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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 :
Analogue Essentials
Low noise, radio frequency amplifier. Anti-aliasing filter. Power amplification.
i.e. The course syllabus.
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.
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.
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.
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
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
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.
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
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
Equivalent Circuit
Cout
eBmBin
min
B
inc
B
inbRBin
in
inin
Rr
rRgRr
gv
R
vi
R
viii
i
vr
||/||
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.
Power Amplifier Stages
Properties : Low voltage gain (usually unity). High current gain. Low output impedance. High input impedance.
Example – An Operational Amplifier
+
-
DifferentialAmp
VoltageAmp
PowerAmp
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.
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
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.
Class A Operating Mode
Time
Iout
One device conducts for the whole of the a.c. cycle.Conduction angle = 360 .
Class B Operating Mode
Time
Iout
Two devices conduct for half of the a.c. cycle each.Conduction angle = 180 .
Class AB Operating Mode
Time
Iout
Two devices conduct for just over half of the a.c. cycle each.Conduction angle > 180 but << 360 .
Class C Operating Mode
Time
Iout
One device conducts a small portion of the a.c. cycle.Conduction angle << 180 .
Class D Operating Mode
Time
Iout
Each output device always either fully on or off – theoretically zero power dissipation.
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.
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.
