance seen at emitter vs base
Input Impedance Seen at Emitter vs. Base

Common Base Stage

Common Emitter Stage

input impedance example
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).