this paper described basics of MOSFET, impedance matching and biasing of transistor. the steps for designing of amplifier is also given in this paper.

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Design and Analysis of Solid State Power Amplifier
at High Frequency Band
Avani M. Patel
M.E. - Communication System
L. J. Institute of Engineering and Technology
Ahmedabad, India
avani7993@gmail.com
Mr. Nimesh Prabhakar
Assistant Professor, PG Dept.
L. J. Institute of Engineering and Technology
Ahmedabad, India
nimpra28@gmail.com
Prof. Anil K. Sisodia
Professor, PG Dept.
L. J. Institute of Engineering and Technology
Ahmedabad, India
ak_sisodia@yahoo.co.in
Abstract—This paper describes the operation, parameters, merits and demerits of the power amplifier. Steps for design procedure of
the power amplifier have been described. Along with that importance of matching and its methods has been described. Different
approaches for biasing and advantage of each have been elaborated. The effect of thermal runaway has been explained. At last
parameters of power amplifier at different high frequency bands has been analyzed.
Keywords—Solid State Device; Power Amplifier; Biasing; Impedance Matching; Thermal Runway
I. INTRODUCTION (HEADING 1)
An amplifier is an electronic circuit that increases the voltage, current or power of a signal. In recent times solid state
power amplifiers using transistor are commonly used due to its good reliability and ease of operation. Amplification can be done
with a vacuum tube or transistor. In transistors the electrons are passed through the semiconductor material therefore named solid
state device. In the power amplifier, voltage and current level of the signal is increased. The power amplifier may be in single or
multiple stages. A linearity and output power are main specifications of designing the power amplifier. Basically, there are four
types of the power amplifier which are class A, class B, class AB and class C.
II. DESIGN METHODOLOGY OF THE POWER AMPLIFIER
A. Parameters of the power amplifier
• Power gain of a two-port network, amplifier is defined as the ratio of the output power to input power [8].
• Bandwidth is the range of frequencies-the difference between the higher frequency and the lower frequency. BW is
measured in hertz.
BW=f2-f1
• Power delivered to the load is known as the output power, which is a strong function of the input power [8].
• The ratio of the ac output power to the dc output power supplied by the battery of the power amplifier is known as
efficiency [1].
• Distortion in power amplifier is evaluated in terms of harmonic power level [8].

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The basic block diagram of the power amplifier is as shown in Fig. 1.
Fig. 1: Block diagram of solid state power amplifier
Here RF input is given to the input circuit and output is taken from the output circuit as shown in the Fig. 1. The transistor is
active component and amplification process is done by that. Here MOSFET is used as the transistor as shown in the Fig. 1.
Impedance matching is done for two reasons. First one is to transfer maximum power to the load and second reason is to protect
the circuit from reflected power. The biasing is a process in which we apply external dc voltages to select the appropriate
operating point.
B. Design Specifications
A weak signal is amplified by using the transistor same as the vacuum tube. The transistor are small in size, light weight, less
power required, used less material and less expensive compare to the vacuum tube. So in most cases the transistor used instead of
the tube.
.
The FET has less noisy and higher input impedance compare to the BJT. So, the FET is most widely used in the power
amplifier compare to the BJT. In the n-p-n, the majority charge carriers are electrons and electrons have higher mobility than
holes. Thus, the n-p-n is preferred over the p-n-p. There are three way to connect the transistor in to the circuit which are
Common Base connection (CB), Common Emitter connection (CE) and Common Collector connection (CC). The CE connection
has higher voltage, current and power gain. So, CE configuration is widely used.

