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.
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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.