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Amplifier Topologies
Most circuit element can be classified into resistive, capacitive element, magnetic
devices including inductor and transformer, and semiconductor devices operate in linear
mode or switch mode.
Circuits operating in a linear mode are usually restricted to signal processing applications
where frequency is not the primary concern.
Hence, magnetic devices can be avoided in linear mode operation, where the
output is in a linear relationship with the input, -the output is directly derived from it
thus able to provide a minimal amount of output distortion.
Since the transistor is constantly operating in its linear region potential high power dissipation
can occur, directly affecting the efficiency of the circuit.
The power is being wasted through the heat dissipation into the air (surrounding).
This is the case for the classical amplifiers such as Class A, B and AB , which all operate
in the linear region of the transistors.
Contrarily, nonlinear devices such as capacitors and magnetic devices, which are the key ingredients
of switch mode operation ideally consume no power.
In switch mode operation, the semiconductor devices are either fully on (saturation region) or off
(cut-off region) and this operation mode results the theoretical zero power dissipation.
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Class A
In class A operation the output transistor conduct continuously for the entire cycle of
signal swing, or the bias current flows in the output devices all the times.
Because of this, Class A amplifiers are single-ended designs with only one type of
polarity output device. Class A is the most power inefficient of all transistor couplings,
averaging only around 20%. Because of this, Class A amplifiers are large, heavy and run very hot.
All this is due to the amplifier constantly operating at full power.
The positive effects of all this is that Class A design are inherently the most linear,
with the least amount of distortion. |
Class A amplifier scheme.
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Class B
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Class B operation is opposite of Class A. Both output devices (transistors) are never allowed to be
on simultaneously, meaning that no bias current flow when the input signal is present.
When a signal is applied, the current flows for one half cycle on one transistor and in the opposite it
flows on the other.
Hereby each output devices is on for exactly one half of a complete sinusoidal signal cycle.
Generally, Class B designs show high efficiency but poor SNR due to the crossover region (zero-crossing)
where the "turn-on"/"turn-off" -time of the transistors introduce distortion.
Thus restricting Class B designs to power consumption critical applications, e.g. battery
operated equipment such as 2-way radio and other communications audio. |
Class B amplifier scheme.
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Class AB
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Class AB operation allows both devices to be on at the same time (Like Class A), but
just barely. The output bias is set so that current flows in a specific output device
appreciably more than a half cycle but less than the entire cycle. That is, only a small
amount of current is allowed to flow through both devices, unlike the complete load current of Class
A design, but enough to keep each device on, thereby avoiding the inherent distortion of Class B designs.
The efficiency of class AB designs is typically 50%, and the linearity is excellent which makes class
AB the most popular audio amplifier design. |
Class AB amplifier scheme.
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Class D
Class D operation is based on high speed switching, hence the term switching amplifier.
Here the output devices are rapidly switched on and off at least twice for each cycle to
afficliate the sampling law by Nyquest.
Theoretically since the output devices are either completely on or completely
off they do not dissipate any power. Consequently class D operation is theoretically
100% efficient, but this requires zero on-impedance switches with infinitely fast
switching times... In practice true efficiencies exceed 90%.
Class D amplifier scheme.
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