BJT Configurations
Voltage Gain
Current Gain
Power Gain
X
X
X
X
X
Common Emitter
Common Collector
Common Base
X
Common emitter: hgh input impedance, for general amplification of
voltage, current and power from low power, high impedance sources.
• BJT Circuits & Limitations
• LTspice
Common collector: aka "emitter follower" for high input impedance
and current gain without voltage gain, as in an amplifier output stage.
Common base: low input impedance for low impedance sources, for
high frequency response. Grounding the base short circuits the
Miller capacitance from collector to base and makes possible much
higher frequency response.
Acnowledgements:
Neamen, Donald: Microelectronics Circuit Analysis and Design, 3rd Edition
LTspice material by Devon Rosner ‘13 (6.101 TA), Engineer, Linear Technology
6.101 Spring 2015
X
Lecture 3
1
6.101 Spring 2015
Lecture 3
General Configuration
2
Transistor Configurations
TRANSISTOR AMPLIFIER CONFIGURATIONS
Common
Emitter
+15V
RL
+15V
R2
RL
R2
+
R2
+15V
+
+
Vin
Common
Base
Common
Collector
6.101 Spring 2015
R1
RE
VOUT
-
[a] Common Emitter Amplifier
Lecture 3
3
6.101 Spring 2015
-
+
Vin
+
+
R1
RE
-
VOUT
-
-
[b] Common Collector [Emitter Follower] Amplifier
Lecture 3
+
+
+
Vin
+
VOUT
+
+
+
R1
RE
-
[c] Common Base Amplifier
4
Base Current – Resistor Divider
Commom Emitter – Hybrid π
TRANSISTOR AMPLIFIER CONFIGURATIONS WITH HYBRID- EQUIVALENT CIRCUITS
COMMON EMITTER AMPLIFER
 0 g m r
+15V
IC
68K
3.7 mA
50
4.0 mA
100
4.2 mA
200
4.3 mA
ib
F
RL
RB
C
300
2N3904
+
gm 
IC
v
vout
+
vin
IC=0.6 mA
_
VTH  26mv
+
IB
Rs
I CQ
VTH
g m v
_
R’s=RB // RS
33K
Make ib small
compared to the
current through R2
b
c
+
r
Rs
+
vin
ib
RB
RL
_
_
See handout: Transistor bias stability
6.101 Spring 2015
Lecture 3
5
R’s=RB // RS
vout
  o ib RL
  o RL
;


vin ib R 's  r   o  1RE  R 's  r   o  1RE
if R 's  r   o  1RE ;
•
•
•
6.101 Spring 2015
Lecture 3
6.101 Spring 2015
vout
  oib RL   o RL


vin ib R's  r  R's  r
if R'S r ; then Av 
  o RL
o
  g m RL
gm
Lecture 3
6
AC Coupled vs DC Coupled Amplifiers
Common Emitter with Emitter Degeneration
Av 
vout
e
Av 
then Av  RL / RE
Input resistance (β+1)RE
Voltage gain reduced by (1+gm RE)
Voltage gain less dependent on β
(linearity)
7
• AC Coupling
– Advantage: easy cascading with DC blocking capacitor, bias stability and stage independent
– Disadvantage: lot’s of R’s and C’s, no DC gain, need large C for low freqency
• DC coupling
– Same gain at DC
– Fewer R’s C’s
6.101 Spring 2015
Lecture 3
8
Gain vs Frequency
Expanded Hybrid π
rx
6.101 Spring 2015
Lecture 3
9
6.101 Spring 2015
Lecture 3
Miller Effect* – Common Emitter
10
Low Pass Filter LPF
log scale
AV (dB)
R
V1
C
V2
0
-3dB
slope = -6 dB / octave
slope = -20 dB / decade
log f
1
Av 
Av 
CM  C [1  g m ( RC RL )]
1
 j XC
V2
j C



