Calculate initial load line of valve RF amplifier

This calculator calculates some key values for operation of a valve RF amplifier with resonant load, and points on an intitial load line for further modelling with the Designing the operating point for a grounded grid Triode Class B RF amplifier.

Key assumptions of the calculator are:

linear device transfer characteristic;
360° conduction angle for Class A;
180° conduction angle for Class B; and
120° conduction angle for Class C.

Class AB RF amplifiers usually operate so close to Class B conduction angle that Class B assumptions are quite reasonable for an initial load line estimate, in fact the greatest source of error is the device non-linearity rather than conduction angle assumptions.

The method used by this calculator is described in detail at Designing the operating point for a grounded grid Triode Class B RF amplifier.

Inputs:

Vs (V)		Po (W)
Vgk (V)		NW eff (%)
Vak min (V)		Class
Vk bias (V)		Number of valves
Configuration

Results:
Vak DC (V)		Ia
Vak (V)		Ia RF
Ia RF / I (pu)		Ia RF RMS
Ia DC / I (pu)		Ia DC
Ia RMS / I (pu)		Rl
Anode eff (%)		Rlaa
Eff (%)
Pa RF
Pa diss
Initial Load Line
Vak	Ia	Vak	Ia

The calculator does not do a lot of error checking, if you enter nonsense, it will produce nonsense. NaN means not a number, check the input values.

All values are peak unless annotated otherwise. RF annotation means Radio Frequency component at the fundamental frequency.

Table 1: Input field descriptions
Input field	Meaning
Vs	Anode DC power supply voltage (loaded)
Vgk	Peak grid - cathode voltage (not required for common cathode configuration)
Vak min	Minimum instantaneous anode - cathode voltage
Vk bias	Cathode bias voltage
Po	Required RF power output
NWeff	Output network efficiency (typically ~95%)
Class	Amplifier class (A, B, C)
Configuration	Amplifier configuration (CC, CG), single ended or push-pull

Calculated values are per device except Rl which in push-pull configuration is anode to anode.

ARRL

The ARRL Handbook has a formula with a bunch of K factors for different classes, and they explain as K = a constant that approximates the (fundamental) RMS current to dc [sic] current ratio appropriate for each class. Their factor K captures more than that, even if poorly. Their method implies that Vak min is proportional to DC supply voltage, but it isn't. The ARRL method is not sensitive to whether a PA is common cathode or common grid configuration. The method described in this article and used in the calculator is a more accurate model than the ARRL formula.

Many other design tools are based on the ARRL's method and K factors, or similar.

Changes

Version	Date	Description
1.01	22/05/2011	Initial.
1.02	01/02/2014	Added push-pull option.
1.03
1.04
1.05

Calculate initial load line of valve RF amplifier

ARRL

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Changes