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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.

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Lecture 9

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  1. 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

  2. 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.

  3. 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.

  4. Tradeoff between Gain and Headroom • To ensure that the BJT operates in active mode, the voltage drop across RC cannot exceed VCC-VBE.

  5. Simple CB Stage Example VCC = 1.8V IC = 0.2mA IS = 5x10-17 A b = 100

  6. Input Impedance of a CB Stage • The input impedance of a CB stage is much smaller than that of a CE stage.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. Av of CB Stage with Base Resistance (VA = ∞) • With base resistance, the voltage gain degrades.

  12. 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.

  13. 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.

  14. Input Impedance Seen at Emitter vs. Base Common Base Stage Common Emitter Stage

  15. 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).

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