Vacuum Tubes
• Vacuum Tubes, BJT or FET?
• Circuit Analysis: Amplifier & Feedback
• Classic BJT Circuits
• NY Times April 14, 2007
p
– “Guitar heroes favor vacuum tube amplifiers in their instruments”
– “Many recording engineers tend to use vacuum‐
tube equipment in their studios”
b
h
d ”
– “Some listeners pay thousands of dollars for high‐
end tube‐based stereo systems and CD players.”
Acnowledgements:
John K Fitch,
Neamen, Donald: Microelectronics Circuit Analysis and Design, 3rd Edition
6.101 Spring 2015
Lecture 5
1
6.101 Spring 2015
Lecture 5
2
Transistor Configurations
BJT or FET
TRANSISTOR AMPLIFIER CONFIGURATIONS
high input impedance, for
general amplification
BJT
R2
– BJT +15V
RL
+15V
low input impedance
for low impedance
sources, for high
frequency response.
R2
+15V
RL
R2
+
• Depends on application
• Amplifiers
lifi
voltage gain = 1 ; use
in amplifier output
stage.
+
+
Vin
R1
VOUT
RE
-
-
[a] Common Emitter Amplifier
+
Vin
R1
RE
-
+
+
+
VOUT
+
Vin
-
-
[b] Common Collector [Emitter Follower] Amplifier
VOUT
+
+
+
+
+
• are
are more linear that MOSFET, better fidelity (low harmonic more linear that MOSFET better fidelity (low harmonic
distortion)
• can drive low impedances at higher power
lower noise with low source impedance
• lower noise with low source impedance R1
RE
-
[c] Common Base Amplifier
– MOSFET
• high input impedance
• Voltage controlled device => lower power
V lt
t ll d d i
l
FET
Common source
6.101 Spring 2015
Lecture 5
3
6.101 Spring 2015
Common drain [Source follower]
Lecture 5
C
Common
gate
4
Gain vs Frequency
Expanded Hybrid π
rx
6.101 Spring 2015
Lecture 5
5
6.101 Spring 2015
Miller Effect* – Common Emitter
Lecture 5
6
Input Impedance and Frequency Response
log scale
AV (dB)
R
V1
V2
C
0
-3dB
slope = -6 dB / octave
slope = -20 dB / decade
log f
1
Av 
Av 
C M  C  [1  g m ( RC RL )]
V2
 j XC
1
j C



V1 R   j X C R  1
j RC  1
j C
1
sRC  1
High frequency cutoff f hi 
fHI or f-3dB
Degrees
PHASE LAG
0o
1
2RC
-45o
-90o
log f
* Agarwal & Lang Foundations of Analog & Digital Electronics Circuits p 861
6.101 Spring 2015
Lecture 3
fHI or f-3dB
7
6.101 Spring 2015
Lecture 2
8
Cascode Amplifer
Low Frequency Hybrid‐ Equation Chart
•
•
•
•
High gain applications
Moderate input resistance
High output resistance
6.101 Spring 2015
Unity gain, low
output resistance
High input resist.
Two transistor amplifer: common emitter (CE) common
emitter (CE)
with a common base
Miller effect avoid by setting gain of CE stage sett
g ga o C stage
low.
CE stage provides high input impedance
Common base (CB) provides gain without Miller effect; base is grounded.
grounded
No Miller effect
Miller effect
High gain, better high
frequency response
Low input resistance
Lecture 5
9
6.101 Spring 2015
Objectives
Lecture 5
10
Three Stage – Push Pull Amplifier
• Perform an analysis into the behavior of a complicated circuit using basic properties of
complicated circuit using basic properties of BJT’s (npn, pnp)
• Calculate gain of the system • Discuss crossover distortion issues in push‐
pull amplifiers
• Understand feedback
6.101 Spring 2015
Lecture 5
11
6.101 Spring 2015
Lecture 5
12
Three Stage Amplifer – Block Diagram
Q3‐Q4 Push Pull
• Q3: Emitter Follower (positve cycle)
• Q4: Emitter Follower (negative cycle)
(negative cycle)
• Overall gain ~1
• Q3 & Q4 V
& Q4 Vbe
are
b are always one diode drop
• Crossover distortion about zero crossing
Feedback
6.101 Spring 2015
Lecture 5
13
6.101 Spring 2015
Lecture 5
14
Q2 – High Gain Stage
Crossover Distortion •
Since last stage is an emitter follower node n4 should be
follower, node n4 should be biased near zero volts.
•
Q2 is a common emitter configuration with a pnp
t
transistor.
it
•
For pnp configuration, reverse the polarity of the voltages from a npn.
 0 g m r
gm 
gm 
I CQ
6.101 Spring 2015
Lecture 5
15
6.101 Spring 2015
I CQ
VTH
15
 4
10
 38 * I CQ
Av  570
Lecture 5
I CQ
VTH  26mv
VTH
Av 
  o RL
o
  g m RL
gm
16
Q1 Analysis
6.101 Spring 2015
Lecture 5
Biasing
17
6.101 Spring 2015
Biasing – Quiescent Point
Lecture 5
18
Biasing – Quiescent Point
• Determine the quiescent point
• For BJT
– assume Vbe = 0.6V
– assume base current neglible assume base current neglible
• Apply KVL, KCL
6.101 Spring 2015
Lecture 5
19
6.101 Spring 2015
Lecture 5
20
Biasing – Quiescent Point
6.101 Spring 2015
Lecture 5
20
Key Observations
Classic BJT Circuits/components
•
•
•
•
•
•
•
• Crossover distortion caused by base emitter voltage.
• Feedback results in overall gain F db k
l i
ll i
independent of β
• Biasing not precise, highly dependent Biasin not pre ise hi hl dependent
on supply voltage (poor supply voltage rejection)
voltage rejection) 6.101 Spring 2015
Lecture 5
21
6.101 Spring 2015
Darlington Pair
Matched transistors
Short circuit protection
Currrent mirror
Schottky diode
Baker Clamp
Vbe multiplier
Lecture 5
22
Matched Pair
MPSA13 Darlington
• Cl
Close electrical and l
i l d
thermal characteristics
• Supermatched p
pairs also available
• Used in differental amplifiers and
amplifiers and measurement equipment
6.101 Spring 2015
Lecture 5
23
6.101 Spring 2015
Lecture 5
24
356 Op‐Amp
Short Circuit Protection
7805 Regulator
7805 Regulator
Short Circuit Protection
• When voltage across R16 exceeds ~0.6 volt, Q14 diverts away base
Q14 diverts away base drive to Q15.
• N
Note Darlington pair t D li t
i
Q15, Q16
6.101 Spring 2015
Lecture 5
25
6.101 Spring 2015
Current Mirror
Lecture 5
26
Schottky* Diode
•
Schottky diode formed with
metal‐semiconductor junction vs
pn junction
•
Lower forward voltage drop: 0.15‐0.45 vs 0.6‐0.7 for pn junction
•
Almost zero reverse recovery times.
•
Used in 74LS series logic Low‐power Schottky
* Walter Schottkly
6.101 Spring 2015
Lecture 5
27
6.101 Spring 2015
Lecture 5
28
Baker* Clamps
Vbe Multipler
• Reduces turn off time by limiting saturation
• Baker Camp with diodes
V
( R1  R2 )
VBE
R2
• Baker clamp with Schottky diode
*named by Richard Baker (1956); drawings from http://en.wikipedia.org/wiki/Baker_clamp
6.101 Spring 2015
Lecture 5
29
6.101 Spring 2015
Lecture 5
30