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C. Classification of the power amplifier
The power amplifier is classified into the four types depending on the position of the operating point on the load line.
1. Class A power amplifier
2. Class B power amplifier
3. Class AB power amplifier
4. Class C power amplifier
D. Impedance matching
The maximum power transfer theorem says that to transfer the maximum amount of power from a source to a load, the load
impedance should match with the source impedance [2]. For maximum power transfer into the transmission line, the line
impedance should be matched with the source and load impedance. If the impedances aren’t matched, maximum power is not
delivered and standing waves are generated along the line. A coaxial cable and twisted pair are the transmission line.
SWR = ZL / Z0 (for ZL > Z0) [2]
SWR = Z0 / ZL (for Z0 > ZL) [2]
Here, Z0 = Characteristic impedance of the transmission line and ZL = the load impedance
Reflection coefficient = (ZL – Z0) / (ZL + Z0) [2]
Return loss (in dB) = 10*log (PIN / PREF) [2]
For perfect matching condition, the SWR = 1, the reflection coefficient = 0 and RL (in dB) = infinite. The S-parameter is the
ratio of the reflected wave to the incident wave. The physical meaning of the S11 = the input reflection coefficient, S22 = the output
reflection coefficient, S12 = Reverse gain and S21 = Forward gain. The matching is done by using L-network, T-network, π-
network or transformer. In the transformer, the losses are high, the circuit is bulky, and the cost is high. But in the L, T and π
network the inductor and capacitor is used. So, the cost is low and the size and weight of the circuit is reduced compare to the
transformer. The power handling capacity is higher in the 30Ω and the loss is minimum at the 77Ω. So, 50Ω is used as the
characteristic impedance of the line because it has low loss and good power handling capacity.
E. Biasing of the transistor
Basically there are three region of the transistor which is as following:
1. Saturation region
2. Cut off region
3. Active region
For amplification process, the transistor must be operated into the active region. In active region, the Emitter-Base (EB)
junction is always forward bias and the Collector-Base (CB) junction is always reverse bias. So for proper amplification process,
the biasing of the transistor is done.
Basically there are three types of the biasing process which are:
1. Fixed Bias
2. Emitter Bias
3. Voltage Divider Bias
In the emitter bias, the stability of Q-point is improved compare to the fixed bias by introducing RE resistor. When the
temperature increased —> ICBO increased —> IC increased —> IERE increased (because IC is approximate to the IE) —> The IB
decreased —> So the IC is also decreased (because IC = β*IB). So due to the RE, IC is not changed when the temperature is
changed. In this biasing, the IC is independent of β due to (β*RE) >> RB condition. So, there is no effect of β and temperature on
the IC.
In the voltage divider bias, the stability of the transistor is very good comparing to other two types of the biasing. In the fixed
bias, the stability factor S = β+1. But the β is very high. So, the fixed bias is highly instable. Dual dc supply is used in the fixed
bias. The stability factor for voltage divider bias and the emitter bias is almost 1.
F. Thermal Runway
When temperature is increased then the ICBO is also increased because ICBO is flow due to the minority charge carrier and this
are depended onto the temperature. The ICBO is double at every 10°C of the temperature. So, the IC is increased due to the ICBO is
increased. The flow of the IC produces heat within the transistor. So, the temperature is again increased and the process is repeated
itself. So, temperature increased —> ICBO increased —> IC increased.

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There is cyclic process. Within few second the IC is high enough to burn the transistor. So to reduce thermal runway heat sinks
are used.
Following are the design steps to make the power amplifier:
Step 1: To select appropriate transistor according specification.
Step 2: To check the stability factor (K>1) of the transistor.
Step 3: To design the input and output matching circuit.
Step 4: To design the input and output side biasing circuit.
Step 5: To simulate the circuit by using appropriate software.
Step 6: To fabricate all components and testing the fabricated circuit.
III. ANALYSIS OF VARIOUS RESEARCH PAPERS AT HIGH FREQUENCY BAND
REPARE YOUR PAPER BEFORE STYLING
In [3], the power amplifier is designed for frequency from 930 to 960MHz. The gain is 15.6dB at the 945MHz frequency and
the gain flatness of this power amplifier is about ±0.1dB in the frequency range of 930-960MHz. The output power is 26.6dBm.
ATF-5019 transistor is used. The stability factor of this transistor is 2.590 at 945MHz frequency as shown in Fig. 3. The ADS
software is used for the simulation purpose. The transistor is biased for class A power amplifier.
Fig. 3: The stability curve of ATF-50189 [3]
In [4], the power amplifier is designed for 0.8 to 4.2GHz frequency range. The gain is 10 ± 1.5dB over the entire operating
frequency range. The output power is 10W. CGH40025F transistor is used. The CGH40025F is used up to 6GHz frequency range
and it gives 13dB gain in the 4GHz frequency. The power added efficiency is 28%. The transistor is biased for class AB power
amplifier.
In [5], the power amplifier is designed for frequency range from 2 to 500MHz. The gain is 22 ± 1.5dB from 2 to 500MHz.
The output power is 5W. They used MRF281Z transistor. The transistor is biased for class AB power amplifier. They used push
pull configuration. The power added efficiency is 43%. The input and output matching is done by using a transformer. The
calculated gain is 21 with less than 1.5dB fluctuation as shown in the Fig. 4.
In [6], the power amplifier is designed for the wide band. This power amplifier is designed for the frequency range from
500MHz to 2500MHz range. The gain is 12dB ± 1dB from 500MHz to 2500MHz. The output power is very flat: 43dBm ± 1dBm
from 500MHz to 2500MHz. The efficiency is 40% at 500MHz and 33% at 2500MHz as shown in Fig. 5. The CGH40025
transistor is used and the stability factor of this transistor is 1.2. The ADS software is used for simulation purpose.

