eem3a – analogue electronics dr. t. collins [email protected]
TRANSCRIPT
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Analogue Electronics ? Who Cares ?
D.S.P. Filter
R.F. Pre-Amplifier
Power Amplifier
Even digital systems usually rely on analogue electronics 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 course 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 amplifier’s bandwidth.
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Active Filters
Passive filter designs consist of a ladder of capacitors and inductors.
Inductors are bulky, expensive and imperfect components – especially when low values are required.
Using operational amplifier designs, inductors can be replaced using a variety of synthesis and simulation techniques.
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Recap : Common-Emitter Amplifier
V 5.121515
mA 1.015
V 5.0
0
0 Assume
CECCC
E
EE
E
B
B
RIRIV
RV
I
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
Col
lect
or C
urre
nt, [
mA
]
Base-Emitter Voltage [V] Base-Emitter Voltage [V]
VBE
IC
vbe
ic
Slope = gm
T
BESC V
VII exp
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Small Signal Operation
•As vin changes, the base-emitter voltage follows, i.e. vin = vbe.
•As vbe changes, the collector current follows, ic = gm.vbe.
•As ic changes, the voltage across Rc follows (Ohm’s law).
•Gain therefore depends on the relationships between vbe & ic and ic & 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 ratio between current and voltage changes.
Since the IC-VBE curve is not a straight line, gm changes with bias current.
T
C
T
BE
T
S
T
BES
BEBE
C
be
cm V
I
VV
V
I
VV
IVV
I
v
ig
expexp
dd
d
d
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Voltage Gain
100250.1
k.25
15dd
dd
mCin
c
c
out
in
out
CCCCC
out
c
out
mbe
c
in
c
gRv
i
i
v
v
v
RRIII
Viv
gv
i
v
i
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Equivalent Circuit
Cout
eBmBin
min
B
inc
B
inbRBin
in
inin
Rr
rRgRr
gv
R
vi
R
viii
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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Power Amplifier Stages
Properties : Low voltage gain (usually unity). High current gain. Low output impedance. High input impedance.
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Example – An Operational Amplifier
+
-
DifferentialAmp
VoltageAmp
PowerAmp
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Power Amplifier Designs
Differences between power amplifier designs :
Efficiency / Power dissipation. Complexity / Cost. Linearity / Distortion.
Power amplifier designs are usually classifiedaccording to their conduction angle.
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Efficiency / Dissipation
The efficiency, , of an amplifier is the ratio between the power delivered to the load and the total power supplied:
S
L
PP
Power that isn’t delivered to the load will be dissipated by the output device(s) in the form of heat.
LSCCED PPIVP
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Conduction Angle
The conduction angle gives the proportion of an
a.c. cycle which the output devices conduct for.
E.g.On all the time 360 On half the time 180 etc.
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Class A Operating Mode
Time
Iout
One device conducts for the whole of the a.c. cycle.Conduction angle = 360 .
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Class B Operating Mode
Time
Iout
Two devices conduct for half of the a.c. cycle each.Conduction angle = 180 .
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Class AB Operating Mode
Time
Iout
Two devices conduct for just over half of the a.c. cycle each.Conduction angle > 180 but << 360 .
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Class C Operating Mode
Time
Iout
One device conducts a small portion of the a.c. cycle.Conduction angle << 180 .
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Class D Operating Mode
Time
Iout
Each output device always either fully on or off – theoretically zero power dissipation.
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Differences Between Classes
Class A : Linear operation, very inefficient. Class B : High efficiency, non-linear response. Class AB : Good efficiency and linearity, more
complex than classes A or B though. Class C : Very high efficiency but requires
narrow band load. Class D : Very high efficiency but requires
low pass filter on load.
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Summary
Multi-stage amplifiers generally consist of a voltage gain stage and a current gain (or power amplifier) stage.
Several operating modes for power amplifiers can be designed.
Major differences between modes are efficiency, complexity and linearity.