The radio frequency signal amplifier (RF PA) is an important part of the transmission system, and its importance is obvious. What is a radio frequency signal amplifier? The following gives us a brief introduction to the concept of RF signal amplifiers and the classification of RF amplifiers.
1.What is an RF signal amplifier?
In the front-end circuit of the transmitter, the radio frequency signal power generated by the modulation oscillator circuit is very small, and it needs to obtain sufficient radio frequency power through a series of amplifiers (buffer stage, central amplifier stage, and final power amplifier stage) before it can be fed. radiate from the antenna. In order to obtain sufficiently large RF output power, it is necessary to use an RF power amplifier. After the modulator generates a radio frequency signal, the radio frequency modulated signal is amplified by the RF PA to meet the power requirement, passed through the matching network, and then emitted by the antenna.
2. RF signal amplifier structure
The entire link of the RF signal amplifier consists of three parts: input matching circuit, output matching circuit and bias circuit.
Regarding the input matching circuit, we can use auxiliary tools, such as ADS, to roughly match it to a certain frequency band. This frequency band is usually a narrow frequency, and then make appropriate fine-tuning to achieve a relatively good target.
DC blocking capacitors are generally required in amplifiers, and their size affects the cutoff frequency of the operating frequency band. Simply put, due to the skin effect, capacitors will exhibit certain high-frequency effects under high-frequency conditions. The capacitor here is not just a simple capacitor, it is equivalent to a high-pass filter. The DC blocking capacitor is usually 100pF, 1000pF or 0.01uF. The smaller the capacitor, the higher the cut-off frequency, and the greater the high-frequency loss. On the contrary, the capacitor capacity The larger the value, the lower the cutoff frequency and the smaller the high-frequency loss.
Looking at the bias part again, the larger the inductance L, the lower the cut-off frequency, but the high-frequency characteristics are poor and harmonics may appear. The smaller the inductance; the higher the cut-off frequency, the better the high-frequency characteristics. Generally, if the inductor here is not used for matching, it is usually above 100nH. The inductor capacity should be larger than the power supply current here. If the power supply current is large, it is necessary to choose an inductor with a larger package.
If the requirements for gain flatness are high, you can give priority to the method of adding a tapered inductor and deploying high-frequency capacitors. BIAS-TEE made in this way can generally meet the requirements.
3. Types of RF signal amplifiers
RF amplifiers can be divided into high-gain amplifiers, low-noise amplifiers, and medium-to-high power amplifiers. At the heart of the amplifier circuit is the microwave transistor.
The operating frequency of RF power amplifiers is very high, but the frequency band is relatively narrow. RF power amplifiers generally use frequency-selective networks as the load loop. RF power amplifiers can be divided into three operating conditions: A (A), B (B), and C (C) according to different current conduction angles.
1. The current conduction angle of Class A amplifier is 360°, which is suitable for small signal and low power amplification;
2. The conduction angle of the Class B amplifier current is equal to 180°;
3. The conduction angle of the Class C amplifier current is less than 180°;
Both Class B and Class C are suitable for high-power working conditions, and the output power and power of Class C working conditions are the highest among the three working conditions.
Most RF power amplifiers operate in Class C, but the current waveform distortion of Class C amplifiers is too large, so they can only be used to use a tuned loop as a load resonant power amplification. Because the tuned loop has filtering ability, the loop current and voltage are still close to sinusoidal waveforms, and the distortion is very small.
The above is the concept, structure and classification of radio frequency signal amplifiers. Radio frequency amplifiers play a vital role in the wireless signal transmission system. Only by understanding the working principles can they be used better. Keywords: remote control RTU