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ACCENT JOURNAL OF ECONOMICS ECOLOGY & ENGINEERING Available Online:www.ajeee.co.in vol 01, Issue 04, August 2016, ISSN -2456-1037 (INTERNATIONAL JOURNAL)

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AN ANLYSIS OF HIGH DATA RATE COMMUNICATION SYSTEM

Prof. Sanjay Shrivastava

Ass. Prof., Guru Ramdas Khalsa Institute of Science & Technology Jabalpur, MP, India Email Id:[email protected]

ABSTRACT

The application of Darlington pair amplifier is supported in a condition, where requirement of high gain at a low frequency is applicable. Recently, Darlington pair amplifier has been testified with high gain bandwidth product for current applications. If we need more current gain and more stability in any applications, Darlington pair amplifier has been suggested to be the best options.

The Darlington pair amplifier has been indicated to possess many important properties such as high speed combined with less power consumption and use of less circuitry. In this review paper, authors summarize the techniques used to improve the gain and bandwidth of the Darlington pair amplifier

Key words: Bandwidth enhancement, Broadband Darlington Amplifier, Circuit, HBT, HEMT, Multi-stage amplifier, Slew rate, Transistor.

1. INTRODUCTION

To switch on a transistor, the output of input current available is very low in case of some applications. This may be observed that a single transistor is unable to pass sufficient current essential by the load. The transistor gain is recommended to get enhanced, if it’s not possible to enhance the input current. This can be done by using a Darlington pair amplifier only [1].

Darlington pair amplifier may be defined as the transistor in which two transistors are kept inside a single packet and whole configuration has high gain and high input impedance Techniques to Improve Darlington Pair Amplifier Applications for High Data Rate. The emitter of the first transistor is coupled to the base of second transistor in such a way that second transistor to amplify the signal which is already amplified by the first transistors and collectors are common . Darlington amplifiers are basic building blocks of high speed communication system, imaging and wide band instrumentation. A single transistor in common source configuration is used as a trans-conductance stage in broad band amplifiers. A Darlington cell and micro-strip line is applicable in trans- receiver design. Darlington technology is applicable in high frequency wireless communication system. Darlington pair amplifier has become standard RF building block for RF and microwave applications,

Above observation shows that gain Darlington pair amplifier is just double or we can say that product of gain of two transistors. The voltage required to switch on a Darlington pair amplifier is two times comparied to one tarnsistors, reason is that there are two base emitter junctions. In this case second transistor should be capable to handle high level of current. Darlington pair amplifier has larger face shift at greater frequency compared to one transistor, which will stop the circuit and circuit become unstable if used in negative feedback.

As shown in first figure i.e. of NPN transistor, the collector terminal of both transistors are common and emitter terminal of TR1 becomes the base of TR2.

In the above configuration base current Ib is increased by a factor β, so collector current become β. Ib and the current gain should be unity, which is represented as equation form.

Above equation shows that the overall current gain β is just double, or expressed as the product gain of individual transistor.

This equation also infers that the current gains of two transistors can be of multiplication to each other. In contrast,

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2 Darlington transistor pair can be defined as a single transistor having high β and maximum input resistance, which is created by the combination of two bipolar transistors . In this review article, authors have attempted to summarize the recent techniques used to improve Darlington pair amplifier and their modern applications.

2. USE OF HIGH ELECTRON MOBILITY TRANSFER (HEMT)

It is a type of field effect transistor (FET), which integrate s junction in the middle of two materials with not the same band gaps as the channel in its place of a doped region. A material arrangement used is GaAs with AlGaAs which depend on the applications. Device shows the better performance if it is incorporated with more indium. GaN-HEMT attracted because of high power performance. HEMT transistors are capable to operate at huger frequency compared to other transistors. Applications of HEMT are satellite receiver, low power amplifiers, cell phone etc. Advantages of HEMT are high gain, high speed, and low noise. A broad band Darlington pair amplifier is designed using fifty nm GaN-on –Sic HEMT process. The design with cascade topology is used to improve the characteristics of small signal gain and maximum output power. The calculated gain is 15 dB at power consumption 330 mW and supply voltage 30 vdc. The size of chip is mentioned as 3.78 mm2 .

2.1. Versions of HEMT

Pesodomorphic HEMT (pHEMT)

A small layer of single material is used to design pHEMT. Too much small layer such as the materials lattice structure, just expenses have been suggested to be fabricated in a different type of materials.

