Monday, September 21, 2009

Class Act! - A little bit on amplifiers

Countdown timer: 2 weeks 1 day to go
Right, there are four main classes of amplifiers. They are:

Class A
Class AB
Class B
Class C

The sample paper, and indeed the June 2009 paper, asked which class of amplifier is the most efficient, and the answer would be Class C. Class C amplifiers produce harmonics but are only suitable for FM and CW transmissions.

Here's what you need to know about the efficiencies of the various classes.

Class A would demonstrate an efficiency of around 25%.
Class B would demonstrate an efficiency of around 50%.
Class C would demonstrate an efficiency of between 60% and 90%.

In Class A, the input voltage signal and output voltage are 180 degrees out of phase.
Class B requires less bias, and current flows in the output circuit for alternate half-cycles only. As I said above, a practical operating efficiency is around 50%.

According to JOhn Bowyer's Towards the RAE, Class C amplifiers are often used in oscillators and tuned r.f. amplifiers. Expected efficiency is 65%-90%. A class C amplifier is rich in harmonics and can be used as a frequency multiplier.

The total gain of an amplifier in B is the sum of the individual gains, expressed in decibels (dB), of each stage.

Output power is often expressed in dBW, which is the power relative to 1W. More on that later.

You might get a question about the output power of an amplifier. For instance, try this:

A carefully designed class C amplifier is taking an input current of 1A at 24V. The expected output would be:
a 24 W
b 16 W
c 10 W
d 5 W

Now we know class C is 65-90% efficient. Now we need to calculate the power. Here's the formula: P = V*I, and its variants: P = I(2)*R, P = V(2)/R, R = V(2)/P, where the (2) means squared.

Here we know the voltage (V) - 24V - and the current (I) - 1A. So 24*1 = 24. We can see that answer a, 24W, is not possible because that would require 100% efficiency. Answer b is the correct answer, being 66% or thereabouts of 24W.

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