AM Linear Amplifiers
Related page Amplitude Modulation
"Incorrectly" in the context below means for linear, non-splattering, service.
AM linear amplifiers are often operated incorrectly. This is especially true in CB service, where virtually 100% are designed and operated incorrectly, but CB operators generally accept wide signals and poor amplifiers. CB amplifiers are often set up to process the AM signal and modify the signal like an RF speech processor would. RF processing makes meters swing, and makes audio sound heavy, but it also creates very wide bandwidths. This is unacceptable for other applications, where bands are policed or where people show respect for other users.
The problem with CB amplifiers is that some of the renegade nonsense theory or improper operation is brought into amateur radio use. This is especially true when amateurs or Hams use CB linears or CB-design style amplifiers on amateur bands.
Characteristics of AM (amplitude modulation)
Perfect AM in this text is considered to be:
With perfect amplitude modulation, the signal has the following characteristics:
Based on the above, a perfect 100-watt PEP transmitter would have:
Keep the above power relationships in mind for correct AM linear planning and operation.
Linear Amplifiers in AM Service
Traditional linear AM amplifiers are a form of efficiency modulation. This occurs because supply voltage is constant and does not vary with modulation. With fixed high voltage, only the supply current varies with drive.
When we vary current in a device with fixed supply voltage, the device normally does not have a square-law power response. We have to change something other than current to obtain full peak envelope power, since PEP is four times the carrier power. This is accomplished with changes in efficiency during modulation.
Efficiency modulation occurs naturally in a properly tuned linear amplifier. The linear amplifier has a constant anode or collector voltage. The constant supply voltage means output device impedance, or E/I of the output device, varies over the RF audio envelope cycle. The output device has highest current on modulation positive peaks, and lowest current on modulation negative peaks.
The output device impedance varies over the RF cycle, being highest at zero power (the full negative modulation peak). Output device impedance is lowest during the positive peak of modulation.
Because the output device impedance varies over the modulation cycle, and because the tank or matching system is fixed at one impedance, matching between the output device and the load varies over the modulation cycle. We want coupling to be perfect at the highest modulation peak, or at the very highest peak envelope power ever presented to the amplifier. This will produce peak efficiency during modulation peaks, where a class-B stage can have over 70% theoretical efficiency (typically it is only around 65%).
The carrier impedance is lower, and amplifier efficiency drops to about half of the peak efficiency. In practice, with excellent amplifier design, peak efficiency is around 60%. This places theoretical maximum carrier efficiency at less than 30%. The reasons are too complex to go into here, but we really should consider 20-25% as a good carrier efficiency.
This means the amplifier output device dissipates at least three times the carrier power as heat when a good amplifier is properly cooled, tuned, and operated.
The following list shows safe limits for properly tuned amplifiers with different tube types, assuming perfect 100% modulated AM signals:
All values are per tube with full airflow
This does not mean an amplifier can actually run the above power levels. The limit for AM power or transmit time is almost always output device cooling. Cooling is usually planned for noise considerations in amateur amplifiers. This means tubes will almost always not take the full absolute maximum power.
The exceptions are with tubes having thin long leads to the envelope, like the 811A and 572B. The 572 and 811 are designed to be convection cooled. They do not require forced-air on seals. 572B and 811A anodes dissipate the same power with or without airflow. The air keeps the envelope and the surroundings cool. As long as the glass envelope is kept below approximately 180 F, and as long as external components around the tube do not have too much thermal rise, 811A's and 572B's will handle full rated dissipation.
This does not apply to glass tubes like the 3-500Z, because the 3-500Z has significant heat conduction from the anode to the anode seal. The 3-500Z is airflow critical because of conducted heat to the seals through the very large diameter and reasonably short connections.
Dissipation in tubes with external anodes is directly tied to airflow, small airflow changes can make noticeable safe dissipation change.
The normal tuning procedure is to match the output device at maximum positive modulation peak. This matches the tube to the load at full peak power. As the modulation positive peak power is reduced, the output device has a progressively higher impedance. This higher impedance from reduced current mismatches the output tube or output device to the tank. The result is a progressive reduction in efficiency as the system moves below the peak positive modulation level, reaching minimum plate, collector, or drain efficiency at maximum negative peak when power output is zero.
The above requirement demands we tune or match any linear amplifier at the absolute maximum peak envelope power that ever appears. If we tune at a lower level and exceed that level on peaks, the amplifier will lose peaks. It will become non-linear. The exception to this is if the amplifier uses a TOF-1 (patent pending) tuning system, in which case improper operation will show during normal speech operation.
When an amplifier is properly tuned at 100% modulation, and only the carrier is present, output device carrier efficiency drops to about half of the device's positive peak efficiency. Let's assume an amplifier has about 70% anode efficiency, with 4% tank and other losses, for 66% total efficiency. At carrier levels, plate efficiency will be about 35%. This means on carrier conditions, 35% of plate input power will be lost as anode heat. Tank losses will be constant percentage, at 4% of the anode RF power. Including tank losses, overall carrier efficiency would be 33.6% with only 1.4% of anode input power appearing as lost power in the tank. Anode heat will be almost twice the heat carrier power output.
Linear amplifiers with high conduction angles only have about 50% efficiency on peaks. Along with a normal design procedure to slightly over-couple the output device, some amplifiers will only have around 20% carrier efficiency.
A reasonably safe general rule for linear amplifiers is output device power dissipation is three times carrier power when amplifying unmodulated carriers, although output device heat can be as low as two-times carrier output power. A legal-limit AM linear could have about 1125-watts anode dissipation during carrier conditions of 375 watts. On positive modulation peaks, output power will be about 1500 watts with 1500 watts of short-term dissipation. This is a reasonable safe estimate.
If a conventional AM linear or screen modulated stage is making more than half of the peak efficiency at full PEP levels when on unmodulated carrier, odds are very good the amplifier will have excessive distortion and splatter.
Linear Amplifiers on AM, or the Difference between Low Level and High Level Modulation
Low level modulation
often has much less
distortion and more
fidelity than high
level modulation of
tetrodes, and low level modulation more
reproduces the audio
input. It is much
easier to have low-distortion high-fidelity audio using
modulation. To be
sure, some of the
cleanest AM BC
built were low level
resulted in high
stations to use more
To be linear all stages must be tuned or loaded at full peak envelope power, plus a little safety factor. In other words if we are going to 1500 watts PEP output, we must load the amplifier stages to 1500 watts carrier or more! After loading at full peak power, carrier is set at less than 25% of the peak power. Failure to do this will result in modulation distortion called "flat-topping". The result will be very wide bandwidth splatter and "downward modulation".
If we are going to run 100-watt AM carrier levels, all stages must be tuned for at least 400-watts of peak power.