j RC  1
V1 R   j X C R  1
j C
1
sRC  1
High frequency cutoff f hi 
fHI or f-3dB
Degrees
0o
1
2RC
-90o
* Agarwal & Lang Foundations of Analog & Digital Electronics Circuits p 861
6.101 Spring 2015
Lecture 3
PHASE LAG
-45o
fHI or f-3dB
11
6.101 Spring 2015
Lecture 2
log f
12
Hybrid‐π Parameters
 q 
g m    IC
 kT 
0  h fe (datasheet)
β
C  Cob (datasheet)
gm
 fT (transit frequency datasheet)
2 (C  C )
gm
 C
2 fT r
rx (low frequency) : datasheet or estimate 50 100
Use max for worst
case cu
C 
(high frequency) : estimate  25
6.101 Spring 2015
Lecture 3
13
6.101 Spring 2015
Lecture 3
14
hfe and High Frequency Limits
2N3904
CE configuration,
VCC +15v
Small signal current gain versus frequency, hfe, of a BJT biased in a
common emitter configuration:
ib 
vbe
 vbe jC
r
h fe 
g m vbe
g m r



ib
1  jr C 1  jr C
For hfe =1 = fT, (transit frequency )
hT 
gm
where C  (c  c )
2C
For 2N3904*, IC =1ma, VCE=10V , cπ=25pF, cμ=2pF
fT 
0.04mho
 240 MHz
2 27 pF
for a gain of g m RL  100 f h 
1
1

 320kHz
2 r g m RL c 2 2.5 K(100)2 pF
Miller effect reduces high frequency limit!
*Lundberg, Kent: Become One with the Transistor p29
6.101 Spring 2015
Lecture 3
15
6.101 Spring 2015
Lecture 3
16
Common Collector (Emitter Follower)
Common Base Configuration
 0 g m r
gm 
I CQ
VTH
VTH  26mv
R’s=RB // RS
Av 
v
g m v
if R's  r   o  1RE ;
•
•
6.101 Spring 2015
Lecture 3
17
6.101 Spring 2015
Common Collector – Emitter Follower Biasing
+15V
7.5 mA
R2
2N3904
1.0 k
7.5 mA
B
+15V
2N3904
IB
RB
VB
7.5 V
Β = 100, iB = 7.5ma/100 =‐ 75µa
Using Thevenin equivalent, +15V
R1 

 R1  R2 
RB = R1||R2, VB = 15
7.5 mA
R2
IDivider
A
8.1 V
VB = IBRB + 0.6V + 7.5V
VB = [75 µA x 10k] + 0.6V + 7.5V
VB = 750 mV + 0.6V + 7.5V
VB = 8.9V
2N3904
R1
1.0 k
[15 R1] ÷ [R1 + R2] = 8.9V
15 R1 = 8.9 x [R1 + R2]
[15−8.9] R1 = 8.9 R2
R1 = 1.44 R2
[R1 x R2] ÷ [R1 + R2] = 10 kΩ
[1.44R2 x R2] ÷ [1.44 R2 + R2] = 10kΩ
R2 = 16.9 kΩ (use 16 kΩ)
R1 = 1.44 R2 = 24.4 kΩ (use 24 kΩ)
6.101 Spring 2015
Lecture 3
Buffer with unity gain
High input resistance driving low output resistance (current gain).
18
Common Collector – Emitter Follower Biasing
B
7.5 mA
then Av  1
Lecture 3

