π - COUPLERS

A class C amplifier draws plate current in the form of very distorted pulses of short duration.
Such an amplifier is always operated into a tuned inductance-capacitance or "tank circuit" which tends to smooth out these pulses by its storage or tank action, into a sine wave of radio-frequency output.
Any waveform distortion of the carrier frequency results in harmonic inteference in higher frequency channels.
Class B and C amplifiers driven hard with short pulses at the peak of the exciting waveform act more as an electronic switch and therefore can convert its DC input to RF output with relativley good efficiency.



For the above graph it is obviously desirable to have a plate circuit with Q>= 10 in order to reduce harmonic radiation.
Q of a tank circuit at parallel resonance = R/Xl or R/Xc
Load impedance for a class B/C amplifier can be shown to be:







F 14MHz Q=10 RF PLATE LOAD RESISTANCE = Eb/2Ib
L or C 1000 1500 2000 2500 3000 3500 4000 4500
C1 pF 112 75 56 45 37 32 28 25
C2 pF 460 355 275 225 175 145 117 85
L uH 1.4 2 2.5 3.1 3.6 4.2 4.7 5.3


Table above is for Ra (Load) of 50 Ω
For 70 Ω termination multiply values of capacitor C2 by 0.72

Wide-spaced Tuning capacitor VOLTAGE RATING
SPACING 0.026" 0.047" 0.065" 0.077" 0.085" 0.177" 0.180"
VOLTAGE RATING 1000 1500 2000 3000 3500 6000 6500

Souce: 1947 ARRL handbook (National Radio Co Advertising)