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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

DESIGN AND SIMULATION OF MULTIBANDPOWER AMPLIFIER

A V Goutham Kumar1*, Shruthi S R1, K Sreelakshmi1 and Shanthi P1

*Corresponding Author: A V Goutham Kumar,goutham001te@gmail.com

As inferred from the name, the main job of a “Power Amplifier” (also known as ‘large signalamplifier’), is to deliver power to the load and it is mathematically given by the product of thevoltage across the load and current flowing through the load, with the output signal power beinggreater than the input signal power. In other words, a power amplifier amplifies the power of theinput signal which is why these types of amplifier circuits are used in audio amplifier outputstages to drive loudspeakers. For successful mobile phone communication, the job of the poweramplifier is to boost the small digitally modulated input signal to an adequate output power levelbefore it is radiated by the antenna. It is the most power consuming device in the whole transmitterchain, and hence, the performance of complete system, is mainly dependent on the performanceof the power amplifier. The main concerns of power amplifier design are higher linearity, efficiency,bandwidth, lower manufacturing cost, and so on.

Keywords: Power amplifier, Reflection coefficient, S11, S21, Spectrum, Harmonics, 1-dBcompression point

INTRODUCTIONAn efficient linear power amplifier is requiredin all the battery-operated radio systems.Some examples of these battery operateddevices are Wide-band Code-DivisionMultiple-Access (W-CDMA), GSM, portablehandsets etc. Since the bias-point and the loadline of amplifiers are optimized for themaximum value of 1-dB compression point, inorder to achieve high power amplification, aconventional power amplifier has highest

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Int. J. Elec&Electr.Eng&Telecoms. 2014

1 Department of Telecommunication, R V College of Engineering, Bangalore, India.

efficiency at the maximum output power anddrops to a lower value as the output power levelis reduced. The power usage efficiency isdefined as the ratio of average output powerover the supplied dc power.

The power amplifier works on the principleof converting the DC power drawn from thepower supply into an AC voltage signaldelivered to the load. Although the amplificationis high, efficiency of conversion of DC powerinput to the AC power output is usually poor.

Research Paper

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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

An ideal amplifier would give an efficiencyrating of 100% or at least the power “IN” wouldbe equal to the power “OUT”. However, inpracticality, this can never be attained, as someof the power is lost in the form of heat and also,the amplifier itself consumes power during theamplification process. Then the efficiency ofan amplifier is given as:

Efficiency = Pout/Pi

Ideal Amplifier: The characteristics of an IdealAmplifier are:

• The amplifiers gain (A), should remainconstant for varying values of input signaland across all frequencies.

• The amplification process must not addnoise to the output signal.

• The amplifier gain should not be affectedby changes in temperature, thus, givinggood temperature stability.

• The gain of the amplifier must remain stableover long periods of time.

Amplifier Classes: Classification of anamplif ier is made by comparing thecharacteristics of input and output signals, bymeasuring the amount of time, in relation toinput signal, for which current flows in the outputcircuit. For a transistor to operate within its“Active Region” some form of “Base Biasing”is required. This small Base Bias voltageadded to the input signal allows the transistorto reproduce the full input waveform at itsoutput with no loss.

Power amplifiers are classified in analphabetical order according to their circuitconfigurations and mode of operation.Amplifiers are designated by different classesof operation such as class “A”, class “B”, class

“C”, class “AB”, etc. These different AmplifierClasses range from a near linear output withlow efficiency to a non-linear output with a highefficiency. No class of operation is “better” or“worse” than any other class. The type ofoperation is determined by the use of theamplifying circuit.

For the purpose of multiband operation,linearity, being the main concern, we intend touse a Class A amplifier.

Class A Amplifier—has low efficiency ofless than 40% but good signal reproductionand linearity.

Amplifier Design: The proposed amplifier isdesigned and verified using the tool – ADS(Advanced Design System), by Agilent. Thetransistors and components used are availableon the library file of the simulation tool.

The power amplifier is a two-stage amplifiercircuit with the input and output matched to 50ohm. The circuit of the power amplifier is asshown in Figure 1. The specifications for thedesigned amplifier are given in Table 1.

The amplifier circuit uses separate biasingcircuits for the drain-source junction and thegate source junction. The inter-stage couplingfor the amplifier, and the input and outputmatching circuits use only inductors and

Vds 10

Vgs –1

Pout(max) 28 dBm-32 dBm (1 W)

Pin 10 dBm (10 mW)

Frequency of Operation 800 MHz and 1.1 GHz

|S11| >30 dB

S21 20-25 dB

Table 1: Specifications

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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

capacitors which can be replaced byequivalent distributed networks, because theperformance of resistors at such highfrequencies is poor.

RESULTSThe parameters measured to determine theperformance of the amplifier are:

• S11

• S21

Figure 1: Power Amplifier Circuit

• Output power vs Input power.

• Gain vs Input power.

• Output power spectrum.

• Power spectral density.

The results obtained were:

The resultant S-parameters agree with themultiband tag used to define the poweramplifier. The S11 value obtains a low at allresonant frequencies.

Figure 2: S11 and S21

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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

Figure 3: Output Power vs Input Power at 800 MHz

The graph of output power vs input power,at a particular frequency suggests themaximum allowable input power. Themaximum allowable input power is the power

at 1-dB compression point. 1-dBcompression point is the point at which thegain of the amplifier falls 1dB below itsmaximum gain.

Figure 4: Output Power vs Input Power at 1.1 GHz

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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

Figures 3 and 4 show the plots of outputpower vs input power at 800 MHz and 1.1 GHzrespectively.

Figure 5: Gain vs Input Power at 800 MHz

Figures 5 and 6 show the plots of Gain vsInput power at 800 MHz and 1.1 GHzrespectively.

Figure 6: Gain vs Input Power at 1.1 GHz

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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

The 1-dB compression point can beobtained from both the Output power vs Inputpower plot as well as the Gain vs Input power

Figure 7: Power Spectral Density at 1.1 GHz

plot. Gain is given by the difference betweenthe Output power and Input power in dB ordBm.

Figure 8: Stability Factor |k|

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Int. J. Elec&Electr.Eng&Telecoms. 2014 A V Goutham Kumar et al., 2014

The power spectral density plot can be usedto determine the strength of the harmonics. Theharmonics should have a strength below 30dBof the parent signal.

The stability factor |k| should always begreater than one. The plot shows that theamplifier is stable for all frequencies after0.2 GHz.

SUMMARYTable 2 provides a summary of the resultsobtained for the designed power amplifier.

Frequency 800 MHz 1.1 GHz

S11 –36.371 dB –33.789 dB

S21 23.486 dB 22.906 dB

Input Power at 1 dBCompression Point 9.6 dBm 12 dBm

Output Power at 1 dBCompression Point 32.053 dBm 33.912 dBm

Gain at 1dB CompressionPoint 22.453 dB 21.912 dB

Table 2: Summary of Results

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