# How can we measure/calculate exactly the conduction angle?

Discussion in 'Mathematics and Physics' started by anhnha, Aug 3, 2013.

1. ### anhnhaMember

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Hi,
I am a bit confused about conduction angle. Here is a picture about conduction angle of amplifier topologies.

With class A and class B the conduction angles are 360° and 180° respectively.

However, I got stuck in geting exactly the conduction angle of class AB and class C.
How can we measure/calculate exactly the conduction angle of these classes?

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2. ### rumpfyActive Member

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In one of your earlier posts about load lines, You has a fet amplifier with a resonant circuit and a load. In that design the bias point for teh DC conditions was a 'Class A' design, because at the operating point there was DC current flowing in the fet. The load line was a resistor and with changes in the input conditions, the output current /voltage followed the load line.
That amplifier was a single stage amplifier.
However, for other requirements such as power, efficiency, or fidelity, other configurations are used.

With Class A, if there is no input signal, the amplifier is consuming current and this may be wasteful.
If a push-pull arrangement is used, the bias point can be shifted so that one half of the AC input wave is amplified in one transistor, and the opposite half cycle can be amplified in the other transistor. This operating condition is known as Class B, and the bias point can be set so that the quiescent current in each transistor is quite low (or zero).
In the case of RF transmitters, another operating point can be chosen where the bias of the transistor set so that the conduction angle is very much less than 180 degree. This bias condition gives short pulses of output power which, in audio terms would be highly distorted, but because the pulses of output power are sent to a resonant load, and the antenna is coupled to the amplifier by specially designed coupling networks, it is possible to pass only the fundamental required frequency to the antenna and the harmonic frequencies are attenuated. The transistor characteristic curves for this operation condition are quite specifically designed for class C operation.
there are other operating points as well.
In high power audio valve amplifiers, I have seen operating points of Class AB1 and AB2 and again, these are specific designs for specific applications. Typically a thermionic valve is not driven into a grid current condition but Class AB2 does drive the valve into the grid current region.
Class C radio frequency valve amplifiers drive the valve well into grid current.

Class A; conduction all the time
Class B; conduction half the time
Class AB/AB1; conduction slightly more than half the time
Class AB2; conduction slightly more than half the time but grid current flows
Class C; conduction less than half the time.
Hope this helps.

3. ### anhnhaMember

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Thanks, rumpfy, for detailed explanation!
That is really not my question but I appreciate it.