Microwave Amplifier Design and Smith Chart Utility for Matching Networks - Chapter 9

Microwave Amplifier Design and Smith Chart Utility for Matching Networks

PathWave Advanced Design System (ADS)

Background

The purpose of this lab is to provide designers with a simple method of developing and designing amplifiers. There are many textbooks available on amplifier theory and design, but they often leave a gap between theory and practical considerations necessary to produce good amplifiers. A good circuit will compare well with simulated data so that minimal post-production fine-tuning is required. This lab combines basic amplifier design theory with practical procedures that are needed to make the design right the first time, minimizing time and effort.

Amplifiers are 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. They also help in keeping the signal well above any present noise so that it can be analyzed easily and accurately. The choice of amplifier topology is dependent upon the individual system requirements and can be designed for low frequency applications, medium to high frequency applications, mm-wave applications, and many more.

Amplifiers can also adopt many design topologies and can be used at different stages of the system. They are classified as Low Noise Amplifiers, Medium Power Amplifiers, and Power Amplifiers. The most common structure tends to be a Hybrid MIC amplifier. The main design concepts for amplifiers, regardless of frequency and system, remain the same and need to be understood very clearly by designers. Specific frequency ranges pose their own unique design challenges and must be handled appropriately. This paper focuses on the design of small signal C- band Hybrid MIC Amplifiers.

DC - FET Curve Tracer Template

This step uses a built-in DC simulation template that sweeps drain-source voltage at different steps of gate- source voltage (also called a nested sweep). The resulting IDS or bias characteristics are plotted automatically using a built-in data display template. This FET will be used for the amplifier.

1. Create a new schematic named FET_curves. Click Show Advanced. Under Insert Template, select ads_templates:FET_curve-tracer.

2. In the new schematic, select the DemoKit_Non_Linear palette and insert the DEMO FET 1 and the DEMO KIT TECH INCLUDE component, as shown in the bottom of Figure 2.

3. The FET text may overlap other text, components, or wires. Move the component text using the F5 keyboard key or the move handle (blue diamond near component text when component is selected). This keeps schematics easy to read.

4. Save the schematic and run the simulation. If a window pops up and warns that the nodes are not named, select Run Anyway.

Compare the status information to the Parameter Sweep and DC Simulation blocks in the schematic to understand how a nested sweep is done in ADS. In this case, the Parameter Sweep is the outer sweep, and the DC Sweep is the inner sweep. Notice that the variables must first be declared in the VAR block before they are used. The initial value is not used for the sweep, as they are automatically overridden with the swept values. The setup for these sweeps is shown back in Figure 2, at the bottom of the schematic view.

In the Data Display plot, the marker value should appear at VDS = 3 V and VGS = -0.5 V with IDS = 75 mA. Select the marker with your cursor and then use the marker arrow icons to move to different values.

Running Optimizations

Set Up an Optimization Controller and Goals

In the remainder of this lab, you will perform an optimization for matching a FET amplifier to 50 Ω. This lab will cover the basics of using the optimizer and goals.

1. Open the schematic view for a2_match_opt. This is the same stable amplifier from the previous section with an added output match circuit formed by L3 and C3. The input match is formed by C1 and L2.

2. In this schematic, the optimization controller and the goals have been set up to save time. These components are located on the Optim/Stat/DOE palette.

3. Open the OptimGoal1 setup window by double clicking on it. Notice that in the Expression field, the value dB(S11) means that when the simulation is performed, S(1,1) will be calculated in dB. This quantity will be used as our goal for the optimization. The analysis is set to SP1, which is the name of the S Parameter block. Frequency will be the swept variable, which means that the goals will be analyzed over the range of frequencies.

In the limit lines, the operator type has been set to < (less than). This tells the optimizer that this goal must be below -15 dB (max value) for a successful optimization. The minimum and maximum frequency are set to 6.5 GHz and 7.5 GHz, which establishes a 1 GHz frequency sweep centered around 7.0 GHz. Note that in the frequency fields, G is used in place of e9, as the default unit is Hertz.

Open the Optimization controller, optim1. The optimization type is currently set to gradient. For this example, a gradient search is more efficient. At the bottom of the screen, the maximum number of iterations is set to 50. If the optimization does not meet the goals within the maximum number of iterations, the optimization will stop.

NOTE: The ‘Save’ parameters on the Optim controller that are set to ‘no’, means that those values will not be written into the dataset.