Transcript EEM3A – Analogue Electronics

EEM3A – Analogue Electronics
Dr. T. Collins
[email protected]
http://www.eee.bham.ac.uk/collinst
Analogue Electronics ? Who Cares ?
Even digital systems usually rely on analogue
electronics in some way. E.g. A “digital” radio :
R.F. PreAmplifier
Power
Amplifier
D.S.P.
Filter
Analogue Essentials
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Low noise, radio frequency amplifier.
Anti-aliasing filter.
Power amplification.
i.e. The course syllabus.
Power Amplifiers
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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
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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
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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
Quiescent Conditions
Assume I B  0
 VB  0
 VE  0.5 V
IE 
VE   15
 0.1 mA
RE
VC  15  I C RC  15  I E RC  12.5 V
Biasing
VBE 
I C  I S exp  
 VT 
0.12
Collector Current, [mA]
10
8
Slope = gm
0.11
ic
6
0.1
IC
vbe
4
0.09
2
VBE
0
0
0.2
0.4
0.6
0.8
Base-Emitter Voltage [V]
1
0.08
0.586
0.590
0.594
Base-Emitter Voltage [V]
0.598
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
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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.
VBE  I S
VBE  I C
ic
dI C
d
gm 


I S exp    exp   
vbe dVBE dVBE
 VT  VT
 VT  VT
Voltage Gain
ic
ic

 gm
vin vbe
vout dVout
d


15  I C RC    RC
ic
dI C dI C
vout vout ic
0.1

  RC g m  25k.
 100
vin
ic vin
25
Equivalent Circuit
rin 
vin
iin
vin ic vin vin g m
iin  iRB  ib 
 

RB  RB

rin  RB ||  / g m  RB || re
rout  RC
Loaded Common-Emitter Amplifier
vout
  g m  RC || RL 
vin
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
+
-
Differential
Amp
Voltage
Amp
Power
Amp
Power Amplifier Designs
Differences between power amplifier designs :
 Efficiency / Power dissipation.
 Complexity / Cost.
 Linearity / Distortion.
Power amplifier designs are usually classified
according to their conduction angle.
Efficiency / Dissipation
The efficiency, h, of an amplifier is the ratio between the
power delivered to the load and the total power supplied:
PL
h
PS
Power that isn’t delivered to the load will be
dissipated by the output device(s) in the
form of heat.
PD  VCE I C  PS  PL
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
Iout
Time
One device conducts for the whole of the a.c. cycle.
Conduction angle = 360 .
Class B Operating Mode
Iout
Time
Two devices conduct for half of the a.c. cycle each.
Conduction angle = 180 .
Class AB Operating Mode
Iout
Time
Two devices conduct for just over half of the a.c. cycle each.
Conduction angle > 180  but << 360  .
Class C Operating Mode
Iout
Time
One device conducts a small portion of the a.c. cycle.
Conduction angle << 180 .
Class D Operating Mode
Iout
Time
Each output device always either fully on or off –
theoretically zero power dissipation.
Differences Between Classes
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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
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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.