Class B and C RF Amplifier Operating Characteristic CurvesAuthor: R.J.Edwards G4FGQ © 16th February 2006
The efficiency of RF power amplifiers and other parameters depend on the plate current operating angle. This is the angle during which plate current flows out of a total of 360 degrees occupied by the grid-drive sine waveform.
This program assists with understanding the grid circuit factors which set operating angle to a particular value. It may be used in practical applications.
Use the curves to assist with choosing a value of -ve grid bias and the RF peak grid driving voltage. The curves will indicate the -ve grid volts which will almost cut off plate current. Also find the tube's amplification factor Mu.
For a particular tube it will be necessary to refer to its plate current versus plate voltage characteristic curves for various values of grid voltage. Select a +ve value of grid-cathode voltage and a plate or screen voltage at which peak plate current will flow at the +ve crest of the operating cycle.
After entering a set of data, vary transconductance until calculated peak plate current coincides with the peak current which actually flows in the tube.
A linear amplifier needs an operating angle of 180 degrees. To achieve this set the -ve grid bias equal to the plate current cut-off value. It is then obvious that plate current is directly proportional to the amplitude of the +ve half-cycle of the grid sine waveform with little distortion. The theoretical maximum efficiency, RF power output / DC power input, is 78 percent but depends on the impedance of the plate load. For Class-B expect efficiencies between 55 to 65%.
At angles less than 180 degrees an amplifier is no longer linear but efficiency increases. Angles may be less than 80 degrees, the lower limit being a function of cathode emission and peak plate current capability. With a tuned plate load power can be extracted at harmonic frequencies. Efficiency at the fundamental frequency, using a triode, can approach 80 percent.
Don't attempt to match a tuned load to the internal plate resistance. The load resistance depends only on peak sinewave volts across it and on plate current.
Triode Mu is from 5 to 100. Beam tetrode and pentode Mu is from 50 to 1000.
Transconductance (or mutual-conductance) is the number of milliamps change in DC plate current for a one volt change in DC control-grid volts.
Using PLATE CURRENT vs PLATE VOLTAGE and GRID VOLTAGE Characteristic Curves
When selecting values of grid cut-off volts, grid bias and grid drive to enter in the program, it will also be necessary to choose values of plate volts, DC screen volts and peak +ve excursion of grid-cathode volts. Peak grid-cathode volts and minimum plate volts can be adjusted to coincide with a tube's data.
The chosen value of plate voltage will be the minimum voltage which occurs at the same instant as peak grid drive. It will be greater than peak grid-cathode volts and, if the tube is a pentode, somewhat less than screen grid DC volts. If the tube is a beam tetrode minimum plate volts will be considerably less.
To match the program to the tube's characteristics, adjust transconductance to make computed peak plate current equal to its actual value and adjust grid drive volts to make peak grid-cathode volts simultaneously equal to its actual value.
Control grid current when the grid-cathode voltage is positive is a small fraction of total cathode current. Control grid and screen grid currents are not included in this program but screen grid power dissipation must be considered.
To complete the design make the tuned plate load resistance equal to the peak plate RF volts divided by the peak sinewave component of plate current.
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