**6. Distributed amplifier**

196 Ultra Wideband – Current Status and Future Trends

circuit and antenna can be built on a single substrate [15].

functions by integrating with antenna such as transponders [16].

• Bandwidth increment and impedance matching improvement

• Development of using active antenna arrays in mobile communications

parameters are reviewed in the next part as the active part of active antenna.

location [15].

are as below [17];

generating

rates in future

impedance [8].

antenna dimensions

incident signal will reflect back toward the transmitter without prior knowledge of its

Transponders are circuits which can be activated by an external explorer system transmitting signal in predefined frequency. In this case, transponder will transmit a response signal to the interrogator. These small low-cost microwave transponders are used for noncontact identifications such as entry systems, toll collection, and inventory control.

Transceivers and millimeter-wave vehicle radars are some other applications which are used respectively for wireless local area networks (WLANs) and for intelligent cruise control.

Additionally AIA can be an ideal choose for designing compact transceivers and transponders for wireless applications. In this case the whole RF subsystem, including active

Active antennas are categorized depending on the function of active circuit integrated with Them. The main functions of the devices in active antenna structures are generating and amplifying RF signals and frequency conversion. Based on previous discussion, the active antenna functions can be categorized into three types comprised of oscillator type, amplifier type and frequency type. This base unites can prepare possibility of more complicated

Some other benefits of using active antenna in microwave and millimeter wave frequencies

• Improvement in sensitivity of receiver antennas and reduction of return loss and

• Possibility of using active antenna arrays in microwave and millimeter wave signal

• Possibility of using active circuits in large antenna arrays which can eliminate the need to complicated RF circuits for phase shifters and advanced control electronics

• Advancing beam steering techniques and their applications in smart antenna concept • Possibility of using them to resolve channel capacity limitations by increasing data

As discussed in the previous part, for UWB applications, there are some limitations in using spiral antenna for broadband applications and it must be optimized to exhibit desirable characteristics. Using active antenna technology is one of effective optimization methods which leads to reduction of return losses and improvement in bandwidth, gain and input

To design a UWB active antenna with desirable parameters, study and design of an appropriate active circuit is an important step. Therfore distributed amplifier structure and Power amplifiers are essential parts of each transmitting system. They are used to amplifying signals to transmit from one point to another. Nonlinear effects of high power signals are the main differences between power amplifiers and the others.

In power amplifier design depending on application, higher efficiency, power and gain can be the main subjects of optimizations and linearity and noise figure are less considered.

Power amplifiers are categorized to different classes depending on active element bias and input/output signal forms.

Travelling wave structures are new methods to design of wideband amplifiers which have vast applications in wideband communications such as wideband travelling wave amplifiers, matrix amplifiers, travelling wave oscillators, mixers and power amplifiers . Concept and basic of travelling wave structures was initially originated by Percival in 1937. In 1940, this method was used to design of wideband vacuum tube amplifiers. But using GaAs MESFET in distributed amplifiers was studied at first by Moser in 1967 and Jutzi in 1969. They designed a distributed amplifier using lumped element technology and showed the ability of these circuits to achieve high gains in a wide frequency band [18].

## **6.1. Features**

Because of the ultra-wide operating band of these amplifiers, they are receiving much attention. In a general amplifier, using parallel transistors lead to increasing the gain which is caused by summation of trans-conductances. But increasing input and output capacitors cause decrease in cutoff frequency. So as is shown in figure (5a), it does not solve the problem, because the multiplication of gain and bandwidth almost remains constant. In a distributed amplifier, low or high cutoff frequencies will be modified by summation of transistors trans-condoctances and realization of additional LC transmission lines in the input/output sides. The result is illustrated in figure (5b) [18].
