power amps 14
TRANSCRIPT
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EE3B1 Analogue Electronics
Dr. T. [email protected]
http://www.eee.bham.ac.uk/collinst/ee3b1
mailto:[email protected]://www.eee.bham.ac.uk/collinst/ee3b1thttp://www.eee.bham.ac.uk/collinst/ee3b1tmailto:[email protected] -
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EE3B1 Structure Content Delivery
18 Lectures (Mondays 12-1, Tuesdays 11-12)
5 Tutorial Sessions (Odd Mondays 4-5) + revision sessions
Online Material
Tutorial Problems
PowerPoint slides
Circuit analysis walkthroughs
Frequently Asked Questions
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Analogue Electronics ? Who Cares ?
D.S.P. Filter
R.F. Pre-Amplifier
PowerAmplifier
Even digital systems usually rely on analogueelectronics in some way. E.g. A digital radio:
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Analogue Essentials
Low noise, radio frequency amplifier.
Anti-aliasing filter.
Power amplification.
i.e. The module syllabus.
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Power Amplifiers Common-emitter amplifiers and
operational amplifiers require high
impedance loads. To drive low impedance loads, a power
output stage is required.
Designs vary in complexity, linearity and
efficiency. Power dissipation and thermal effects
must be considered.
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Low Noise and R.F. Amplifiers Pre-amplifier stages are the most prone to
noise as the signal level is so low.
Careful design minimises interference.
Common-emitter amplifiers can have a
disappointingly low upper cut-off frequency. Steps can be taken to extend an amplifiers
bandwidth.
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Active Filters Passive filter designs consist of a ladder of
capacitors and inductors.
Inductors are bulky, expensive and imperfectcomponents especially when low values arerequired.
Using operational amplifier designs, inductorscan be replaced using a variety of synthesisand simulation techniques.
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Recap : Common-Emitter Amplifier
V5.121515
mA1.015
V5.0
0
0Assume
CECCC
E
EE
E
B
B
RIRIVR
VI
V
V
I
Quiescent Conditions
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Biasing
0 0.2 0.4 0.6 0.8 10
2
4
6
8
10
0.586 0.590 0.594 0.5980.08
0.09
0.1
0.11
0.12
Collect
orCurrent,[mA]
Base-Emitter Voltage [V] Base-Emitter Voltage [V]
VBE
ICvbe
ic
Slope = gm
T
BESC
V
VII exp
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Small Signal OperationAs vin changes, the base-emitter
voltage follows, i.e. vin = vbe.As v
bechanges, the collector
current follows, ic = gm.vbe.As ic changes, the voltage across
Rcfollows (Ohms law).
Gain therefore depends on therelationships between vbe & ic andic & vout.
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Mutual Conductance, gm Mutual conductance, gm, is simply the slope
of the IC-VBE curve.
It is not a physical conductance, just the ratiobetween current and voltage changes.
Since the IC-VBE curve is not a straight line,g
m
changes with bias current.
T
C
T
BE
T
S
T
BES
BEBE
C
be
cm
V
I
V
V
V
I
V
VI
VV
I
v
ig
expexpd
d
d
d
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Voltage Gain
10025
0.1k.25
15d
d
d
d
mCin
c
c
out
in
out
CCC
CC
out
c
out
m
be
c
in
c
gRv
i
i
v
v
v
RRIII
V
i
v
g
v
i
v
i
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Equivalent Circuit
Cout
eBmBin
min
B
inc
B
in
bRBin
in
inin
Rr
rRgRr
gv
R
vi
R
v
iii
i
vr
||/||
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Loaded Common-Emitter Amplifier
LCmin
out RRgv
v||
i.e. Low load impedance low gain or high gm.But, high gm low re low rin.
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Common-Emitter Limitations It is often not possible to meet a specification
using a single amplifier stage
High voltage gain AND high current gain canbe incompatible
Solution: Multi-stage amplifiers using:
Differential amplifiers for input
Common-emitter amplifier for voltage gain
Power amplifier for current gain
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Example An Operational Amplifier
+
-
Differential
Amp
Voltage
Amp
Power
Amp
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Review Topics Focus on review of 1st and 2nd year material.
In particular
Common-Emitter Amplifier Small signal analysis
Mutual Conductance
Emitter resistance
etc.