12-09-2017, 11:20 AM
The Universal Mobile Telecommunications System (UMTS) is the preferred third generation (3G) communication standard for mobile communications and will provide worldwide coverage, convenient software technology and a very high data rate. High data rate, in particular, requires the use of efficient bandwidth modulation schemes, such as quadrature phase shift modulation (QPSK). But modulation schemes such as QPSK, in turn, require a very linear power from the output of the transmitter power amplifier to meet spectral requirements. A linear power amplifier, traditionally, has a very low energy efficiency. Poor energy efficiency directly affects operating costs and causes heat-up problems in base station transmitters. Therefore, the designer of the power amplifier is forced to interchange between linearity and efficiency. As a result of this trade-off, a Class AB power amplifier is most commonly used in QPSK-based systems. AB-class power amplifiers provide acceptable linearity at efficiency values around 45-50%. This commitment is not a satisfactory solution, but it is inevitable when using traditional techniques of power amplifier design.
This thesis details the use of a class F amplifier with carefully selected polarization points and harmonics to overcome this problem. Class F amplifiers are generally considered to be very high efficiency amplifiers (80% or more added power), where high efficiency is obtained by the use of harmonic traps (LC filters or quarter-wavelength transmission lines) which provide adequate open or short terminations) for the generated harmonics. By doing this, a square wave drain voltage and a semi-sinusoidal peak drain current out of phase of 180 ° are produced. Since there is only one drain voltage or one drain current at a given time, the power dissipation is ideally zero resulting in a theoretical efficiency of 100%. These very high efficiency values are often associated with poor linearity. However, linearity can be improved to meet design standards but compromising efficiency. Even after this is done, the efficiencies are generally 10 to 15% greater than a traditional Class AB power amplifier with a similar linearity performance. Therefore, efficiency can be improved without affecting linearity by using Class F power amplifiers.
In order to verify this theory, a Class-AB and Class-F power amplifier are designed using Motorola's high-voltage, side-diffusion metal oxide semiconductor (LDMOS) transistor. The choice of polarization points and the design of harmonic traps are very critical for class F performance and, therefore, were designed after careful consideration. The designs were simulated in the Agilent Advanced Design System (ADS) and the simulated results were compared for three different power levels, ie the maximum power, 3 dB below the peak power and 6 dB below the peak power. At all these power levels it was observed that Class-F and Class-AB power amplifiers have a very similar linearity performance, while Class-F power amplifiers show a 10% performance improvement compared to amplifiers Power Class-AB.
This thesis details the use of a class F amplifier with carefully selected polarization points and harmonics to overcome this problem. Class F amplifiers are generally considered to be very high efficiency amplifiers (80% or more added power), where high efficiency is obtained by the use of harmonic traps (LC filters or quarter-wavelength transmission lines) which provide adequate open or short terminations) for the generated harmonics. By doing this, a square wave drain voltage and a semi-sinusoidal peak drain current out of phase of 180 ° are produced. Since there is only one drain voltage or one drain current at a given time, the power dissipation is ideally zero resulting in a theoretical efficiency of 100%. These very high efficiency values are often associated with poor linearity. However, linearity can be improved to meet design standards but compromising efficiency. Even after this is done, the efficiencies are generally 10 to 15% greater than a traditional Class AB power amplifier with a similar linearity performance. Therefore, efficiency can be improved without affecting linearity by using Class F power amplifiers.
In order to verify this theory, a Class-AB and Class-F power amplifier are designed using Motorola's high-voltage, side-diffusion metal oxide semiconductor (LDMOS) transistor. The choice of polarization points and the design of harmonic traps are very critical for class F performance and, therefore, were designed after careful consideration. The designs were simulated in the Agilent Advanced Design System (ADS) and the simulated results were compared for three different power levels, ie the maximum power, 3 dB below the peak power and 6 dB below the peak power. At all these power levels it was observed that Class-F and Class-AB power amplifiers have a very similar linearity performance, while Class-F power amplifiers show a 10% performance improvement compared to amplifiers Power Class-AB.