EE105 Fall 2007Lecture 10, Slide 1Prof. Liu, UC Berkeley Lecture 10 OUTLINE BJT Amplifiers (cont’d) – CB stage with biasing – Emitter follower (Common-collector.

EE105 Fall 2007 Lecture 10, Slide 1 Prof. Liu, UC Berkeley

Lecture 10

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

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

EE105 Fall 2007 Lecture 10, Slide 2 Prof. Liu, UC Berkeley

Biasing of CB Stage

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ECm

in

out

in

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X

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RRg

v

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v

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inSEmE

Ein

Sin

inX v

RRgR

Rv

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

Em

E

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EE105 Fall 2007 Lecture 10, Slide 3 Prof. Liu, UC Berkeley

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

EE105 Fall 2007 Lecture 10, Slide 4 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 10, Slide 5 Prof. Liu, UC Berkeley

Example of CB Stage with Bias Design a CB stage for Av = 10 and Rin = 50. • Rin = 50≈ 1/gm if RE >> 1/gm Choose RE = 500

• Av = gmRC = 10 RC = 500• 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 = 52A• CB is chosen so that (1/(+1))(1/CB) is small compared to 1/gm at

the frequency of interest.

VCC = 2.5VIS = 5x10-16 A= 100VA = ∞

EE105 Fall 2007 Lecture 10, Slide 6 Prof. Liu, UC Berkeley

Emitter Follower (Common Collector Amplifier)

EE105 Fall 2007 Lecture 10, Slide 7 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 10, Slide 8 Prof. Liu, UC Berkeley

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 Vout = Vin .

1vA

AV

EE105 Fall 2007 Lecture 10, Slide 9 Prof. Liu, UC Berkeley

Small-Signal Model of Emitter Follower

• The voltage gain is less than 1 and positive.

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EE105 Fall 2007 Lecture 10, Slide 10 Prof. Liu, UC Berkeley

Emitter Follower as a Voltage Divider

AV

EE105 Fall 2007 Lecture 10, Slide 11 Prof. Liu, UC Berkeley

Emitter Follower with Source Resistance

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EE105 Fall 2007 Lecture 10, Slide 12 Prof. Liu, UC Berkeley

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).

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xin Rr

i

vR )1(

AV

EE105 Fall 2007 Lecture 10, Slide 13 Prof. Liu, UC Berkeley

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).

EE105 Fall 2007 Lecture 10, Slide 14 Prof. Liu, UC Berkeley

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.

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1inCmv RRgA speakerRRgA Cmv

EE105 Fall 2007 Lecture 10, Slide 15 Prof. Liu, UC Berkeley

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.

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mout R

R

gR ||

1

1

EE105 Fall 2007 Lecture 10, Slide 16 Prof. Liu, UC Berkeley

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.

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EE105 Fall 2007 Lecture 10, Slide 17 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 10, Slide 18 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 10, Slide 19 Prof. Liu, UC Berkeley

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.

EE105 Fall 2007 Lecture 10, Slide 20 Prof. Liu, UC Berkeley

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.

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m

S

C

in

out

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Rg

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CE stage Small-signal equivalent circuit

Simplified small-signal equivalent circuit

EE105 Fall 2007 Lecture 10, Slide 21 Prof. Liu, UC Berkeley

Amplifier Example #2• AC grounding/shorting and Thevenin transformation

are needed to transform this complex circuit into a simple CE stage with emitter degeneration.

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m

S

C

in

out

RRR

Rg

RRR

v

v

1

1

21 1

1

||

EE105 Fall 2007 Lecture 10, Slide 22 Prof. Liu, UC Berkeley

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.

1

1 1

2

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gR

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EE105 Fall 2007 Lecture 10, Slide 23 Prof. Liu, UC Berkeley

Amplifier Example #4

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• 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.

EE105 Fall 2007 Lecture 10, Slide 24 Prof. Liu, UC Berkeley

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

Amplifier Example #5

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1

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21