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:
Table above is for Ra (Load) of 50 Ω
For 70 Ω termination multiply values of capacitor C2 by 0.72
Souce: 1947 ARRL handbook (National Radio Co Advertising)
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)