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Amplifier
Design Made Simple
By Anurag Bhargava, Application
Engineer EEsof EDA, Agilent Technologies, Inc.
Abstract
The purpose of this paper is to demonstrate a simple procedure
for amplifier design. There are many available references
on amplifier theory and design, but they often leave a gap
between theory and practical considerations that should
be understood by a designer to produce a good amplifier
circuit that compares well with the simulated data so that
minimal or almost no post production tuning is required
for the Amplifier. This paper tries to collect basic theory
of Amplifier design as well as the practical procedure that
needs to be adopted for making design first time success
so that designers could save their time and efforts. This
paper focuses more on CAD aided design procedure to design
amplifiers because CAD software has become a necessity for
a design house to design accurately and shorten time to
market. The design process utilized in this paper makes
use of Agilent's Advanced Design System (ADS) software.
Introduction
The amplifier is an integral part of any communication system.
The purpose of having an amplifier in a system is to boost
the signal to the desired level. It also helps to keep the
signal well above noise so that it can be analyzed easily
and accurately. Choice of amplifier topology is dependent
on the individual system requirements. Amplifiers can be
designed for low-frequency applications, medium- to high-frequency
applications, mm-wave applications, and so on. Depending
upon the system in which they are used, amplifiers are classified
as low-noise amplifiers, medium-power amplifiers, power
amplifiers, and so on. The most common structure that still
finds application in many systems typically is a hybrid
MIC (Microwave Integrated Circuit) amplifier.
The main design concepts for amplifiers apply regardless
of frequency and system, and designers need to understand
them very clearly. Specific frequency ranges in particular
pose their own unique design challenges.
This paper focuses on design of a small-signal C-band hybrid
MIC amplifier. The method described here is equally valid
for other amplifiers operating in other frequency ranges,
with minor design changes.
Amplifier Theory
Before beginning to design an amplifier, the designer must
have a basic understanding of things like amplifier stability
and matching conditions. These are discussed in the following
section. There are many references available on basic amplifier
concept and design. The procedure presented in this paper
is taken from one of them. [1]
Stability Condition
Stability analysis is the first step in any amplifier design.
The stability of an amplifier or its resistance to oscillate
is a very important consideration during design and can
be determined from S-parameters, the matching networks,
and the terminations. In a two-port network, oscillations
are possible when either the input or output port presents
a negative resistance.[1]
This occurs when
which for a unilateral device occurs when
Unconditional stability of the circuit is the goal of
the amplifier designer. Unconditional stability means that
with any passive load presented at the input or output of
the device-the circuit should not become unstable; in other
words, it should not oscillate. In general, for a linear,
two-port device characterized by S-parameters, the two necessary
and sufficient conditions to guarantee unconditional stability
are a) K >1 and b) | D | < 1, where
Matching Conditions
The amplifier could be matched for a variety of conditions
such as low noise applications, unilateral case and bilateral
case. The formulae for each condition follows.[1]
Optimum Noise Match:
The matching for lowest possible noise figure over a band
of frequencies require that particular source impedance
be presented to the input of the transistor. The noise optimizing
source impedance is called as Gopt, and is obtained from
the manufacturer's data sheet. The corresponding load impedance
is obtained from the cascade load impedance formula.
The common source configuration is normally chosen for
the highest gain per stage. If the stability factor K>1,
the network gives MAG. If K<1, the network could cause
oscillations. In other words, Gmax is infinite and given
as
This should be avoided by locating the region of instability
in the GS and GL planes.
CAD-Oriented Design Procedure
The CAD-oriented design procedure consists of the following
steps, which are described individually:
• DC Analysis
• Bias circuit design
• Stability analysis
• Input and Output matching network design
• Overall Amplifier performance optimization
Amplifier Specifications
• Frequency Band: 5.3 GHz - 5.5 GHz
• Gain: 13 dB (min)
• Gain Flatness: +/- 0.1 dB (max.)
• Input/Output Return Loss: < -15 dB
• DC Power Consumption: 50 mW (max.)
• Output P1dB point: +5 dBm (min.)
