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Introduction to Power Amplifier Pulse Modulation
1.1. Switch Modulation
For small-signal amplifier circuits, pulse modulation can be achieved using a switch. A typical circuit is shown in Figure 1, where the input is a continuous wave and the output is a modulated signal. High-speed pulse modulation with a high on/off ratio can be achieved through proper switch and driver circuit design. It's important to note that because the amplifier remains operational even when the switch is off, system losses are relatively high, requiring careful heat dissipation design. Furthermore, if switch modulation is used in the receiver link design, the wideband noise generated by the constantly operating amplifier can affect the receiver's sensitivity.
1.2. Direct Pulse Modulation
For high-power signals, direct pulse modulation circuits are generally used, as shown in Figure 2. The modulated signal controls the power amplifier's operating state through a modulator, converting the continuous wave input RF signal into a modulated pulse signal output. Because this modulation method directly alters the amplifier's operating state, it results in a high output signal on/off ratio and low system heat dissipation requirements. Furthermore, compared to switching modulation, direct modulation can "force-shut down" the amplifier, thus reducing the impact on the receiver system. Direct modulation is further divided into gate modulation and drain modulation depending on the modulation position.
1.2.1. Gate Modulation
Gate modulation refers to modulating the gate voltage (Vg) of the power amplifier die to control the amplifier's operating state, changing the conduction and pinch-off of the amplifier's drain-source current, thereby modulating the output signal. Generally, the amplifier's gate current is small, typically in the mA range, making its drive circuit relatively easy to design and achieving fast rise and fall edges, resulting in a fast overall circuit response time. However, its drawbacks are also significant. When the gate voltage switches to the cutoff region, the drain-source current is pinched off. As the drain voltage increases, the power amplifier's operating state can easily enter the breakdown region, leading to amplifier burnout, posing a significant risk.
1.2.2. Drain Modulation
Drain modulation refers to changing the amplifier's operating state by altering the drain voltage state of the amplifier die, thereby modulating the drain current. Drain modulation is a type of current modulation. During normal amplifier operation, the power supply needs to provide the required operating current, which can range from several amperes to tens of amperes. Therefore, a MOSFET is needed to improve the current drive capability of the pulse modulator. Depending on the type of MOSFET, modulation circuits can be divided into NMOS and PMOS circuits, as shown in Figures 3 and 4, respectively.
The NMOS modulation circuit uses an N-type metal-oxide-semiconductor (MOS) as the switching device. When the gate voltage of the NMOS transistor exceeds a certain value of the source voltage, the MOS transistor conducts, and the power amplifier is in operation. When the gate voltage is equal to or less than the source voltage, the MOS transistor is cut off, and the power amplifier is in a non-operating state. Generally, the drain voltage of the power amplifier is the highest voltage of the system; therefore, the NMOS driver circuit must include a boost circuit, which increases the complexity of the circuit design.
The maximum gate drive voltage of a PMOS modulation circuit is Vd (drain voltage), eliminating the need for an additional boost circuit. This results in a simpler circuit with higher stability and a smaller, more compact driver design. However, compared to NMOS transistors, PMOS transistors have higher internal resistance and slower turn-off time.
Generally, to meet the large instantaneous current required for drain modulation, a storage capacitor is typically incorporated at the MOS transistor's power input. During the amplifier pulse pause, the power supply charges this capacitor, providing the necessary drive current during the pulse operation. This significantly reduces the amplifier's external power requirements, thereby reducing the size of the pulse modulation amplifier and improving its overall efficiency. However, it's important to note that this large capacitor can affect the falling edge of the modulation signal; therefore, a suitable capacitance value must be selected in practical applications.
