IMPROVING THE GAIN-BANDWIDTH PERFORMANCE OF DISTRIBUTED AMPLIFIERS

SUMMARY

The conventional distributed amplifier (CDA) is a strong alternative for wide-band amplification because of general performances; very good gain flatness, very good input and output impedance matching, a compact structure, a phase response approximately linear with frequency which means nearly frequency-independent group delay. In some cases, this circuit can provide better noise or distortion performance than the other techniques for wide-band amplification. All these features have caused the CDA to be widely used in microwave electronics.

There are many works in the literature to improve the performance of the CDA. Losses of the input and the output artificial transmission lines cause the most undesired effects on the gain-frequency performance. The loss of the input line is rather effective and causes the bandwidth of the CDA to decrease.  The loss of the output line affects the gain.

Some works have been made to reduce the losses of the CDA. Compensation of losses by means of negative resistance circuits (NR) is one of these works. But the negative resistance circuit proposed in the literature is not flexible for design and realization. In this thesis, a modification on the NR is presented to obtain a better performance and a more flexible design. The losses in the CDA occur mostly in the input line (the gate line for FET’s). Therefore, an NR should be placed in the gate line. The input of the NR proposed in the literature is the drain port and this is a disadvantage for the compactness of the CDA. In this thesis, a new NR whose input is the gate port is proposed in order to get rid of the disadvantage. The new NR helps the design and realization of NR compensated CDA’s become easier.

One of the methods to increase the upper frequency limit of distributed amplifiers is to use bandpass filter structured artificial lines in place of lowpass structured ones. The bandpasss structure proposed in the literature has a serious disadvantage; the bandwidth of a DA with this structure is lower than that of a conventional DA using the same active component. In this thesis, we present a DA with a different BP structure to overcome this problem. Methods are given to improve the frequency behaviour of the CDA using the BP structure presented in this thesis. At the same time, the negative resistance circuit given in this thesis is used to improve the performances in the middle frequency region and high frequency region. As a result, a new bandpass filter structured distributed amplifier circuit has been obtained. Design features of the new circuit are given and its performance is investigated in detail.

a) CDA

b) NRBPDA

Figure.1 The printed circuit boards of the CDA and the NRBPDA

 

Figure.2 Gain-frequency performances of the CDA and the NRBPDA

It has been explained above that the CDA is one of the most frequently investigated and realised broadband circuits. However the maximum gain obtainable from the CDA is limited to about 7-14dB. To overcome this disadvantage, in the literature, a novel broadband amplifier called the cascaded single-stage distributed amplifier (CSSDA) has been presented. The CSSDA relies on the same basic concept as the CDA: the artificial transmission line. The gain of a CSSDA is significantly higher than that of the CDA designed for a given number of active devices. Also higher power efficiency can be obtained from the new amplifier. The CSSDA which has high gain advantage, does not have a compact structure. This is the most serious disadvantage of the amplifier. The CDA which has a compact structure, is an advantageous amplifier for the MMIC realization. This advantage helps the CDA achieve a much higher bandwidth than the CSSDA. 

In the literature, no sufficient work on the gain-frequency behaviour of the CSSDA has been made. In this thesis, the basic formulae and characteristics for the gain-frequency behaviour are given. On the other hand, artificial lines of the CSSDA are lowpass structured lines in the traditional design methods. When bias components are taken into account, it will become apparent that the artificial lines are similar to bandpass structured lines. Therefore some part of the lower frequency region in the gain-frequency band is not utilized. In this thesis, employment of the bandpass structured artificial lines in the CSSDA is proposed in order to overcome this drawback. Hence, better gain-frequency performance can be obtained from the CSSDA.

Figure-3 A schematic representation of the bandpass structured CSSDA

 

a) 

b)

Figure-4 Frequency responses of the BP and LP CSSDAs for two different designs

With the works made in this thesis, it is aimed to improve the gain-frequency behaviours of the CDA and the CSSDA. In view of this aim, a new NR is introduced in order to compensate for the losses in the gate line of the CDA and a BP structure is proposed in order to shift the gain-frequency band. Then, the gain-frequency behaviour of the CSSDA is investigated and it is shown that the CSSDA using BP structured artificial lines can supply wider gain-frequency band. The distributed amplifiers are of the most effective circuits for wide-band amplification. It is thought that the advantages of distributed amplifiers are increased by means of the works given in this thesis.

 

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