Neutralization Problems of High Frequency Power Amplifiers

1. Introduction

For high-power high-frequency power amplifiers with vacuum electron tubes as core amplifier components, in theory, the control effect of the electron tube grid voltage on the cathode is completely achieved by the electric field generated by the grid voltage on the electrons emitted by the cathode. However, in fact, due to the existence of the inter-electrode capacitance of the electron tube, especially the inter-electrode capacitance between the plate and the grid, the grid loop and the plate loop of the electron tube are coupled with each other, causing the adverse effects of direct conduction and reaction of the high-frequency power amplifier circuit, thereby causing the high-frequency power amplifier to work unstably. The degree of influence of the plate-grid capacitance on the high-frequency power amplifier circuit is related to the frequency of use of the amplifier. In long-wave transmitters, due to the low frequency, the influence of the plate-grid capacitance can be ignored; for medium-wave machines, its influence must be considered. For short-wave machines, especially ultra-short-wave transmitters, not only the influence of the inter-electrode capacitance must be considered, but also the influence of the lead inductance. This is mainly because as the operating frequency increases, the capacitive reactance of the inter-electrode capacitance will decrease, while the inductive reactance of the lead inductance will be generated and gradually increase. Therefore, in actual work, effective measures must be taken to adopt the method of neutralizing circuit to eliminate the harm of the inter-electrode capacitance of the electron tube and the equivalent inductance of the line lead to the high-frequency power amplifier, so as to make the high-frequency power amplifier operate safely and stably.

2. Adverse effects of interelectrode capacitance

2.1 Straight-through effect

As shown in Figure 1-1(a), this is a common cathode circuit of an electron tube. In addition to the useful excitation voltage Ug and the plate resonant circuit, there are also parasitic parameters such as the inter-electrode capacitance caused by the component structure, the plate-gate capacitance Cag between the plate and the gate, the plate-cathode capacitance Cak between the plate and the cathode, and the gate-cathode capacitance Cgk between the gate and the cathode.

Cak and Cgk can be combined into input and output circuits respectively, and Cag is connected between the two circuits. In this way, part of the high-frequency current generated by the excitation signal is directly sent to the plate circuit through Cag, causing a voltage drop at both ends of the resonant circuit. The equivalent circuit is shown in Figure 1-1(b). The higher the operating frequency, the greater the impact. This phenomenon is called shoot-through.

The adverse effect caused by the shoot-through effect is that when the plate current of the electron tube is cut off, due to the existence of the shoot-through, a part of the high-frequency current generated by the excitation signal will be directly sent to the plate circuit through Cag, so that the current of the plate circuit cannot be completely cut off. When there is amplitude modulation, 100% amplitude modulation cannot be obtained, causing distortion of the amplitude modulation signal, and also increasing the power consumption of the excitation signal.

2.2 Counteraction

Before discussing the reaction of the inter-electrode capacitance of the electron tube to the high-frequency power amplifier circuit, we first explain the typical and widely used circuit form of the electron tube high-frequency power amplifier. The electron tube high-frequency power amplifier uses the cathode of the electron tube as the high-frequency common point. The signal is sent between the grid and the cathode, and output from the plate and the cathode. It uses the resonant circuit as the load and works in the Class C (high efficiency) state. It has a high power gain, and the plate resonant circuit form mostly uses a parallel resonant circuit. To achieve the maximum power output of the high-frequency power amplifier, it is mainly to complete the tuning of the plate circuit capacitance and inductance to meet the conditions of its parallel resonance. When the circuit resonates, the voltage and current parameters in the circuit have the following characteristics: the grid voltage and the plate voltage are 1800 in opposite phases; the plate current DC component and the gate current DC shunt change in opposite directions, and the minimum plate current DC component and the maximum gate current DC shunt should appear at the same time.

Next we will discuss the reaction of the inter-electrode capacitance of the electron tube on the high-frequency power amplifier circuit. The reaction is that part of the plate current is fed back to the gate circuit of this stage through Cag, causing the input circuit impedance to change and the effect of detuning.

As shown in Figure 1-1 (a), the reaction current is:

The effect of the reaction current on the excitation voltage can be expressed by admittance, that is:

Since the phase difference between the gate loop voltage and the plate loop is zero at resonance, we have:

This means that the reverse effect is that the input admittance becomes capacitive, and the value of the input capacitance is:

It changes the input impedance of the amplifier, and the operation becomes unstable due to the detuning of the previous plate circuit. For the plate current of this stage, the minimum plate current and the maximum grid current do not appear at the same time during resonance due to the detuning of the grid. This is the reaction of the inter-electrode capacitance of the electron tube to the high-frequency power amplifier circuit.

In addition to the inter-electrode capacitance of the electron tube, there will be other stray couplings between the plate and the gate. In addition, the layout of the plate and gate components, and the high and low potentials between each slot will also produce parasitic coupling similar to the inter-electrode capacitance.

3. Methods to eliminate the adverse effects of interelectrode capacitance

The inter-electrode capacitance of the electron tube, the parasitic coupling between the amplifier levels and the plate and grid, will affect the stable operation of the high-frequency power amplifier due to direct conduction and reaction. There are several ways to eliminate this adverse effect:

●Use a neutralization circuit. This means adding another circuit to the original circuit, which has an opposite effect to that of Cag, to offset its influence on the circuit.

● Choose a quadrupole or pentode tube with better isolation effect. Although the Cag is not large, it may still cause instability, so a neutralization circuit must also be added.