By using above techniques the manufacturer of transistors with greater band gap difference than other wise probable, giving them better presentation. A paper has reported that 250 nm AlGaAsd-

InGaAspHEMT technology can be used to improve bandwidth and gain flattening in broad bandwidth feedback Darlington pair amplifier. The gain bandwidth product is 217 GHZ. This amplifier has shown about 3.3 times better than unity gain frequency of process but the possibility of base resistance and base to emitter capacitance acting as a limiting factor of bandwidth of a Darlington pair amplifier [1]. Another work has also suggest that novel Darlington cascode feedback amplifiers is designed using 0.5 μm E-mode GaAspHEMT technique, which may increase the output power and lower down the chip area and up to 60 percent improvement in bandwidth [5]. Next, a finding suggests the use of e- mode pHEMT to disable implementation of FET Darlington. Further, authors indicate that this process can bring down the supply operation and increasing the overall capability. In comparison to InGaAP-based, Darlington pair amplifier e-mode pHEMT Darlington pair amplifier can show twice a better IP-3-Bandwidth product with less voltage operation power.

Metamorphic HEMT (mHEMT)

A buffer layer is used between two materials with different lattice structure.

This is an improvement of pHEMT technology. Buffer layer is typically made up of ALLnAs. This configuration provides high gain and low noise. A medium power amplifier has been reported to be designed with millimeter wave monolithic integrated circuit (MMIC) by using 50nm metamorphic high electron mobility transistor (mHEMT).

In this design a grounded coplanar waveguide (GCPW) technology is used to get high gain and high output power. The gain in case of two stages is 13.4dB and output power is 9.5 dbm. A report shows the design of first stacked FET monolithic microwave integrated circuit (MMIC) power amplifier. These different circuits are fabricated on 50nm gate length metamorphic high electron mobility

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3 transistors (mHEMT). This stacked configuration has shown more gain compared to cascade one.]. A low noise amplifier (LNA) has been designed, which is used for water vapor detection in atmosphere. In this paper, low noise amplifier has been suggested to advance the current state of the art for InGaAsmHEMT technology. The noise figure is 3.5 dB and gain is 24 dB.

3. HETERO-JUNCTION BIPOLAR TRANSISTOR (HBT) TECHNIQUE IN DARLINGTON PAIR AMPLIFIER

HBT has been defined as a kind of bipolar junction transitory (BJT) with the capability of creating hetero-junction and suggested to use different semiconductor materials for emitter and base junction. By using HBT, BJT has been suggested to handle signals with high frequencies. Such transistors find frequent applications in modern ultra-fast circuits, predominantly radio frequencies (RF) systems, high power efficiency based applications like RF power amplifiers in cellular phones. Reports on HEMT-HBT, HBT-HEMT, HEMT-HEMT, HBT-HBT combinations are designed. Among four configurations HEMT-HBT Darlington pair amplifier validates good input/output return loss as the finest gain band width product. HBT-HEMT has large design flexibility. HBT is good for showing high trans-conductance and suitable for high gain design[2]. Microwave circuit is designed to provide outline to fully exploit to full voltage swing capability of SiGs HBT technologies and RF operation created hit carrier damage. A fully calibrated TCAD model are used to create SiGs HBT cascode amplifiers based hot carrier effects in hot carrier damage is being suggested by not showing failure of a SiGe HBT cascade[3]. A Darlington pair amplifier of 0.5-30 GHZ by employing InGaP/GaAs HBT of two μm methodologies is designed to increase the bandwidth of Darlington pair amplifier by using the device size ration and series

inductor. This amplifier has maximum figure of merit (FOM) among various existing HBT Darlington pair amplifier. This ampfier is used in modern broadband communication systems reasoned by showing low DC power requirement, wide spread bandwidth and compressed size of chip[19]. HBT Darlington pair amplifier is designed to decrease voltage control oscillator (VCO) phase noise and enhanced tank voltage swing, which is fabricated on InGaP/GaAs HBT MMIC procedure. This design gets tuning range of 26 percent and phase noise -103.5 to -98.5 dBC/HZ and offset frequencies is 100 KHZ.

4. CONCLUSIONS

In conclusion, using the small size transistor in design, gain bandwidth is increased at higher frequencies, at the cost of degraded linearity of the Darlington amplifier. It may be possible to decrease the power consumption by decreasing the supply voltage if no constraint is put on REFRENCES

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