Linearisation of single transistor amplifiers by operating under a quasi-balanced mode
Student thesis: Master's Thesis
Related Research Unit(s)
Present and future, mobile communication systems are adopting the more spectrally efficient modulation format due to the increasing number of users in a limited frequency spectrum. Modulation formats such as π¼ - DQPSK was chosen for use in Japan's PHs and QPSK was chosen for use in the 3rd generation mobile communication system. Due to the band-limited channel which results in a non-constant envelope, amplification of these types of signals requires the operation of transistors in either class A or class AB. This class of operation is undesirable because it gives lower efficiency and hence shorter operating time for battery operated mobile handsets. An alternative is to drive the transistor harder into its gain compression point, which unfortunately results in spectral re-growth or Intermodulation Distortion (IMD). The spectral re-growth when radiated causes co-channel interference to adjacent channels in the same cell site and co-channel interference to adjacent channels in neighboring cell sites. Spectral re-growth is a type of In-band distortion that can not be removed using practical filters. This type of distortion is generated by the non-linear elements that are characteristic to the transistors. Any means of reducing this spectral re-growth is thus of great interest in maximising network performance and increasing the mobile handset operating time. In the past, many researchers have used different methods to reduce the spectral re-growth, such as feedforward, feedback, inter-stage second harmonic amplification method and predistortion methods, but these methods suffer from problems such as instability, the need of additional high gain amplifiers and high loss. The method using feedback, feedforward and inter-stage second harmonic amplification methods are difficult for use at circuit level due to their complex structure. They are suitable for use in the linearisation of base station power amplifiers where size and efficiency are not as important a requirement. To find a linearisation method that can be applied to mobile handset power amplification, circuit level designs must be used. The predistortion method is a method that can be applied at circuit level, but the loss can be prohibitively high. To solve these deficiencies, a new linearization method of operating a single transistor under a Quasi-Balanced mode in the saturation region or gain compression region, combined with controlled second harmonic terminations to reduce the input nonlinearity of transistors at circuit level is presented in this thesis. By connecting a parallel varactor diode for a GaAs FET and parallel diode for a Silicon BJT respectively, transistors are forced to operate under a Quasi-Balanced mode. Under this mode of operation, the input non-linearity of the transistors is reduced. Second harmonic termination can be combined with a Quasi-Balanced mode operation for GaAs FET to compensate for both the AM-AM and AM-PM distortion, and can be used to confirm the Quasi-Balanced mode in Silicon BJTs. Moreover the method using second harmonic termination is also an individual method which can be used to linearise the power amplifier at circuit level. These methods have the advantages of low loss, inherent stability and is simple to implement. It can be applied to both GaAs FETs and BJTs and is a linearization method that is suitable for circuit miniaturization. An improvement of up to 15dB was achieved for the 3rd order a.c.p. , and since the linearisation is performed at the input there was no reduction in the maximum output power.
- Transistor amplifiers