● Use the frequency doubling method. Since the resonance frequencies of the grid and plate loops of the frequency doubling device are far apart, the direct pass and reaction will be greatly weakened. However, this method has limited application and is generally only used in exciters.

●Use gate grounding circuit, that is, gate-to-ground circuit.

In this circuit, the plate-grid capacitance is no longer the main coupling element between the plate circuit and the grid circuit. The coupling element is the inter-electrode capacitance between the plate and the cathode. Because of this advantage of the grid-ground circuit, it is widely used. However, even if the plate-cathode capacitance is very small, it is sometimes necessary to add a neutralization circuit for the plate-cathode capacitance when the operating frequency is very high.

4. Adjustment of neutralization circuit

For the safety of high-frequency power amplifier equipment and to eliminate the hazards of tube inter-electrode capacitance and line lead equivalent inductance, especially large and medium-sized high-power amplifier transmitters, during debugging, you must first adjust the neutralization circuit before you can add board voltage to make the high-frequency power amplifier work.

In actual work, due to manufacturing, installation and tube parameter errors, the actual inter-electrode capacitance, neutralization capacitance, lead inductance and other values ​​cannot be a fixed value. Therefore, the neutralization element is generally made adjustable so that it can be adjusted appropriately according to the actual situation.

The general approach is to start by eliminating the direct effect and then eliminate the reverse effect.

Here are a few ways to adjust and neutralize.

4.1 Grid current concave neutralization method

When adjusting, use the amplifier's own grid current meter as an indicator, do not apply plate pressure, turn on the filament, and apply appropriate excitation voltage. Observe the grid current meter and tune the plate circuit. If the neutralization in the circuit is not perfect, the change of the grid current meter is as shown in Figure 3-1.

Principle: When the plate loop is tuned to the grid excitation voltage frequency, the power delivered to the plate loop by the direct effect is the largest, resulting in the voltage of the previous plate loop (the grid loop of this stage) being the smallest, thereby reducing the excitation voltage. Because the grid current is proportional to the grid excitation voltage, when the plate loop is adjusted, there is a grid current depression, and it is the smallest at the resonance point.

Adjustment method: adjust the neutralizing capacitor to slowly increase the grid current. At the same time, the front stage circuit must be kept in a resonant state. Repeat the adjustment until the direct pass is completely eliminated. That is, adjust the plate circuit near the resonance point, and the grid current no longer changes. Why do we need to adjust repeatedly? Because the neutralizing capacitor is introduced, its impedance constitutes a part of the front stage plate circuit. Therefore, when adjusting the neutralizing capacitor, the front stage plate circuit also undergoes detuning changes. Therefore, when adjusting the neutralizing capacitor to eliminate the direct pass effect, other tuning elements of the front stage plate circuit must be adjusted at the same time to keep the front stage in a resonant state, that is, adjust the front stage plate circuit so that the grid current is always at the maximum point.

4.2 Observation of plate-to-electrode tank voltage method

Principle: Theoretically, after the direct pass is offset in the power amplifier circuit, there should be no high-frequency voltage in the plate slot of the neutralization stage power amplifier without plate pressure. Therefore, in practice, this high-frequency voltage can be monitored in a variety of ways, and the neutralization capacitor can be adjusted to minimize the high-frequency voltage monitoring indication. At this time, the value of the neutralization capacitor is the neutralization point, so it is considered that the neutralization has been adjusted.

Method: Connect an oscilloscope or high-frequency voltmeter at both ends of the power amplifier plate slot as a monitoring instrument, add excitation voltage, filament voltage can be added or not, and no plate voltage is added.

First, reduce the neutralizing capacitor to the minimum, tune the plate slot circuit, and make the monitoring have obvious high-frequency voltage indication, then gradually increase the neutralizing capacitor and observe the high-frequency voltage indication until the high-frequency voltage indication is minimum. As in the previous method, when the adjustment of the neutralizing capacitor affects the resonance of the front-stage plate circuit, the front-stage plate circuit tuning element must be repeatedly adjusted to keep it in a resonant state.

4.3 Grid flow reverse check method

After eliminating the direct-through phenomenon according to the above two methods, in order to more carefully check whether the neutralization is perfect, it is necessary to further check whether there is any counter-reaction.

Principle: Theoretically, in a well-neutralized high-frequency power amplifier circuit, the tube plate loop is in a resonant state, the loop voltage reaches the maximum value, while the tube plate voltage is the minimum value, and the grid voltage reaches the maximum value, so the minimum plate current and the maximum grid current should appear at the same time as shown in Figure 3-2. If there is a reaction, the input impedance of the grid will change with the reaction current, causing the working state of the previous stage to change. Therefore, when tuning the plate loop, the minimum plate current and the maximum grid current do not appear at the same time.

Method: Add normal plate voltage, grid voltage and excitation voltage to the power amplifier circuit, rotate the plate loop capacitance near the tuning point, and observe the plate current and grid current at the same time. If the minimum plate current and the maximum grid current do not appear at the same time, the neutralizing capacitor should be readjusted until they both appear at the same time.

5. Conclusion

Due to the existence of the structural inter-electrode capacitance of the electron tube, parasitic coupling will inevitably occur in the high-frequency power amplifier with it as the core amplification element, causing the unstable operation of the amplifier. The introduction of the neutralization circuit can effectively eliminate the adverse effects caused by the inter-electrode capacitance, so that the high-frequency power amplifier can work normally and stably.