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Lecture 10

Lecture 10. ANNOUNCEMENTS Alan Wu will hold an extra lab session tomorrow (9/28), 2-4PM The post-lab assignment for Experiment #4 has been shortened! 2 pgs of notes (double-sided, 8.5”×11”) allowed for Midterm #1. OUTLINE BJT Amplifiers (cont’d) CB stage with biasing

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Lecture 10

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  1. Lecture 10 ANNOUNCEMENTS • Alan Wu will hold an extra lab session tomorrow (9/28), 2-4PM • The post-lab assignment for Experiment #4 has been shortened! • 2 pgs of notes (double-sided, 8.5”×11”) allowed for Midterm #1 OUTLINE • BJT Amplifiers (cont’d) • CB stage with biasing • Emitter follower (Common-collector amplifier) • Analysis of emitter follower core • Impact of source resistance • Impact of Early effect • Emitter follower with biasing Reading: Chapter 5.3.3-5.4

  2. Biasing of CB Stage • RE is necessary to provide a path for the bias current IE to flow, but it lowers the input impedance.

  3. Reduction of Input Impedance Due to RE • The reduction of input impedance due to i1 is undesirable because it shunts part of the input current to ground instead of to Q1 (and RC).  Choose RE >> 1/gm , i.e.ICRE >> VT

  4. Creation of Vb • A resistive voltage divider lowers the gain. • To remedy this problem, a capacitor is inserted between the base and ground to short out the resistive voltage divider at the frequency of interest.

  5. Example of CB Stage with Bias VCC = 2.5V IS = 5x10-16 A b = 100 VA = ∞ Design a CB stage for Av = 10 and Rin = 50W. • Rin = 50W ≈ 1/gm if RE >> 1/gm  Choose RE = 500W • Av = gmRC = 10  RC = 500W • IC = gm·VT = 0.52mA • VBE=VTln(IC/IS)=0.899V • Vb = IERE + VBE = 1.16V • Choose R1 and R2 to provide Vb and I1 >> IB, e.g.I1 = 52mA • CB is chosen so that (1/(b+1))(1/wCB) is small compared to 1/gm at the frequency of interest.

  6. Emitter Follower (Common Collector Amplifier)

  7. Emitter Follower Core • When the input voltage (Vin) is increased by Vin, the collector current (and hence the emitter current) increases, so that the output voltage (Vout) is increased. • Note that Vin and Vout differ by VBE.

  8. Unity-Gain Emitter Follower • In integrated circuits, the follower is typically realized as shown below. • The voltage gain is 1 because a constant collector current (= I1) results in a constant VBE; hence DVout = DVin .

  9. Small-Signal Model of Emitter Follower • The voltage gain is less than 1 and positive.

  10. Emitter Follower as a Voltage Divider

  11. Emitter Follower with Source Resistance

  12. Input Impedance of Emitter Follower • The input impedance of an emitter follower is the same as that of a CE stage with emitter degeneration (whose input impedance does not depend on the resistance between the collector and VCC).

  13. Effect of BJT Current Gain • There is a current gain of (+1) from base to emitter. • Effectively, the load resistance seen from the base is multiplied by (+1).

  14. Emitter Follower as a Buffer • The emitter follower is suited for use as a buffer between a CE stage and a small load resistance, to alleviate the problem of gain degradation.

  15. Output Impedance of Emitter Follower • An emitter follower effectively lowers the source impedance by a factor of +1, for improved driving capability. • The follower is a good “voltage buffer” because it has high input impedance and low output impedance.

  16. Emitter Follower with Early Effect • Since rO is in parallel with RE, its effect can be easily incorporated into the equations for the voltage gain and the input and output impedances.

  17. Emitter Follower with Biasing • A biasing technique similar to that used for the CE stage can be used for the emitter follower. • Note that VB can be biased to be close to VCC because the collector is biased at VCC.

  18. Supply-Independent Biasing • By putting an independent current source at the emitter, the bias point (IC, VBE) is fixed, regardless of the supply voltage value.

  19. Summary of Amplifier Topologies • The three amplifier topologies studied thus far have different properties and are used on different occasions. • CE and CB stages have voltage gain with magnitude greater than one; the emitter follower’s voltage gain is at most one.

  20. Amplifier Example #1 • The keys to solving this problem are recognizing the AC ground between R1 and R2, and using a Thevenin transformation of the input network. CE stage Small-signal equivalent circuit Simplified small-signal equivalent circuit

  21. Amplifier Example #2 • AC grounding/shorting and Thevenin transformation are needed to transform this complex circuit into a simple CE stage with emitter degeneration.

  22. Amplifier Example #3 • First, identify Req, which is the impedance seen at the emitter of Q2 in parallel with the infinite output impedance of an ideal current source. • Second, use the equations for a degenerated CE stage with RE replaced by Req.

  23. Amplifier Example #4 • Note that CB shorts out R2 and provides a ground for R1, at the frequency of interest.  R1 appears in parallel with RC; the circuit simplifies to a simple CB stage with source resistance.

  24. Amplifier Example #5 • Note that the equivalent base resistance of Q1 is the parallel connection of RE and the impedance seen at the emitter of Q2.

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