DC Analysis
Based on the frequency range and the gain requirement, the
CFY67-08 HEMT device was selected for the present amplifier
design. The first analysis that needs to be performed is
the DC simulation to find out the right bias points for
the amplifier. Figure 1 shows the DC analysis
results for the above mentioned device. Based on the DC
power consumption requirement (50 mW), bias points are selected
as Vgs=-0.1V and Vds=3V, which provides the drain current
of 15 mA.
Amplifier bias circuit design is dependent on the frequency
range requirements of the amplifier. For example, if the
amplifier will be used for low-frequency applications, then
a choke (inductor) is used. Getting discrete inductors at
microwave frequencies is difficult, however, so a high-impedance,
quarter-wavelength line (l/4) at center frequency is the
best possible choice which when designing a bias network.
Be aware, however, that often this l/4 is followed by a
resistor or a bypass capacitor, adding extra length to the
l/4 line. Designers sometimes don't account for this additional
length, which can cause some of the desired RF frequency
power to be dissipated in this branch, affecting the gain
and frequency response of the amplifier. The calculated
l/4 line needs to adjusted by taking these extra elements
into account.
One probable and commonly used method is to place a radial
stub immediately after l/4 high impedance bias line. This
helps to achieve proper isolation at desired RF frequency,
no matter what component is added after l/4 long bias line.
Figure 2 shows the circuit design for
the bias circuit where it could be seen that high impedance
l/4 bias line is immediately followed by a Radial stub and
then by a resistor and capacitor to ground. Sub-circuits
X2 and X3 were created for input and bias networks respectively
as shown in Figure 3 for further simulations.
Stability Analysis
Stability analysis is a very important aspect of any active
circuit design and it is equally important in amplifier
design, too. Most of the broadband amplifier devices are
unstable and need to be stabilized before we can match input
and output impedances and proceed with amplifier design.
There are various stability configurations which could be
used to stabilize the circuit, the most popular being using
resistive loading of the circuit. The choice is made depending
upon the region of stability and type of amplifier being
designed. Figure 3 shows one of the techniques
to stabilize the circuit.
One output resistor was used at the output side of the amplifier
and then the value of that resistor was tuned to achieve
the proper stability. Figure 4 shows the
results after stabilization.
Input and Output Matching Network Design
After the circuit is stabilized in the broadband range we
can start the design of the input and output matching networks
to achieve the desired specification of the amplifier. Designers
must use proper layout footprint modeling of the lumped
components in schematic simulation to account for the discontinuities
which the signal will undergo in the practical circuit.
This should accompany each lumped components, and is quite
important while designing amplifiers in the microwave range.
Choosing the matching network's topology mainly depends
on the bandwidth of the amplifier. The designer chooses
between single-stub and double-stub matching networks. Simulated
input and output impedances, which need to be matched with
50 ohms, were given as 6.8-j7.1 ohm and 24.5-j24.56 ohm,
respectively, in the present amplifier design at the center
frequency of 5.4 GHz.
A double-stub approach was used to design the input and
output matching networks to achieve the desired input and
output return losses for the present amplifier design. Figures
5 and 6 show the input and output
matching networks that were designed using the matching
networks synthesis utility available in ADS software.
Overall Amplifier Performance Optimization
The only thing remaining now in amplifier design is to connect
all the sub-networks together and see the overall amplifier
performance and to optimize the overall circuit if needed.
Figure 7 shows the complete layout of the designed
amplifier and Figure 8 shows the amplifier
linear simulation results after performing the S-parameter
simulation in ADS. These were obtained after minimal manual
tuning of the matching stub lengths to achieve the desired
results after connecting all the blocks together.
Figure 9 shows the output and input 1-dB compression
point after performing the XdB simulation in ADS software.
XdB simulation is a variant of Harmonic Balance simulation
in ADS. It helps designers perform various compression point
simulations in a single step to find out the output power
at various compression points, such as 1 dB, 3 dB, 5 dB,
and so on.
Conclusion
This article shows that amplifiers are easily designed if
a well defined procedure is followed. Designers can save
time in fine tuning and optimizing the amplifier performance.
Table 1 summarizes the desired and simulated results. Future
work includes statistical analysis and electromagnetic simulation
of this amplifier to ensure that the amplifier works in
all the real world conditions and tolerances.
References
1. Microwave Transistor Amplifiers: Analysis and Design,
Gonzalez, Guillermo, Prentice Hall, 1984.
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