Output Stages and Power Amplifiers
Output stage delivers the output signal
to the load without loss of gain due to
Low output resistance
R.F. PreAmplifier
D.S.P.
Power
Amplifier
Filter
Example – An Operational Amplifier
+
-
Differential Voltage
Amp
Amp
Power
Amp
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.
Properties of Power Amplifier Stage :
R Low voltage gain (usually unity).
R High current gain.
R Low output impedance.
R High input impedance.
Differences between power
amplifier designs :

Efficiency / Power dissipation.

Complexity / Cost.

Linearity / Distortion.
Power amplifier designs are usually
classified according to their conduction
angle.
Conduction Angle
The conduction angle gives the
proportion of an a.c. cycle which the
output devices conduct for.
E.g.
 360 °
On half the time  180 °
On all the time
etc.
Class A Operating Mode
Iout
Time
One device conducts for the whole of the
a.c. cycle.
Conduction angle = 360 .
The Class A stage must be biased at a
current greater than the amplitude of the
signal current.
Class B Operating Mode
Iout
Time
Two devices, each conducting for half
of the a.c. cycle.
Conduction angle = 180 .
Class AB Operating Mode
Iout
Time
Two devices, each conducting for just over
half of the a.c. cycle.
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.
Example: The built-in speaker in a PC is driven
by a Class D type “on/off’ circuit.
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.
Complex and expensive to get high
quality results.
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 supply requirements and
transistor power dissipation ratings
depend on the efficiency.
Power that isn’t delivered
to the load will be dissipated
by the output device(s) in the
form of heat.
PD  PS  PL
 V CE I C
(for amplifier
shown)
Class A Amplifier Efficiency
To calculate efficiency, must calculate load power, PL,
and the supplied power, PS.
Consider the emitter follower shown:
The average load power will be
The total avg. supply power is
Because:
Average
power drawn from  ve supply :
PS (  ve )  V CC I C  V CC I
Average
power drawn from  ve supply :
PS (  ve )  V CC I
The efficiency is
So, peak efficiency is when output
maximum.
Worst efficiency is when output
is at its
= 0.
Maximum output voltage swing is ±VCC.
Maximum output current swing is ±IE = ±I.
So :
and max. efficiency occurs for
and is 25%
In practice, the theoretical peak value of
would
not be reached without distortion, so practical max.
efficiency is between 10 and 20%.
Low efficiency results in Class A being rarely used in
large-power applications (> 1W).