vccee321/spring99/lect/lect15mar5.pdf · • calculate the voltage gain, current gain, input...
TRANSCRIPT
Lecture 15-1
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
RB
RC
RE
-VEE
C1
C2
C3
RL
vs
RS
Lecture 15-2
Common Collector (Emitter Follower) Amplifier
• Without RC there is no need for C3
VCC
RB RE
-VEE
C1
C2
RL
vs
RS
Lecture 15-3
Emitter FollowerExample
• Calculate the voltage gain, current gain, input resistance and output resistance
• Assume capacitors are infinite
• 1) Calculate dc operating point
+10V
100kΩ 10kΩ
-10V
10kΩ vs
10kΩ
β 100=
VA 100=
Lecture 15-4
Emitter Follower Example
• 2) Establish small signal model
100kΩ
10kΩ 10kΩ
vs
10kΩ
rπ βibro
Lecture 15-5
Amplifier Input Resistance, Rib
100kΩ
10kΩ 10kΩ
vs
10kΩ
rπ βibro
• 3) Calculate input resistance
Lecture 15-6
Transistor’s Input Resistance, Rib
100kΩ
10kΩ 10kΩ
3kΩ βib
119kΩ
• “Reflect” impedances into base from emitter for simplified input resistance calcuation
Rib
Lecture 15-7
Amplifier Input Resistance, Ri
100kΩ
10kΩ 10kΩ
3kΩ βib
119kΩ
• RB is part of the amplifier circuit, and adds to the input resistance to the transistor
Ri
• Amplifier input resistance is limited by RB in this circuit. Why not make it bigger?
Lecture 15-8
Emitter Follower --- Buffer
100kΩ
10kΩ RL
vs
RS
rπ βibro
• 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?
vi
Lecture 15-9
Emitter Follower --- Buffer
10kΩ RL
re
αie
• Further voltage division for the amplifier stage
• Most easily seen using other small signal model
vi
ro
Lecture 15-10
Analysis by Inspection
• Experienced analog designers just analyze the circuit directly by inspection
+10V
100kΩ 10kΩ
-10V
10kΩ
vs
10kΩ
Lecture 15-11
Analysis by Inspection
• What if RL is much less than RE?
+10V
10kΩ
-10V
RL
vs
10kΩ
Lecture 15-12
Emitter Follower Current Gain
100kΩ
10kΩ 10kΩ
vs
10kΩ
rπ βib
119kΩ
• No voltage gain, but acts as a buffer to drive small impedance loads
• Provides good current amplification
Lecture 15-13
Current Gain
100kΩ
10kΩ 10kΩ
vs
10kΩ
rπ βib
119kΩ
Lecture 15-14
Output Resistance, Ro
100kΩ
10kΩ
0
10kΩ
rπ βibro
• 4) Calculate output resistance
Ro
Lecture 15-15
Output Resistance, Ro
Lecture 15-16
SPICE Results
Q1CustomIIN
0.000 pA
C21E-6F+ -
RB100E3Ω
+ -
10VVCC
IO
0.000 pA
C11E-6F+ -
+ -10VVEE
RS10E3Ω
IC
839.882 µA
+ SINVSSIN
+
-
VIN
-758.300 mV
RE10E3Ω
RL10E3Ω
+
-
VOUT
0.000 pV
+
-
VINAC
0.000 pV
dc solution
Lecture 15-17
SPICE Results
voltage gain
time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms
-10
0
10
mV
VINAC VOUT
Lecture 15-18
SPICE Results
current gain
time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms
-1
0
1
uA
IO IIN
Lecture 15-19
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
I
-VEE
C1
vs
RS
vo
Lecture 15-20
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
I
-VEE
C1
vs
RS
vo
Lecture 15-21
Current Buffer
• Can be used as a current buffer
• Macromodel:
io
αiivi voRoRi
ii
Lecture 15-22
Small Signal Equivalent Circuit
• Since the base is grounded, we would probably select the T-model from Lecture 13
ic
E
B
C
αie
ie
ib
re
• What parameters are being ignored in this simplified T-model? And what is the potential impact?
Lecture 15-23
Common Base Small Signal Analysis
Lecture 15-24
Common Base Small Signal Analysis