PRESENTED BY:HRUDYA

Amplifier Classes of Operation and Biasing Networks

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

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%

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%

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 lessHence efficiency of Class C

amplifier is almost 100%.

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%.

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

Contd.......

Biasing networks

Biasing networks are needed to set appropriate operating conditions for active devicesThere 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

Passive biasing

Simple two element biasing.

blocking capacitors CB

and RFCs to isolate RF path

Very sensitive to collector current variations

Passive biasing

Voltage divider to stabilize VBE

Freedom to choose suitable voltage and current settings. (Vx,

Ix)Higher component

count, more noise susceptibility.

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.

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

DC and RF Equivalent Circuits for the active biasing network.

FET biasing