Output Stages and Power Amplifiers Output stage delivers the output signal to the load without loss of gain due to Low output resistance D.S.P. Filter R.F. Pre- Amplifier Power Amplifier

Example – An Operational Amplifier + - Differential Amp Voltage 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 :  Low voltage gain (usually unity).  High current gain.  Low output impedance.  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. On all the time  360 ° On half the time  180 ° etc.

Class A Operating Mode Time I out 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 Time I out Two devices, each conducting for half of the a.c. cycle. Conduction angle = 180 .

Class AB Operating Mode Time I out Two devices, each conducting for just over half of the a.c. cycle. Conduction angle > 180  but << 360 .

Class C Operating Mode Time I out One device conducts a small portion of the a.c. cycle. Conduction angle << 180 .

Class D Operating Mode Time I out 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, , of an amplifier is the ratio between the power delivered to the load and the total power supplied: Power that isn’t delivered to the load will be dissipated by the output device(s) in the form of heat. Power supply requirements and transistor power dissipation ratings depend on the efficiency.

Class A Amplifier Efficiency To calculate efficiency, must calculate load power, P L, and the supplied power, P S. Consider the emitter follower shown: The average load power will be The total avg. supply power is Because:

and max. efficiency occurs for and is 25% Low efficiency results in Class A being rarely used in large-power applications (> 1W). Maximum output voltage swing is ±V CC. Maximum output current swing is ±I E = ±I. So : The efficiency is So, peak efficiency is when output is at its maximum. Worst efficiency is when output = 0. In practice, the theoretical peak value of would not be reached without distortion, so practical max. efficiency is between 10 and 20%.