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Classes of Amplifiers.
You knew it had to happen sooner or later. I have to tell you what the difference is between class A, AB1, AB2, B and C amplifiers. So hang on tight because here we go.
Class A Amplifier.
The resistance coupled amplifiers we have already studied are class A. In such an amplifier plate current flows over the entire input wave cycle. A resistance coupled amplifier won't work right in any other class. Single ended power amplifiers, such as those used in All American 5 radios are always class A. The theoretical maximum efficiency is 50% but in practice, consider yourself lucky if you can get 20. The average, DC, plate current will NOT change from zero signal to maximum power output. If it does, the tubes are not running in true class A. I don't care what the tube manuals say. Many audiophiles favor class A outputs because they deliver lower distortion. However a well designed AB1 output can sound just as good.
Remember the definition. Class A, the plate current of both tubes is flowing uninterrupted over the entire cycle.
Class B Amplifier.
Hold it, what happened to AB1 and AB2? As you may already know or have guessed class AB is a hybrid of pure class A and B. After we learn what class B is then we will go back to see what those AB classes are all about.
In ideal class B the plate current is zero when there is no input signal. When the signal appears each tube conducts over half of the cycle. The two tubes work alternately so the whole cycle is amplified. This assumes that the plate current versus grid voltage is a perfect straight line going right down to zero. Unfortunately, it is not. In order to avoid large amounts of distortion as the task of amplifying is being handed off from one tube to the other we have to allow some plate current to flow in the absence of input signal. The average, DC, plate current for maximum power output can be from 5 to 10 times the zero signal current. The class B amplifier that was the modulator for my AM kilowatt used a pair of 810 tubes which are power triodes. The idling current (plate current in the absence of audio signal) was about 60 milliamps and the peak current for 100% modulation was about 500 milliamps. (The plate voltage was 2000 volts.) The theoretical maximum efficiency for a class B amplifier is 78.5 % but in the real world expect about 60. In class B amplifiers the grid must always be driven positive for part of the cycle. That means that the grid draws current over that part of the cycle and a transformer must be used between the driver tubes and the power output tubes instead of resistance-capacitance coupling. A power amplifier with a transformer coupled driver is shown in Figure F below.
Just as a side note, a very popular transmitter used by small, 250 watt, AM broadcast stations and made by Collins Radio company was a pair of 810s for the RF power amplifier and another pair for the modulator. Stations running this rig were easy to spot, they had a terrific sound.
Remember the definition. Class B, the plate current of each tube flows for approximately 1/2 of the cycle and the two tubes work alternately.
Class AB1 Amplifier.
Class AB is intermediate between A and B. The 1 in AB1 means that grid current does not flow, the tube grids are not driven positive. A class AB1 amplifier will produce much less distortion than a class B amplifier using the same tubes. The distortion figures are often comparable to class A. The rise in plate current from zero signal to maximum power may be as little as 10 % to as high as 70 %.
Remember the definition. Class AB1, somewhere between A and B but the grids are NEVER driven positive.
Class AB2 amplifier.
Class AB2 is closer to B than to A. The 2 means that the grids ARE driven positive and grid current will flow. An output stage in which there is grid current such as AB2 or B must be transformer coupled, as shown in Figure F, instead of R-C coupled. If R-C coupling IS used the grid behaves like the plate of a diode and rectifies some of the audio signal which makes the grids more negative. Instead of class AB2 you would have a kind of moving class AB1 and a lot more distortion than you had counted on. If you were to attempt class B with R-C coupling, the extra grid bias could actually bias the tubes beyond cutoff and lead to heavy distortion. The rise in average, DC, plate current may be anywhere from 50 % to 200 %.
Remember the definition. Class AB2, somewhere between A and B but the grids ARE driven positive and there will be grid current.
Class C amplifier.
Class C amplifiers are never used in audio. This section is given in the name of completeness. They are used as radio frequency power amplifiers because of their high efficiency, approximately 80 %. They conduct plate current over 125 degrees, or less, of the input cycle. A resonate L-C circuit restores the wave to the familiar sine wave. The efficiency is so high because the tube conducts current only when the instantaneous plate voltage is low and is cut off when the plate voltage is high. That means the tube dissipates little power and most of it goes to the antenna.