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Figure 4: The measured and simulated power gains from 2 to 500MHz [5]
Fig. 5: The graph of the output power P1dB and efficiency [6]
In [7], the power amplifier is designed and simulated for 380MHz. The bandwidth is 10MHz. The gain of this power
amplifier is 11dB. The efficiency is 20%. The output power is 31dB. The MRF 134 transistor is used. The stability factor of this
transistor is 1.161 at 380MHz frequency as shown in the Fig. 6. The ADC software is used for simulation purpose. The transistor
is biased for class AB power amplifier.
Fig. 6: The stability graph of the MRF 134 in ADS [7]
Fig. 7: Graph of harmonics vs. frequency in the ADS software [7]

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IV. CONCLUSION
Various important parameters of the power amplifier at the high frequency band have been discussed. Effect of these
parameters on the performance has been analyzed. Based on application and specification, the solid state amplifier can be
designed to achieve the desired performance.
References
[1] V. K. Mehta and Rohit Mehta, “ Principles of Electronics ” , S. Chand & Company Ltd., Publication, 2008.
[2] http://electronicdesign.com/communications/back-basics-impedance-matching-part-1
[3] Congjie Wu and Yalin Guan, “Design and Simulation of Driver Stage Power Amplifier”, IEEE conference on Advanced & Technology in Industry
Applications, pp 1265-1267, DOI: 10.1109/WARTIA.2014.6976512,2014.
[4] Basem M.Abdrahman, Hesham N. Ahmed and Mahmoud E. Gouda, “Design of a 10W, highly linear, Ultra wideband power Amplifier Based on GaN
HEMT”, IEEE conference on Engineering & Technology, pp 1-5, DOI: 10.1109/ICEngTechnol.2012.6396134, 2012.
[5] Sungcheol You, Kyunhoon Lim, Jaeyong Ch, Minchul Seo, Kyungwon Kim, Jaewoo Sim, Myungkyu Park and Youngoo Yang, “A 5W Ultra-Broadband
Power Amplifier Using Silicon LDMOSFETs”, IEEE conference on Asia Pacific Microwave, pp 1116-1119, DOI: 10.1109/APMC.2009.5384391, 2009.
[6] Jaewoo Sim, Jaeyeon Lim, Myoungkyu Park, Wenwoo Kang and Bak-IL Mah, “Analysis and Design of wide-band Power Amplifier Using GaN ”, IEEE
conference on Asia Pacific Microwave, pp 2352-2355, DOI: 10.119/APMC.2009.5385457, 2009.
[7] G. Anusha, A. Sai Suneel and M. Durga Rao, “Design and Simulation of Solid State Power Amplifier with ADS for Pulsed Radar Applications”, IJIRCCE,
vol. 3, issue: 6, pp 6276-6281, ISSN: 2320-9801, 2015.
[8] Inder J. Bahl, “Fundamentals of RF and Microwave transistor amplifiers”, A John Wiley & Sons, Inc., Publication, 2009.