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Lecture 7

Lecture 7. ANNOUNCEMENTS MIDTERM #1 will be held in class on Thursday, October 11 MIDTERM #2 will be held in class on Tuesday, November 13. OUTLINE BJT Amplifiers (cont’d) Biasing Amplifier topologies Common-emitter topology Reading: Chapter 5.1.2-5.3.1. Biasing of BJT.

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Lecture 7

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  1. Lecture 7 ANNOUNCEMENTS • MIDTERM #1 will be held in class on Thursday, October 11 • MIDTERM #2 will be held in class on Tuesday, November 13 OUTLINE • BJT Amplifiers (cont’d) • Biasing • Amplifier topologies • Common-emitter topology Reading: Chapter 5.1.2-5.3.1

  2. Biasing of BJT • Transistors must be biased because • They must operate in the active region, and • Their small-signal model parameters are set by the bias conditions.

  3. DC Analysis vs. Small-Signal Analysis • Firstly, DC analysis is performed to determine the operating point and to obtain the small-signal model parameters. • Secondly, independent sources are set to zero and the small-signal model is used.

  4. Simplified Notation • Hereafter, the voltage source that supplies power to the circuit is replaced by a horizontal bar labeled VCC, and input signal is simplified as one node labeled vin.

  5. Example of Bad Biasing • The microphone is connected to the amplifier in an attempt to amplify the small output signal of the microphone. • Unfortunately, there is no DC bias current running through the transistor to set the transconductance.

  6. Another Example of Bad Biasing • The base of the amplifier is connected to VCC, trying to establish a DC bias. • Unfortunately, the output signal produced by the microphone is shorted to the power supply.

  7. Biasing with Base Resistor • Assuming a constant value for VBE, one can solve for both IB and IC and determine the terminal voltages of the transistor. • However, the bias point is sensitive to  variations.

  8. Improved Biasing: Resistive Divider • Using a resistive divider to set VBE, it is possible to produce an IC that is relatively insensitive to variations in , if the base current is small.

  9. Accounting for Base Current • With a proper ratio of R1 to R2, IC can be relatively insensitive to . However, its exponential dependence on R1 // R2 makes it less useful.

  10. Emitter Degeneration Biasing • RE helps to absorb the change in VX so that VBE stays relatively constant. • This bias technique is less sensitive to  (if I1 >> IB) and VBE variations.

  11. Choose a value of IC to provide the desired small-signal model parameters: gm, r, etc. Considering the variations in R1, R2, and VBE, choose a value for VRE. With VRE chosen, and VBE calculated, Vx can be determined. Select R1 and R2 to provide Vx. Bias Circuit Design Procedure

  12. Self-Biasing Technique • This bias technique utilizes the collector voltage to provide the necessary Vx and IB. • One important characteristic of this approach is that the collector has a higher potential than the base, thus guaranteeing active-mode operation of the BJT.

  13. Self-Biasing Design Guidelines (1) provides insensitivity to . (2) provides insensitivity to variation in VBE .

  14. Summary of Biasing Techniques

  15. PNP BJT Biasing Techniques • The same principles that apply to NPN BJT biasing also apply to PNP BJT biasing, with only voltage and current polarity modifications.

  16. Possible BJT Amplifier Topologies • There are 3 possible ways to apply an input to an amplifier and 3 possible ways to sense its output. • In practice, only 3 out of the possible 6 input/output combinations are useful.

  17. Common-Emitter (CE) Topology

  18. Small Signal of CE Amplifier

  19. Limitation on CE Voltage Gain • Since gm = IC/VT, the CE voltage gain can be written as a function of VRC , where VRC = VCC - VCE. • VCE should be larger than VBE for the BJT to be operating in active mode.

  20. Voltage-Gain / Headroom Tradeoff

  21. I/O Impedances of CE Stage • When measuring output impedance, the input port has to be grounded so that vin = 0.

  22. CE Stage Design Trade-offs

  23. Inclusion of the Early Effect • The Early effect results in reduced voltage gain of the CE amplifier.

  24. Intrinsic Gain • As RC goes to infinity, the voltage gain approaches its maximum possible value, gm × rO, which is referred to as the intrinsic gain. • The intrinsic gain is independent of the bias current:

  25. Current Gain, AI • The current gain is defined as the ratio of current delivered to the load to current flowing into the input. • For a CE stage, it is equal to .

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