Amplifier Classes of operation
Electronic amplifiers are grouped into various classes and sub-classes according to the type of work they are intended to perform. The difference between the various classes is determined primarily by the value of average bias employed and the maximum value of the exciting signal to be impressed upon the input element. In the case of the transistor amplifier, there is a bias current present through the base and the emitter, and it's relative magnitude to the input signal, and it's effect on collector current, determine the amplifier class. In the case of the vacuum tube amplifier, the bias is usually expressed as that voltage placed on the grid relative to the cathode. The effect of this bias, and the excitation voltage, upon the plate current determining the class of operation for the amplifier circuit.
Electronic amplifiers use an "active device" to take a power supply voltage and/or current potential and modulate that potential with a smaller input voltage or current. Transistor active devices can be thought of as "current amplifiers", and vacuum tube active devices can be thought of as "voltage amplifiers". I will use the vacuum tube "voltage example" to describe the various classes.

Class A
A Class A amplifier is an amplifier biased and supplied with excitation of such amplitude that plate current flows continuously (360 degrees of the exiting wave shape) and grid current does not flow at any time. Such an amplifier is normally operated in the center of the grid-voltage / plate-current transfer characteristic curve, and gives an output waveshape which is a substantial replica of the input waveshape.
Class A1
A Class A amplifier in which grid current does not flow over any part of the input wave cycle.
Class A2
A Class A amplifier operated under such conditions that the grid is driven positive over a portion of the input voltage cycle, but plate current still flows over the entire cycle.

Class AB1
An amplifier operated under such conditions of grid bias and excitation voltage that plate current flows for more than one half the input voltage wave cycle, but less than the complete cycle. i.e. The operating angle of plate current flow is appreciably greater than 180 degrees, but less than 360 degrees. The suffix "1" indicates that grid current does not flow over any portion of the input cycle.

Class AB2
An amplifier operated under similar conditions as AB1, but where the excitation voltage is of such amplitude that grid current does flow over an appreciable portion of the input wave cycle.

Class B
A Class B amplifier is biased to cut off of plate current (without excitation voltage present) so that plate current flows over only one half of the input voltage cycle. The operating angle of plate current flow is 180 degrees. Grid current flows. The magnitude of grid current at a given point on the input wave cycle depends on the instantaneous voltage at that point. Higher input voltage points having higher grid current magnitudes. This "variable input impedance" requires a stiff driver capable of delivering some power to the input of such an amplifier stage.

Class C
A class C amplifier is biased to a value greater than that required for plate current cutoff. Excitation voltage is of such magnitude that grid current flows over all of the period of the input wave cycle. The angle of plate current flow is less than 180 degrees, actually as low as 120 degrees, up to about 180 degrees. These amplifier circuits also require power to drive them

Amplifier circuits
The active device(s) in an amplifier stage can be connected to the input signal and output load in many different ways. Some amplifier stages employ several active devices in a single current path to form exotic topologies that defy description. I will limit my notes here to general terms relative to the more easily described and implemented circuits.

Voltage amplifiers
These amplifiers are used to increase the magnitude of a signal to a useful level. Examples would be RF amplifiers connected to increase the small signal developed at the antenna terminals, and the amplifier used to increase the small signal voltage produced by a microphone element for audio. These amplifiers are usually of the "Class A" type. The active device is selected for its ability to increase these signals without adding signals of its own, such as noise and microphonics. Operating bias and current through the device are usually selected to optimize the signal to noise ratio. Many devices are available that are designed specifically for use in a "pre-amplifier" of this type.

Power Amplifiers
Amplifiers that are required to deliver power to a load, such as an antenna or loudspeaker, are grouped into two general categories: Single Ended and Push Pull. There are advantages and disadvantages to each of these types. Most RF power amplifiers operate Class AB, or near Class B, for SSB and can operate Class C for constant amplitude modes such as FM.

Where variable amplitude signals are amplified, such as an audio signal, two devices can be connected in push pull to maintain the waveform envelope shape, one tube amplifying the positive half of the waveform, and the second tube amplifying the negative half. The main issue determining the selection of a circuit type being distortion level toleration, and power efficiency. By using two tubes in push pull, and biased to cut off (as Class B), the average power consumed by the circuit can be reduced compared to a parallel connection of the same two devices producing the same level of distortion in the waveform.

Active devices have been developed specifically for use in class B amplifier stages. The 811, 838, and 809 vacuum tubes are examples. These tubes have a relatively fine grid mesh and thus a high amplification factor or "mu". More importantly, this causes less "no signal" plate current to flow than would otherwise flow for a given plate voltage (than if the grid pitch were more open). This allows the tubes to operate without a bias supply in Class B for specific signal and plate voltages.

Normal voltage amplifiers are not suited for use as drivers for class B amplifiers. Power is consumed in the grid(s) of a class B stage. High transconductance triodes tubes make good audio stage drivers. The, 6SA7, 6B4, 2A3, and 417 have relatively high transconductance, and low output impedances, and can be used in audio circuits without negative feedback to drive class B stages.

Pentodes have better power efficiency, but much higher distortion characteristics, and higher output impedances. Use of negative feedback can reduce the output impedance and reduce the distortion in such a stage.

An important factor for consideration for a power amplifier is it's output impedance. For best efficiency, the load must be matched to the amplifier output impedance. Rarely does this load happen to be the same value as the impedance of the active device, so some sort of matching circuit must be used. Tube audio power amplifiers usually use a transformer. An extreme example would be a small power pentode driving a 3.2 ohm speaker where the transformer primary is rated at 10K with a 3.2 ohm secondary.

RF amplifiers can use transformers, but the PI network is more common in modern tube systems. L-networks, T-networks and combinations of these are used in both transistorized and tube type R.F. matching systems. Often the transistor power stage has a lower output Z than the load. Some very creative systems have been used to match these amplifiers to their loads and reduce harmonic content, all in one circuit.

Summary
Electronic amplifiers have come a long way since the first "Audion" triodes were produced. Much has been done in the development of cathode metallurgy, element geometry, and general vacuum tube design. The modern vacuum tube is capable of long life and low distortion amplification with simple, if power hungry circuitry.

Transistors can produce efficient power and voltage amplification without the added power drain of a filament. Careful consideration must be given to the smaller voltage and current tolerances encountered with these handy devices.

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