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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

Biasing of CB Stage

- RE is necessary to provide a path for the bias current IE to flow, but it lowers the input impedance.

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

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.

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.

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.

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 .

Small-Signal Model of Emitter Follower

- The voltage gain is less than 1 and positive.

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).

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).

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.

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.

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.

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.

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.

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.

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

Amplifier Example #2

- AC grounding/shorting and Thevenin transformation are needed to transform this complex circuit into a simple CE stage with emitter degeneration.

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.

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.

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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