A
R1
•
•
 o  1 RE
 o  1ib RE
vout


;
vin ib R 's  r   o  1RE  R's  r   o  1RE
•
With R1 = 24kΩ, R2 = 16 kΩ, the current through the voltage divider is 15 ÷ [40 kΩ] = 375 µA.
•
The 75 µA base current is 20% of 375 µA.
•
With R1 = 2 kΩ, will need a divider current that is ~ 4.1 mA. (75 µA is only ~2% of 4.1 mA, which is negligible)
•
The voltage drop across R2 will be [15 V –
8.1 V] = 6.9 V; R2 = 1.7 kΩ
•
But input impedance will be low = ~890Ω
•
Use bootstrapping configuration
7.5 mA
= 24.4 kΩ (use 24 kΩ)
19
6.101 Spring 2015
Lecture 3
20
Low Frequency Hybrid‐ Equation Chart
Introduction to LTspice
High gain applications
Moderate input resistance
High output resistance
6.101 Spring 2015
Unity gain, low
output resistance
High input resist.
High gain, better high
frequency response
Low input resistance
Lecture 3
21
Acknowledgment: LTspice material by Devon Rosner (6.101 TA), Engineer, Linear Technology
6.101 Spring 2015
SPICE
Lecture 4
22
Netlists
• Simulation Program with Integrated Circuit Emphasis
• Developed in 1973 by Laurence Nagel at UC Berkeley’s Electronics Research Laboratory
• Dependent on user defined device models
6.101 Spring 2015
Lecture 4
23
6.101 Spring 2015
Lecture 4
24
LTspice
LTspice
•
•
•
Developed in 1998 by Mike Engelhardt at Linear Technology Corporation
GUI, simulator and schematic ‐> netlist for SPICE
Free with lot’s of models!
6.101 Spring 2015
Lecture 4
• Available Windows and MAC OS X 10.7+
• Windows version easier to use. • For MAC users install Windows version under VMware or use • WINE (short for Wine Is Not an Emulator)
25
6.101 Spring 2015
Getting Started
6.101 Spring 2015
Lecture 4
Lecture 4
26
Component to Menu Item 27
6.101 Spring 2015
Lecture 4
28
Net Labels
Adding Other Components
• Labeling nets avoid drawing wires.
• Use nets for power supply voltages, ground, circuit‐wide signals.
6.101 Spring 2015
Lecture 4
29
6.101 Spring 2015
Op‐Amps
6.101 Spring 2015
Lecture 4
Lecture 4
30
Editing Components
31
6.101 Spring 2015
Lecture 4
32
Component Values
Basic Voltage Source
1 MEG = 1 megohm resistor
1 = 1 farad capacitor (not 1F)
Basic voltage source: used for DC supply voltage
or bias voltage.
6.101 Spring 2015
Lecture 4
33
6.101 Spring 2015
Lecture 4
34
Device Models
Advanced Voltage Sources
Many device models provided by Ltspice (BJT,
MOSFETS, diodes.
6.101 Spring 2015
Voltage source can produce arbitrary test
signals using a PWL (Piece Wise Linearly)
test file.
Lecture 4
35
6.101 Spring 2015
Lecture 4
36
Simulation: Transient
Voltage/Current vs Time
6.101 Spring 2015
Lecture 4
Simulation Stop Time
37
6.101 Spring 2015
Lecture 4
38
What Should My Circuit Do?
Parameters in Simulation
• Before running simulation you should have an ideal of how the circuit behaves.
• Simulation is a verification tool – circuit solver. 6.101 Spring 2015
Lecture 4
39
6.101 Spring 2015
Lecture 4
40
Low Pass Filter
Analyze this circuit
• Use low pass filter (LPF). Sallen‐Key filter is a second order filter. (A second order filter attenuates by a factor of one fourth for every doubling of frequency.)
• For low frequencies, capacitors are open circuit –
op amp feeds signal through.
• For high frequencies, capactiors act as shorts.
fc 
1
2RC
R = 10K
C = 1n (1nf)
fc= 16 kHz
6.117 IAP 2015 Lecture 3
6.101 Spring 2015
Lecture 4
41
42
Transient Simulation
6.101 Spring 2015
Lecture 4
Low Pass Filter Simulation
43
6.101 Spring 2015
Lecture 4
44
AC Simulation
6.101 Spring 2015
Lecture 4
AC Simulation
45
6.101 Spring 2015
Math In LTspice
6.101 Spring 2015
Lecture 4
Lecture 4
46
Transient Analysis
47
6.101 Spring 2015
Lecture 4
48
More Fun
6.101 Spring 2015
Lecture 4
AC Simulation
49
6.101 Spring 2015
Finishing Touches
6.101 Spring 2015
Lecture 4
Lecture 4
50
Finishing Touches
51
6.101 Spring 2015
Lecture 4
52