Common Collector (Emitter Follower) Amplifier

• Gain is never better than unity, however, has some desirable input and output impedance characteristics --- acts as a buffer

VCC

RC C3

RS C1

C2

vs RB RE RL

-VEE

Lecture 15-1 Common Collector (Emitter Follower) Amplifier

• Without RC there is no need for C3

VCC

RS C1

C2

vs RB RE RL

-VEE

Lecture 15-2 Emitter FollowerExample

• Calculate the voltage gain, current gain, input resistance and output resistance • Assume capacitors are infinite • 1) Calculate dc operating point

+10V β = 100 10kΩ VA = 100

Ω vs 100k 10kΩ 10kΩ

-10V

Lecture 15-3 Emitter Follower Example

• 2) Establish small signal model

10kΩ

π Ω r β r vs 100k ib o

10kΩ 10kΩ

Lecture 15-4 Amplifier Input Resistance, Rib

• 3) Calculate input resistance

10kΩ

π Ω r β r vs 100k ib o

10kΩ 10kΩ

Lecture 15-5 Transistor’s Input Resistance, Rib

• “Reflect” impedances into base from emitter for simplified input resistance calcuation

3kΩ 100kΩ βi Rib b

10kΩ 119kΩ 10kΩ

Lecture 15-6 Amplifier Input Resistance, Ri

• RB is part of the amplifier circuit, and adds to the input resistance to the transistor

Ω Ω 3k β Ri 100k ib

10kΩ 119kΩ 10kΩ

• Amplifier input resistance is limited by RB in this circuit. Why not make it bigger?

Lecture 15-7 Emitter Follower --- Buffer

• Even though the load resistance is 10kΩ, the input resistance is much higher

• This is a desirable feature of a buffer amplifier, especially if RS is large, or RL is small

• Is Rin big enough for our example? What is vi?

RS

π Ω v r β r vs 100k i ib o

10kΩ RL

Lecture 15-8 Emitter Follower --- Buffer

• Further voltage division for the amplifier stage • Most easily seen using other small signal model

α ie

vi re

10kΩ r o RL

Lecture 15-9 Analysis by Inspection

• Experienced analog designers just analyze the circuit directly by inspection

+10V

10kΩ

Ω vs 100k 10kΩ 10kΩ

-10V

Lecture 15-10 Analysis by Inspection

• What if RL is much less than RE?

+10V

10kΩ

vs 10kΩ RL

-10V

Lecture 15-11 Emitter Follower Current Gain

• No voltage gain, but acts as a buffer to drive small impedance loads • Provides good current amplification 10kΩ

π Ω r β vs 100k ib

10kΩ 119kΩ 10kΩ

Lecture 15-12 Current Gain

10kΩ

π Ω r β vs 100k ib

10kΩ 119kΩ 10kΩ

Lecture 15-13 Output Resistance, Ro

• 4) Calculate output resistance

10kΩ

π Ω r β r 0 100k ib o

10kΩ Ro

Lecture 15-14 Output Resistance, Ro

Lecture 15-15 SPICE Results

VCC dc solution 10V + - IC

839.882 µA RS C2 Q1 10E3Ω 1E-6F IIN Custom +- 0.000 pA C1 VINAC VSSIN VIN RB 1E-6F IO + + SIN + 100E3Ω +- 0.000 pV -758.300 mV 0.000 pA VOUT - - RE + 10E3Ω 0.000 pV - RL 10E3Ω

VEE + -10V

Lecture 15-16 SPICE Results voltage gain

time 0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms 10 mV

0

-10

VINAC VOUT

Lecture 15-17 SPICE Results

current gain

time 0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms 1 uA

0

-1

IO IIN

Lecture 15-18 Common Base Amplifier

• Gain is close to unity • Very low input impedance --- great for impedance matching (e.g. 50 ohms)

• Large output impedance -- approximately RC • Great high frequency behavior -- more on this later...

VCC

RC

vo

RS C1 I vs

-VEE

Lecture 15-19 Current Source Biasing

• Instead of resistors, a current source is used to bias the transistor in this example • Current sources can be used to bias other amplifier types too • Building a current source is less expensive than building a resistor on an IC --- we’ll be addressing IC issues such as this much more extensively beginning with the next lecture VCC

RC

vo

RS C1 I vs

-VEE

Lecture 15-20 Current Buffer

• Can be used as a current buffer • Macromodel:

i i io

vi R α R v i ii o o

Lecture 15-21 Small Signal Equivalent Circuit

• Since the base is grounded, we would probably select the T-model from Lecture 13

C ic α ie B ib r ie e E

• What parameters are being ignored in this simplified T-model? And what is the potential impact?

Lecture 15-22 Common Base Small Signal Analysis

Lecture 15-23 Common Base Small Signal Analysis

Lecture 15-24