Existing wireless communication standards cover several frequency bands spread over quite a wide range, e.g. WCDMA, WiFi, GPS, Bluetooth, etc. and next-generation high data rate wireless communication systems such as 4G or software defined radio offer completely new ways to access information and services. To provide higher data speed and bandwidth, RF transceivers in this next-generation communications are expected to offer higher RF performance in both transmitting and receiving circuitry to meet this new quality of service. This tremendous growth in multi-band and multi-standard wireless communication systems has stimulated the development of compact and low cost single device operating at multiple frequency bands as opposed to the more conventional multiple devices, one for each frequency band. An advantage of multiple devices with one for each band is that each can be optimized for their assigned bands while their disadvantages are larger size and higher cost. With larger numbers of multiple devices there is a point when there is not much improvement in performance due to the increase in loss with the increase in switching complexity. It becomes necessary to use wideband/tunable RF front-end circuits to cover these different global standards. The main future components are the active circulator and power amplifiers (PAs). These multi-band and multi-standard circuits used for future software defined radio which requires multimode operation are the ideal application for this topology. They are easily reconfigurable by software which allows improved spectral efficiency and faster deployment of new standards as well as existing wireless systems. This dissertation, firstly, begins with chapter 1, the introduction of existing and future cellular front-end circuit. The active circulators and PAs are reviewed on its key parameters and important future application. Three main sections are included in this dissertation and each section proposes the new active circulator and PAs in its chapters. Section I: Wideband phase inverters and its application on power amplifier In chapter 2, two 180° phase inverters built into the microstrip structure are proposed in the rat-race hybrid. These phase inverters with a 90° line replaces the conventional 270° line in the hybrid, which results in a size reduction and widening of the bandwidth in both phase inverters. In microwave components, a large ground plane has the advantages of acting as an effective reflector in antenna systems and providing excellent heat sinking in high power applications. Experiments show that these proposed hybrids have a good performance from 1.5 to 2.5 GHz (fo = 2 GHz) and from 1.2 to 2.8 GHz (fo = 2 GHz) respectively. The bandwidth is increased from 10 % to 50 % and 80 % respectively, and a 43% size reduction is obtained by comparison with the conventional rat-race hybrid. These hybrids, therefore, are the important passive components used in push-pull power amplifiers and antennas. In chapter 3, a 90° coupler and a 270° transmission line are implemented using the proposed phase inverter in chapter 2 and they are integrated at the input and output of carrier and peaking amplifiers in Doherty power amplifier (DPA) respectively without affecting DPA's size. The two amplifiers, therefore, are excited with 180° phase shift forming the push-pull amplifier, and the amplified signals are combined with the cancellation of the even-mode linearity, resulting in suppression of the second harmonic as well. By selecting proper gate biases for the carrier and peaking amplifiers, the IMD3 generated by the two amplifiers is cancelled. Experiment shows that its power-added efficiency (PAE) using uplink WCDMA signal is greater than 40 % at the adjacent channel leakage ratio (ACLR) around -40 dBc. Compared to the conventional Class AB amplifier, this proposed DPA improves around 7 dB in ACLR under the same output power and PAE. Section II: Tunable active three-way/quasi-circulator In chapter 4, a tunable lumped-element Wilkinson power divider is proposed since Wilkinson power divider is recognized as quasi-circulator with high power handling, high isolation and low noise figure. This tunable quasi-circulator is realized by the electronically tunable microwave impedance transformers controllable by varactor diodes. This quasi-circulator possesses a tunable frequency range over a bandwidth of 53 % different from the conventional Wilkinson power divider. Experimental results show that it can operate over the tunable frequency range from 1.1 to 1.9 GHz with more than 50 dB isolation between the isolated ports. Chapter 5 proposes a new active tunable three-way circulator and both theoretical analyses and experimental verifications are performed in this chapter. The conventional active/passive circulator's structures have a trade-off between the power handling and noise figure, however, this proposed active three-way circulator's structure maintains both performance. It is a three-port non-reciprocal device which allows signal transmission from one port to an adjacent port in one direction only, and does not allow the signal to pass in the other direction. Three two-port networks are connected together to form this circulator. This two-port network is compact, formed by a FET together with a feedback inductor. Both narrowband and tunable design are performed. Experimental results show that the narrowband design achieves the isolation more than 35 dB, the insertion losses of around 2 dB and the return losses better than 10 dB at each port at 1.8 GHz. The tunable design shows its tunable frequency range from 1.15 to 1.85 GHz (bandwidth of 47 %) with the isolation more than 20 dB, the insertion of around 2 dB and return losses better than 10 dB at each port, which is controlled by the varactor diodes added at the input and output of the FET. The output power is linearly proportional to the power operation while isolation is degraded under high power operation. The 15-dB isolation is achieved when the input power is 24 dBm and the noise figure is around 5 dB. This compact structure, therefore, is suitable for use in the transmit/receive (Tx/Rx) front-end with a sufficient isolation, power handling and noise figure. Section III: Distributed amplifier topology on power combining and wideband/tunable active quasi-circulator A high efficiency power combining technique based on distributed amplifier's (DA) topology is proposed in chapter 6 to allow efficient distributed combining of FETs output power without the use of multi-way power combiners. The proposed topology uses a quarter-wave short-circuit stub or open circuit to replace the conventional drain line lossy dummy load. This topology is able to achieve power equalization among the FETs at RF/microwave frequencies. This design method ensures that optimum loadlines are achieved for all FETs and the efficiency obtained is comparable to a conventional single-transistor Class A power amplifier using the same FET type. Different stages of DA have different optimum loadline resistance for power combining. This optimum characteristic resistance of the drain output transmission line presented here obtains the power-matched condition for all FETs. It is demonstrated at 2 GHz with 1-stage, 2-stage, and 3-stage DAs. Experimental results show that the power-added efficiency (PAE) of these three DAs is greater than 35 % at the 1-dB gain compression point (P1dB). The 3-stage DA demonstrates an output power of 27.4 dBm at P1dB with power combining efficiency (PCE) around 90 %. In chapter 7, a wideband equalization technique for use in a quasi-circulator is presented. The technique utilizes both gain and phase equalization, which provides wideband frequency operation. The circulator is realized by configuring three heterojunction bipolar transistors (HBTs) together with an equalizing phase shifter. Both theoretical analyses and experimental validations are performed in this chapter. Experimental results show that the proposed quasi-circulator has insertion losses of around 0 dB, return losses better than 10 dB for all ports and minimum isolation of around 15 dB from 0.8 to 2.2 GHz. In chapter 8, a tunable quasi-circulator is proposed based on wideband equalization technique in chapter 7 and is comprised of three single stage distributed amplifiers (DAs), in which two are for wideband transmission while the remaining one is for wideband signal cancellation. This quasi-circulator has wideband frequency operation and by adjusting the bias current, tunable isolation enhancement for isolation between port 1 and port 3 is achieved. Experimental results show that this tunable quasi-circulator has insertion losses of around 0 dB, return losses better than 10 dB at each port, minimum non-tunable isolation of around 15 dB from 0.8 to 2.2 GHz and tunable isolation of more than 40 dB between isolated ports from 0.8 to 2.2 GHz. Finally, in chapter 9, a conclusion of the research work and a vision for future work is given. The main contribution of the research presented in this dissertation is to reduce the size and simplify the circuit to give multi-band and multi-standard operation used for future software defined radio or replacing the existing complex circuits, therefore, it will be adaptable to the current multi-band standards and foreseeable standards in future.