# 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 A B C Vk bias (V) Configuration common cathode common grid single ended push-pull Results: Vak DC (V) Ia Vak (V) Ia RMS Ia RF / I (pu) Ia DC Ia DC / I (pu) Ia RF Ia RMS / I (pu) Rl Anode 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.

 Input field Meaning Vs Anode DC power supply voltage (loaded) Vgk Peak grid - cathode voltage (not requiredfor 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.