Radio frequency power amplifiers (RFPAs) are an essential component of various wireless transmitters. In the front-end circuitry of a transmitter, the RF signal generated by the modulation oscillator circuit has very low power. It needs to pass through a series of amplification and buffer stages, intermediate amplification stages, and a final power amplification stage to obtain sufficient RF power before it can be fed to the antenna for radiation. To obtain sufficiently high RF output power, an RF power amplifier must be used.
Types of RF amplifiers
Radio frequency amplifiers can be classified into high-gain amplifiers, low-noise amplifiers, and medium-to-high power amplifiers. The core of the amplifier circuit is the microwave transistor.
Radio frequency (RF) power amplifiers operate at very high frequencies but have relatively narrow bandwidths. RF power amplifiers generally use frequency-selective networks as the load circuit. RF power amplifiers can be classified into three operating states—A, B, and C—based on their current conduction angle.
Class A amplifiers have a conduction angle of 360°, making them suitable for small-signal, low-power amplification. Class B amplifiers have a conduction angle of 180°, while Class C amplifiers have a conduction angle of less than 180°.
Both Class B and Class C are suitable for high-power operation, with Class C having the highest output power and efficiency among the three operating modes.
Most radio frequency power amplifiers operate in Class C, but the current waveform distortion of Class C amplifiers is too great, so they can only be used for resonant power amplification with a tuned circuit as the load. Because the tuned circuit has filtering capabilities, the circuit current and voltage are still close to a sine wave, and the distortion is very small.
RF amplifier structure
The entire link consists of three main parts: the input matching circuit, the output matching circuit, and the bias circuit. For the matching circuit, we can use auxiliary tools, such as ADS, to roughly match a certain frequency band, which is usually a narrow band. Then, by making appropriate fine adjustments, we can achieve relatively good performance.
The image above shows AVAGO's MGA30889 series product. The matching circuit is already perfectly matched inside the chip. We only need to add appropriate DC blocking capacitors, as shown in the figure. C7 and C8, L1 and C8 constitute the DC bias circuit, and C1, C2 and C3 are power supply filter capacitors.
DC blocking capacitors are typically needed in amplifiers, and their size affects the cutoff frequency of the operating band. Simply put, due to the skin effect, capacitors exhibit a certain high-frequency effect at high frequencies. Here, the capacitor is not just a simple capacitor; it is equivalent to a high-pass filter. DC blocking capacitors are usually selected as 100pF, 1000pF, or 0.01uF. The smaller the capacitor, the higher the cutoff frequency and the greater the high-frequency loss. Conversely, the larger the capacitor, the lower the cutoff frequency and the smaller the high-frequency loss.
Looking at the bias section, the larger the inductance L, the lower the cutoff frequency, but the poorer the high-frequency characteristics and the more likely harmonics will appear. The smaller the inductance, the higher the cutoff frequency and the better the high-frequency characteristics.
If the inductor here is not for matching purposes, it is usually above 100nH. The inductor capacitance should be greater than the supply current. If the supply current is large, an inductor with a larger package must be selected.
If a high degree of gain flatness is required, a tapered inductor can be added in combination with a high-frequency capacitor. This method of BIAS-TEE usually meets the requirements.
