Interstage Coupling Methods

Author: J.B. Hoag

There are two commonly used methods of coupling between stages in a transmitter. These are of the capacitive and link methods.

Figs. 31 A and 31 B. Direct, capacity-coupling between a driver and an amplifier

Typical circuits are shown in Figs. 31 A, B, C, D, and E. In the first two, the condenser C serves as a coupling device and also as a blocking condenser to keep the d.c. plate voltage of the driver off the grid of the amplifier stage. The amount of voltage exciting the amplifier can be controlled by moving the tap on the driver's tank coil. In Fig. 31 B, the two taps should be equidistant from the center of the coil. Capacitive coupling

is not particularly satisfactory at the higher frequencies. In this case, link coupling is used. The latter reduces the effects of the amplifier tube capacitances on the L/C ratio of the tank circuit of the driver. With link coupling, the various stages can be constructed as separate units and then assembled on a common rack. The link coupling coils consist of a turn or two of wire mounted close to that point of the tank circuit where the r.f. potential is least.

Figs. 31 C and 31 D. Link coupling between a driver and an amplifier

In the single-ended driver, feeding the push-pull amplifier of Fig. 31 D, one link coil is mounted at the bottom of the driver's tank circuit, and the other is at the center of the amplifier's grid tank.

Fig. 31 E. Complete symmetry in coupling

In the double-ended driver of Fig. 31 E, complete symmetry can be attained. The wires between the coils of the link are closely spaced and parallel to each other. A method of changing the coupling between the link and the tank coils is necessary. This can be accomplished by spreading the turns of wire, rotating them, or moving them farther away from the tank coil.