An RF power amplifier is an electronic device that changes a low-power radio frequency signal to a higher-power signal. An RF transceiver device has both a receiver and a transmitter. RF power amplifiers are typically designed for half-duplex operation, but there are full-duplex modules available, the latter being more expensive due to their greater complexity.
RF power amplifiers are most often used in a selection of different applications, including TV and transmission, radar, and wireless communications. The most often used combinations of these are categorized from A to F, for frequencies from very low (VLF) to microwave. These RF power outputs most often range from only a few milliwatts (mW) to megawatts (MW).
Benefits of Modern RF Devices
Modern RF devices offer many benefits. Primary among these are higher currents, lower voltages, and lower load resistance.
The parameters that can define an RF amplifier include:
DC Voltage Supply
Perhaps the best thing about an RF device is that PA depends on a number of factors. Better yet, when these factors are optimized it depends on the bias points of the RF amplifier. Depending on these, tradeoffs can be made with output power, efficiency, linearity, and others.
As discussed above, RF is based a lot on power classes. These include:
Class A Amplifiers
This type of amplifier is biased, so its current flows constantly. The transistor it uses is small enough that it avoids the transistor cutting it off. For example, the angle of conduction is 360 degrees, which means that it can use the input cycle to operate a full cycle. This ability makes the Class A the most linear of all the different types of amplifiers. That said, no transistor is always going to be perfectly linear. One of the shortcomings for this is that the output signal will never be a perfect copy of the input signal.
Class B Amplifiers
With a Class B amplifier, the transistor’s angle of conduction is about 180 degrees. This will lead the transistor to only conduct about half the time on either a negative or positive half-cycle of an input signal.
Class AB Amplifiers
Lacking quiescence for this kind of transistor, there is no bias point outside 10 to 15 percent of iCmax. In cases such as these, the transistor should be on for more than half a cycle, or less than a input signal's full cycle.
Class C Amplifiers
This is an amplifier that has a conduction angle for a transistor that is considerably less than 180.
Class D Amplifiers
A Class D amplifier has a switching circuit that causes a square voltage waveform and half-sinusoidal current to be generated. Two or more transistors are used in Class D power amplifiers to create a square drain-voltage waveform. The fundamental frequency loads only when the output filter passes.
Class E Amplifiers
This type of single transistor is operated as a switch. The sum of the RF and DC current charge is the culmination of the voltage waveform.
Class F Amplifiers
This type boots the output and efficiency of the signal by employing harmonic resonators in the output network to shape the drain waveforms.
NPR and PA Linearity
PR linearity is also measured by Noise Power Ratio (NPR). The NPR has been a standard since before the days of frequency division multiplexed (FDM) telephone networks. This is only a measurement of quietness in a system. This is especially a multi-channel system that is unused, when there is random activity in others.
Test Setup, Measurement, and Noise Power Rate (NPR)
Without a notch filter, the noise power of the RMS inside the notch is evaluated by a narrowband receiver. When the notch filter is on, and the remaining noise is calculated.
Linearity Measurement and Crest Factor (CF)
Another method of computing an amplifier's linearity is by using a CF measurement. CF is a ratio of peak-to-average power. CF is similar to an NPR measurement since amplifier input is band-limited noise to keep it with a signal about the same and what an amplifier would see in day to day use.
Complementary Cumulative Distribution Function (CCDF)
A CCDF provides insight into the performance of the amplifier. It allows power increases to certain predetermined levels, the CF at these grades considerably, showing a substantial degree of compression.
At each level of power, the CCDF curve indicates the period of time the signal spends over the average level of power. The CCDF curve also indicates probability of the signal power as it will be over the average power level.
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