# class A power transistor amplifier

Discussion in 'Homework Help' started by markmc001, Mar 10, 2008.

1. ### markmc001 Thread Starter Member

Mar 10, 2008
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0
Hi all,
I have a problem with an assignment question with regards to classes of amplifiers. The question asks to explain the meaning of Class A and Class B amplifiers with reference to a power transistor amplifier. I would have no problem explaining the difference between the two in terms of a voltage amplifier, but am unsure of the differences in this application. Can anyone help??

Thanks,
Mark

(p.s. I'm new here. Hi!!)

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
search in google for class A and class B amplifiers !!!!

3. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
From wikipedia:

Angle of flow or conduction angle
Power amplifier circuits (output stages) are classified as A, B, AB and C for analog designs, and class D and E for switching designs based upon the conduction angle or 'angle of flow' Θ of the input signal through the amplifying device, that is, the portion of the input signal cycle during which the amplifying device conducts. (If the device is always on, Θ = 360°.) The angle of flow is closely related to the amplifier power efficiency. The various classes are introduced below, followed by more detailed discussion under individual headings later on.

Class A
100% of the input signal is used (conduction angle Θ = 360° or 2π). Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A, which means that the output devices are always in the conduction region. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones).
Class B
50% of the input signal is used (Θ = 180° or π). In Class B, there are two output devices (or sets of output devices), each of which conducts alternately for exactly 180 deg (or half cycle) of the input signal.
Class AB
More than 50% but less than 100% is used. (181° to 359°, π < Θ < 2π). Class AB amplifiers are a compromise between Class A and B, which improves small signal output linearity; conduction angles vary from 180 degrees upwards, selected by the amplifier designer. Usually found in low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency, or other designs where both linearity and efficiency are important (cell phones, cell towers, TV transmitters).
Class AB1 applies to tube or transistor amplifiers in class AB where the grid or base is more negatively biased than it is in class A.
Class AB2 applies to tube or transistor amplifiers in class AB where the grid or base is often more negatively biased than in AB1, and the input signal is often larger. When the drive is high enough to make the grid or the base more positive, the grid or base current will increase. It is possible depending on the level of the signal input for the amplifier to move from class AB1 to AB2.
Class C
Less than 50% is used (0° to 179°, Θ < π). Popular for high power RF amplifiers, Class C is defined by conduction for less than 180° of the input signal. Linearity is not good, but this is of no significance for single frequency power amplifiers. The signal is restored to near sinusoidal shape by a tuned circuit, and efficiency is much higher than A, AB, or B classes of amplification.

http://en.wikipedia.org/wiki/Electronic_amplifier

Some good pics to explain operation visually

Steve

Jul 17, 2007
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Mar 10, 2008
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