Staying neutral

At the heart of radio transmission is the oscillator. Without an oscillator to generate a basic RF signal, which can be multiplied as many times as required to reach the desired operating frequency, transmitters would make good boat anchors. The use of controlled oscillation in an electrical environment makes the system function. Unfortunately, because the average tube or transistor stage has so many random potential inductive (L) and capacitive (C) circuits associated with it, it is often easy for undesired and usually unwanted oscillations to be generated in an amplifier stage.



Figure 1. For inductive neutralization, coil L1 provides a path to the grid. Capacitor C1 blocks the high voltage. The inductive reactance of L2 is adjusted to cancel the capacitive reactance of the grid and plate elements.

Sometimes these random L/C circuits will resonate and develop unwanted RF voltages. Depending on the physical characteristics of the tube, RF can be developed between various tube elements, often depending on the relative voltages on these elements. An often unanticipated problem can be caused by coupling between the plate and grid elements.

In the early days of radio when triode tubes were the mainstay of radio receivers, the simplest and most popular type of receiver was the regenerative. It used an early version of feedback that controlled external coupling between the plate and grid circuits. By adjusting the amount of feedback — positive in this case — the sensitivity and gain of the stage could be controlled. It was controlled until the positive feedback became so high that the stage broke into oscillation. Many years ago, it was common to hear whistles and squeals on a station because another ham-fisted listener turned his regeneration controls up too far and into oscillation. In the 1920s, receiver feedback control was usually accomplished by varying the coupling by means of a knob that physically changed the spacing between the grid and plate coils.

Tubes lend themselves more easily to the demonstration and discussion of power-stage neutralization. Despite increasing use of solid-state devices in transmitters, there are still many transmitters using power tubes in their final amplifiers. Some transmitter handbooks contain information on neutralizing power stages in transmitters. Simply replacing a failed final tube with a new tube may not return a transmitter to full normal operation, and it is essential to check neutralization when replacing a PA tube.

Although it seems reasonable to assume that all tubes of a given type will have the same internal inter-electrode capacities, this is not necessarily so. New models and different manufacturers usually show some differences. Many chief engineers have simply pulled out the bad tube and popped in a new one without checking that the stage is still properly neutralized. I have seen transmitters in which tube types have been changed (with similar base connections) without checking neutralization. Then they wonder why the signal doesn't sound quite right.

The fact that a stage may not be neutralized, is unstable and is generating a spurious frequency may not be noticed by a careless operator. This oscillation does not usually occur at the station's licensed frequency (the tuned frequency of the output circuit), but at a slightly lower frequency. This may or may not be passed by the antenna system.



Figure 2. With capactivie neutralization, the tank coil L1 is center-tapped and one end provides a 180-degree voltage differential. Capacitor C2 not only blocks the high voltage, but also provides a broad range of adjustment to cancel any internal tube coupling.

The power tube is a high-impedance input device, while the transistor has a low input impedance and generally is less susceptible to spurious signal generation. Undesired feedback in a stage is usually due to the presence of unintended magnetic coupling, produced by poor equipment design, or feedback/coupling between the plate and the grid in the final amplifier. The former effect can be corrected by careful stage redesign. Oscillation because of a tube's internal capacitance coupling requires external inductive or capacitive neutralization.

Neutralization requires the introduction of an out-of-phase voltage of equal amplitude to the grid of the tube. Figure 1 shows a method of inductive neutralization. There is no control of feedback voltage; it's basically the circuit used in the old regenerative receivers. It is more difficult to vary the feedback voltage in this system of neutralizing. For this reason this method is not often used.

Capacitive neutralization is shown in Figure 2. Capacitor C2 controls the feedback from the plate to the grid circuit. This circuit is generally easier to adjust than inductive neutralization and is convenient to use if a tube type is changed. Basically, a center-tapped PA tank coil is used with B+ being fed to the center tap, with one end to the tube plate and the other through the neutralizing capacitor to the grid.

There are several different methods of obtaining the necessary out-of-phase signal.

Screen grid tubes

As tube and stage amplification figures increased, the screen grid tube was developed in an effort to avoid the need for neutralizing circuits. An additional grid with a positive voltage lower than that of the plate is interposed between the control grid and the plate. This is known as the screen grid. This is connected to ground directly or through a large capacitor. This low-reactance grid effectively shields the control grid from the plate and reduces the capacity between these two elements. This system operates efficiently in lower gain stages, but when stage gain is increased, it is necessary to add a neutralizing circuit.

The high-efficiency of a high-gain screen grid stage requires that no RF coupling occurs between the plate and the control grid. The circuit is similar to that used for a triode and varies depending on the level of feedback required.

In solid-state amplifiers, a low-frequency parasite occasionally occurs. This sometimes appears as noise adjacent to the operating frequency. This is not as common these days, however it is good to know about it in case some unusual phenomena should appear in your signal.


E-mail Battison at batcom@bright.net.


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