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Lecture 11

Lecture 11. Bipolar Amplifiers (1). Output/Input Impedance. It is desirable to maximize the input impedance and minimize the output impedance of the amplifier. Measurement of Input/Output Imepdance. Not connected to any external source. Disable the effect of any input voltage

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Lecture 11

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  1. Lecture 11 Bipolar Amplifiers (1)

  2. Output/Input Impedance It is desirable to maximize the input impedance and minimize the output impedance of the amplifier.

  3. Measurement of Input/OutputImepdance Not connected to any external source Disable the effect of any input voltage Source. • Apply a test voltage (Vx) • Measure the resulting current (IX) • Calculate Vx/IX

  4. Summary of Impedances Seen at Terminals of a Transistor (Into the base) (Into the collector) (Into the emitter)

  5. DC and Small-Signal Analysis • 2-step analysis: • DC analysis • Small signal analysis • (Premise: the change in IC due to the signal must remain small)

  6. Operating Point Analysis and Design

  7. Simple Biasing Assumed VBE and β → IB→IC →VC Comments: IB is sensitive to VBE. IC is very sensitive to β.

  8. Iteration Solution (Kick off iteration with VBE=0.7) IC=βIB

  9. Example 1 Assumed: RB=1 Mohms RC=2 Kohm IS=6.734e-15 Beta=155

  10. ADS Simulation IB=11.3 uA IC=1.76 mA Beta=155

  11. β Variation Variation of gm implies that the gain of the amplifier will vary significantly as a function of beta.

  12. Resistive Divider Biasing

  13. Back of the Envelope Calcuation Assume the base current is negligible, i.e. negligible compared to the current in R1. IC does not depend on Beta!

  14. Negligible Base Current VBE=R3/(R1+R3)VCC VBE=5.75/(5.75+17)2.5V=0.631 V I(R1)/IB=110uA/2.10=52.38

  15. Sensitivity to Component Variation 1% error in R2 leads to 14 % error in IC. 5% error in R2 leads to 85 % in IC.

  16. Biasing with Emitter Degeneration If R2↑→VP↑→IE↑→VRE↑→small ∆VBE →Small ∆IC An error in VX due to inaccuracies in R1, R2 and VCC is absorbed by RE, leading to a smaller change in VBE.

  17. Design Rules VRE should be > 100 mV.

  18. ADS Simulation IB=12.4 uA IC=1.91 mA Beta=154

  19. Sensitivity to Component Variation 1% error in R2 leads to 1.05 % error in IC. 5% error in R2 leads to 5.7 % error in IC.

  20. Misc.

  21. What if base current is not ignored!

  22. Iterative Method Initial assumption: VBE=0.7 V

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