Driver Transformers

Requirements for class B modulator driver transformers are unusually difficult to satisfy. The transformer load is non-linear, for grid current is far from sinusoidal. Although the average load is low, the driver tube must deliver instantaneous current peaks; otherwise distortion will appear in the modulator audio output and therefore in the r-f envelope. The grid current peaks contain harmonic currents of higher order, and to insure their appearance in the modulator grid current an extension of the driver transformer frequency range at both ends is required: on the high-frequency end because of the decreased leakage inductance necessary to allow the higher currents to flow, and on the low-frequency end to prevent transformer magnetizing current, itself non-linear, from loading the driver tube so that it does not deliver the peak grid power. If the driver tube is a pentode or beam tube, it is usually loaded with resistance to minimize current variations. Driver transformers are usually step-down because the grid potentials are relatively low.

These conditions require transformers of exceptionally large size. For low (1 to 2 per cent) overall harmonic distortion, driver transformer design becomes impractical, and it is advantageous to dispense with driver transformers entirely. This is accomplished by the cathode follower circuit (Fig. 150), which for a push-pull amplifier takes the form of a symmetrical pi-filter.

Fig. 150. Cathode follower driver circuit.

The two input chokes connect the driver tube cathodes to ground and carry their plate current. Coupling capacitors connect these chokes to the modulator tube grid chokes, which carry modulator grid current. Sizes of chokes and coupling capacitors are chosen to give approximately constant impedance from the lowest modulation frequency up to the higher harmonics of the highest frequency, and choke capacitance is reduced to preclude pronounced resonance effects throughout the frequency range.

Fig. 151. Rear view of exciter cubicle for 50-kw broadcast transmitter.

In Fig. 151, the filter components are mounted in the exciter cubicle; a transformer for this purpose would be too large to locate internally.

The cathode follower circuit is advantageous in another way. Leakage inductance in a driver transformer causes high-frequency phase shift between driver and grid voltage, which does not exist in the coupling capacitor scheme. Since inverse feedback is often applied to audio amplifiers to reduce distortion, the absence of phase shift is a great advantage. The low frequency at which phase shift appears must be kept below the audio band, but this can be done without excessively large components.