1. Amplifier Classes of Operation
and Biasing Networks
PRESENTED BY:
HRUDYA

2. Amplifier Classes
 Amplifiers are classified according to the
portion of the input cycle the active device
conducts current
 This is referred to as the conduction angle and
is expressed in degrees
 Single-ended audio amps are operated in Class
A where the device conducts for 360°
 Push-pull amps can be a Class B if one of the
two devices is conducting at all times
 Most audio power amps operate in Class AB - a
compromise between Class A and Class B

3. Class A
 The active device conducts






current 100% of time .
Current flows in the output
circuit throughout the input
signal period.
Conduction angle (output voltage
swing) is 360 degree.
Operating point is established at
the centre of the load line .
Therefore practically no
distortion of amplified output
But half of the supply voltage,
drop across the transistor and
current through the transistor is
half of the saturation value.
Hence efficiency of class A
amplifier is only 50%

4. Class B
 Output current flows only for half





the period of the input signal.
Conduction angle is only 180
degree.
Operating point is selected at or
very near the cut-off point on the
load line .
There is distortion of output
voltage wave due to clipping.
Power drain is much less .(Since
during the half cycle of the input
signal, when there is no
conduction, transistor bias
current is zero.)
Hence efficiency of class B
amplifier is only 78.5%

5. Class C
 Output current flows less




than half the period of the
input signal.
Conduction angle less than
180 degree..
There is maximum
distortion of output voltage
wave due to clipping.
Power drain is much less
Hence efficiency of Class C
amplifier is almost 100%.

6. Class AB
 Output current flows for more




than half the period of the
input signal but not
throughout the cycle.
Conduction angle is more than
180 but less than 360 degree.
Operating point is selected such
that it is neither at cut-off nor
at the centre of load line, but
lies somewhere in-between .
There is distortion of output
voltage wave due to clipping.
Hence efficiency of class B
amplifier is more than 50% but
less than 78.5%.

7. Load and power supply current waveforms as a
function of conduction angle.

8. Contd.......

9. Biasing networks
Biasing networks are needed to set appropriate
operating conditions for active devices
There are two types:
• Passive biasing (or self-biasing)
– resistive networks
– drawback: poor temperature stability
• Active biasing
– additional active components (thermally
coupled)
– drawback: complexity, added power
consumption

10. Passive biasing
 Simple two element
biasing.
 blocking capacitors CB
and RFCs to isolate RF
path
 Very sensitive to
collector current
variations

11. Passive biasing
 Voltage divider to
stabilize VBE
 Freedom to choose
suitable voltage and
current settings. (Vx, Ix)
 Higher component
count, more noise
susceptibility.

12. Active biasing
 Disadvantages of passive
baising networks are


Poor temparature stability
Sensitive to changes in
transistor parameters
 Base current of RF BJT (Q2)
is provided by low-frequency
BJT Q1
 Excellent temperature
stability (shared heat sink)
 High component count, more
complex layout,Additional
Circuit Board Space
 Added power requirements.

13.  In all biasing networks , the operating conditions of the transistor at rf
frequencies are entirely independent of dc configuration.
 At dc
 Capacitors - rep open circuit
 RFC
- short circuit
 At ac
 Capacitors – short circuit
 RFC
– open circuit

14. DC and RF Equivalent Circuits for the active
biasing network.

15. FET biasing