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:

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.

Vs (V)   Po (W)
Vgk (V)
  NW eff (%)
Vak min (V)   Class
Vk bias (V)      
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.


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.



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

© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.