Class A, AB & C Operation of Tetrode Single-Ended RF AmplifiersAuthor: R.J.Edwards G4FGQ © 12th February 2001
A complete RF sine waveform cycle extends over an angle of 360 degrees. The part of the cycle during which anode current flows is known as the 'operating angle.' This depends on the screen-grid-to-control-grid amplification factor, on grid DC bias, on the DC screen voltage, and on the amplitude of RF grid drive volts. DC and RF anode voltages have little effect on the operating angle.
Linear amplification, where the RF output envelope is proportional to the input envelope, is obtained only when the operating angle is 360 degrees, or when the amplifier is tuned and the operating angle is 180 - 190 degrees. DC to RF power conversion efficiency is poor at 360 degrees but moderately good at 180 degrees.
When non-linearity is acceptable as in FM and CW transmitters, the angle can be reduced to 100 degrees or less where high efficiency is obtainable. Very small angles are eventually restricted by a tube's peak cathode current capabilities.
This program allows the operating angle to be entered directly. Biasing conditions in the grid circuit are then computed. For A,B or C operating conditions all computed values apply at the peak envelope power output.
Notes on Program Input Data
The anode/control-grid amplification factor of screen grid tubes is very high. It ranges from 100-2000. If not known enter 400. It is very non-critical.
Screen-grid/control-grid amplification factors are in the range 4 to 25. If not known initially enter 8. It can be readjusted later to make tube behaviour conform to expectations. Its value is critical and should be obtained from the tube manufacturer's data sheets. Screen Mu is approximately the +ve DC screen voltage divided by the -ve DC grid voltage needed just to cut off cathode current.
Perveance has the dimensions of a conductance. It is a constant depending on a tube's cathode and grid geometry. It determines cathode current and transconductance. Doubling perveance is equivalent to connecting two identical tubes in parallel. Initially enter perveance = 1. Then readjust such that peak anode current approximates the manufacturer's value at the same electrode potentials.
For efficient DC-to-RF power conversion the lowest instantaneous anode to cathode voltage should be a small proportion of the anode's DC supply. However, if the anode voltage falls much lower than the screen then anode current will be diverted to the screen and the screen's maximum power rating may be exceeded. Beam tetrodes are better than pentodes in this respect and the instantaneous anode voltage may be allowed to fall to 50% or less of the DC screen volts.
For a given tube and set of conditions, the maximum instantaneous control grid voltage sets the peak cathode current. The program will not accept a -ve value for this parameter. Therefore the computed grid drive signal amplitude is never less than the grid's DC bias. So when operating in Class-A conditions, prevent a positive grid bias by setting maximum instantaneous grid volts to zero.
Operating angle can be varied in conjunction with perveance until peak current, power input and power output agree with values taken from tube data graphs when the grid is at its most +ve voltage simultaneously with the anode voltage being at its least +ve value.
The program always computes anode load and grid drive requirements for when the amplifier is running at peak envelope power. Once the amplification factors and perveance have been matched to a particular tube the DC voltages and operating angle can be varied to investigate operation under a wider range of conditions.
Tank circuit operating Q is normally in the range 8 to 14. Lower values may not adequately reject harmonics of the fundamental. Higher values cause increasing power dissipation in the tank coil itself.
Notes on Computed Output Data
When the control grid voltage is +ve with respect to the cathode it will itself draw current. Grid current operating angle is always less than the anode's. The program computes grid current angle, fundamental frequency component of grid current, required RF drive power, the DC component of grid current and the bias resistor value. Grid current through this resistor automatically provides the required -ve grid bias when the tube is used in CW non-linear conditions, or as an oscillator, or as an anode modulated amplifier.
If a linear amplifier has a constant DC bias supplied by a power unit then the bias resistor = 0 ohms, but RF grid drive power, etc., will remain unchanged.
Component values of a Pi-matching network are computed as an alternative to the tuned tank and usually it has the same Q as the tank. But in some circumstances there are Pi-match Q values which are not possible. This occurs when the tuning capacitor values are seen to differ. However the required Pi impedance match is always correct even if a particular value of Q is not available.
The source or generator dynamic internal resistance is the value seen looking back into the amplifier's 50-ohm power output socket. It is of interest when undesirable back-and-forth reflections/echos occur on long antenna feedlines. Its high value illustrates the non-existence of a 50-ohm "conjugate match".
To reduce the number of input data items the program assumes screen current is always approximately 1/12th of anode current. Actually it depends on the ratio of screen grid wire diameter to wire spacing. A coarse grid intercepts a smaller proportion of electrons. There may be errors in estimating screen dissipation.
To enter an accurate value of the greatest instantaneous +ve value of control grid volts for a particular peak anode current, the tube manufacturer's data curves will be helpful. If this is not checked there may be errors in grid drive estimates.
Cathode current is assumed to follow the law Ik=(Vg1+Vg2/Mu)^1.3 Computed transconductance is an average value over Vg1 voltage range. It is available as a rough check on closeness of the tube model to an actual tube when the transconductance of the actual tube is known.
Computed power output and grid drive data applies to the lower frequency end of the frequency range in which a tube is normally designed to operate. Various factors such as electron transit-time and parasitic reactances degrade performance at the high frequency end of the range. The drive power needed, the power dissipated within the tube and in its envelope increase. The DC-to-RF power-conversion efficiency falls off.
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