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 analysed 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.

• 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.
• 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 in a design and can be determined from S-parameters, the matching networks, and the terminations. Unconditional stability of the circuit is the goal of the amplifier designer. Unconditional stability means that with any passive load – the input or output of the device – the circuit should not become unstable; in other words, it should not oscillate.
• 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 optimising source impedance is called as Gopt, and is obtained from the manufacturer’s data sheet.
• CAD-Orientated Design Procedure. The CAD-orientated 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 optimisation.
• Amplifier Specifications are: frequency band: 5.3GHz–5.5GHz; gain: 13dB(min); gain flatness: ±0.1dB(max.); input/output return loss: <-15dB; dc power consumption: 50mW(max); output P1dB point: +5dBm(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. Fig.1 shows the dc analysis results for the above mentioned device. Based on the dc power consumption requirement (50mW), bias points are selected as Vgs=-0.1V and Vds=3V, which provides the drain current of 15mA.
• Bias circuit design. 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 centre 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 do not 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.
• 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 stabilised before we can match input and output impedances and proceed with amplifier design.

There are various stability configurations which could be used to stabilise 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.

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. Fig.2 shows the results after stabilisation.

Input and Output Matching Network Design. After the circuit is stabilised 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 modelling 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 50ohms, were given as 6.8-j7.1ohm and 24.5-j24.56ohm, respectively, in the present amplifier design at the centre frequency of 5.4GHz.

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. Figs.? and ? show the input and output matching networks that were designed using the matching networks synthesis utility available in ADS software.

• Overall Amplifier Performance Optimisation. The only thing remaining now in amplifier design is to connect all the sub-networks together and see the overall amplifier performance and to optimise the overall circuit if needed. Fig.3 shows the complete layout of the designed amplifier and Fig.4 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. Fig.5 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 1dB, 3dB, 5dB, and so on.

Conclusion

Amplifiers are easily designed if a well defined procedure is followed. Designers can save time in fine tuning and optimising the amplifier performance. The following table summarises the desired and simulated results.

Anurag Bhargava is an application engineer at EEsof EDA, part of Agilent Technologies. For more information, visit http://eesof.tm.agilent.com