Loaded Common-Emitter Amplifier i.e. Low load impedance  low gain or high g m. But, high g m  low r e  low r in. Ideal amplifier has high gain, high r in, low r out. Impossible with a single stage –> multi-stage amps

Example – An Operational Amplifier + - Differential Amp Voltage Amp Power Amp

Power Amplifier Stages Properties : Low voltage gain (usually unity). High current gain. Low output impedance. High input impedance.

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. (More on this later)

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.

Complexity / Cost If you want a cheap simple solution, you ideally want: Low component count (fewer transistors) Easy design (no hard sums) Easy set-up / calibration Of course, this probably won’t be the case for the best power amplifiers

Linearity / Distortion For an ideal power amplifier: The voltage gain is unity The output voltage exactly equals the input voltage For real power amplifiers: The voltage gain is slightly less than one The input-output relationship might not be perfectly linear Other sources of distortion can be present (e.g. cross-over distortion) Non-linearity causes harmonic distortion which, if large enough, can be audible and annoying

Amplifier Classes: 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 .

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

Class AB Operating Mode Time I out Two devices conduct for just over half of the a.c. cycle each. 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.

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 : Potentially very high efficiency but requires low pass filter on load.

Summary 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.