EECS 105 Fall 2003, Lecture 18
Lecture 18:
Bipolar Single Stage Amplifiers
Prof. Niknejad
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Lecture Outline





BJT Amps
BJT Biasing
Common Emitter Amp
Common Base Amp
Common Collector Amp
–


Department of EECS
AKA Emitter Follower
β Multiplier Concept
Emitter Degeneration
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Bipolar Amplifiers
Common-emitter amplifier:
Biasing: adjust
VBIAS = VBE so
that IC = ISUP.
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Small-Signal Two-Port Model
Parameters: (IC = 1 mA, β =100, VA = 3V)

25mV
Rin  r 
 100
 2.5k
gm
1mA
3V
Rout  ro || roc 
|| roc  3k||roc  3k
1mA
Gm  g m 
Department of EECS
1mA
1
 S  40mS
25mV 25
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Base Amplifier
To find IBIAS, note that
IBIAS = IE = - (1/F)IC
Common-base current
gain Ai = - F
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
CB Input Resistance
Summing currents at the input node:
small
1

it    g m  vt
 r

v
it   g m v  (vo  vt ) g o  0
r
1
1

 gm

vt  1
1
25mV
Rin     g m   
 gm  

 25
it  r
gm
1mA
 


Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
CB Output Resistance
put roc back in
after finding vt / it
note
polarity
Same topology as CG amplifier, but with r || RS rather than RS
Rout  roc ||  ro 1  g m (r || RS  
RS  r
Rout  roc ||  ro 1  g m r  
Rout  roc || ro 1   
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Output Impedance Details



First draw small signal equivalent circuit with transistor and
simplify as much as possible
Then (if needed) add the small signal equivalent circuit
If frequency is low, get rid of caps!
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Output Impedance Calculation
it

r

v
g m v

ro
vt

RS
vt  (v )
it  g m v 
ro
v  it ( RS || r )
vt  RS || r
it   g mit ( RS || r )  
it
ro
ro

RS || r  vt
it 1  g m ( RS || r ) 

ro  ro

Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Base Two-Port Model
Why did we consider it a current amp?
Current Amp :
• Unity Current Gain (-1)
• Small Input Impedance
• Large (huge!) Output Impedance
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Collector Amplifier
DC Bias:
output is one
“VBE drop” down
from input
“Emitter Follower”
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Collector Input Resistance
vt  it r  (   1)it ( RL || ro || roc )
Rin  r  (   1)( RL || ro || roc )
Rin  r  (   1) RL
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Collector Output Resistance
r
v

vt
Divider between vt and v : 
r  RS
it  g m v  v r1  vt (ro || roc ) 1  0

1
it  vt gm  r
Department of EECS

 r 
1

v
(
r
||
r
)

 t o oc
 r  RS 
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Collector Output Res. (cont)

1
it  vt gm  r

 r 
1

v
(
r
||
r
)

 t o oc
r

R
S 
 
 1 
it  vt  g m r  1 

r

R
S 
 
vt r  RS
R out  
it
 1


Looking into base of emitter follower: load
impedance larger by factor β+1
Looking into emitter of follower: “source”
impedance smaller by factor β+1
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Collector Voltage Gain
KCL at the output node: note v = vt - vout
vout roc || ro



 g m  r1 (vt  vout )
vout roc || ro
vout

roc || ro
Department of EECS

1

1

1
 g m v  v r1




 g m  r1  g m  r1 vt
vout  vt
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Common-Collector Two-Port Model
typo: RL
Voltage Amp :
• Unity Voltage Gain (+1)
• Large Input Impedance
• Small Output Impedance
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Summary of Two-Port Parameters
Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Typical “Discrete” Biasing
VCC

RC
R1

R2
A good biasing scheme must
be relatively insensitive to
transistor parameters (vary
with process and temperature)
In this scheme, the base
current is given by:
VB  VCC
RE

R1
R1  R2
The emitter current:


R1
I E  VCC
 VBE ,on  / RE
R1  R2


Department of EECS
University of California, Berkeley
EECS 105 Fall 2003, Lecture 18
Prof. A. Niknejad
Gain for “Discrete” Design

Rin  (   1)(r  RE )
 (   1) RE
vs
RC
R1
~ vs RC / RE
R2
vs / RE
~ vs
RE
Rin 2  (   1) RE || R1 || R2



Let’s derive it by
intuition
Input impedance can
be made large
enough by design
Device acts like
follower,
emitter=base
This signal generates
a collector current
Can be made large to couple
All of source to input (even with RS)
Department of EECS
University of California, Berkeley