Microwave Circuit Design: Lab 5
© B. Pejcinovic, P. Wong, O. Woywode

Introduction

This lab investigates how trade-offs between gain and noise figure affect the design of an amplifier.

Design Specifications

Block diagram of amp

Figure 1: Block diagram of an amplifier

You are to design a single-stage amplifier that has a noise figure (NF) of 1.8 dB and an available gain (GA) of 20 dB. The design frequency is f = 1 GHz.

The core of the schematic is a special low noise amplifier (LNA) subcircuit. MDS supplies a pre-built LNA subcircuit (called LS_amplifier) as part of its demo files. You still have to add a few extra components to the LS_amplifier subcircuit to create the final LNA subcircuit.

There is a 50 ohm source impedance on the input side of the LNA and a 50 ohm load on its output. To keep things simple, use ell circuits (i.e., capacitors and inductors) instead of microstrip for the input and output matching networks. You may also ignore coupling capacitors.

Accessing the Pre-built LNA Subcircuit

If you already have both the samplelib and Learn_amp icons on your main MDS screen, then you may skip the following procedures. If you do not have those icons visible, then perform the following procedures to load the necessary MDS library files:

Loading samplelib:

Activate the file browser utility either by choosing [MB:TOOLS/BROWSE-OPEN] or by clicking [SUI:Browse/Open]. The 'Browse/Open' dialog window appears. At the top of the window, click [Unix System] to view your Unix directories. Next click [Browsing Options…]. When the 'Browsing Options…' window appears, click [Libraries] to view only MDS libraries and then click [OK] to exit.

In the center of the 'Browse/Open' window, you should now see a scrollable list of available MDS libraries. Scroll down the list, select the samplelib library, and click [OK]. The 'samplelib: FILE' window will appear. Close that window.

Loading Learn_amp:

Activate the file browser utility again. In the 'Browse/Open' dialog window, click [Browsing Options…]. Click [Working] to view your current working directory and then click [OK] to exit.

In the center of the 'Browse/Open' window is an input box labeled Directory path, which shows the name of your current directory. Below that is a scrollable list of directories and files that you can click on to navigate the directory system. Erase whatever text is in the input box and type /pkgs (i.e., a forward slash character followed by the directory name pkgs) In the scrollable list area, successively click on directory names in this order:

hp_hfds-7.0/ --> mds_b.07.00/ --> hp85150/ --> demos/

Once you are in the demos/ directory, scroll down the list, select the Learn_amp file, and click [OK]. The 'Learn_amp: FILE' window will appear. Open the LS_amplifier icon and examine the low noise amplifier schematic. When you are done, close the open windows.


Creating the Final LNA Subcircuit


Assignment

Create the LNA subcircuit that contains both the LS_amplifier subcircuit and its DC power supply.

Circuit construction

LNA Subcircuit

Figure 2: LNA subcircuit (LS_amplifier + power supply)

LNA Noise Characteristics


Assignment

You will determine the noise figure characteristics of the LNA subcircuit.

Circuit construction

LNA test circuit

Figure 3: LNA test circuit

NOTE: The circuit in Figure 3 was created using a template that has all the necessary noise and stimulus controls pre-configured. Do not try to build the circuit using your standard construction techniques. Perform the following steps instead.

Simulation

NOTE: Instead of using the standard 'Simulation Setup' dialog window to configure the simulation, you will edit the parameters of the noise and stimulus control boxes that reside on the circuit page.

Output

Items to turn in

Questions

  1. According to the noise figure plots, what is the minimum noise figure NFmin? (NFmin in the plot)
  2. What is the equivalent unnormalized noise resistance Rn? (RN in the plot)
  3. What is the stability of the LNA subcircuit at 1 GHz? Explain how the stability regions of the LNA subcircuit might affect your choice of source and load reflection coefficients.

Matching Network Design


Assignment

The amplifier design requirements are NF = 1.8 dB and GA = 20 dB at f = 1 GHz. Find the source and load reflection coefficients that will accomplish this goal. Design the corresponding input and output matching networks using ell circuits.

NOTE: This is a paper design only. You do not need to construct and simulate the IMN and OMN.

Design considerations

One way to approach this design is to plot the 1.8 dB noise figure circle and the 20 dB available gain circle on the same Smith chart. The intersection points between the two circles will tell you the required source reflection coefficient.

Items to turn in

Questions

  1. What are the values for Gamma_s and Gamma_L (in magnitude & phase format) that fulfill the design requirements of NF = 1.8 dB and GA = 20 dB at 1 GHz?
  2. If you were to redesign the amplifier circuit to achieve NF = NFmin, what would be the maximum gain (dB) of the amplifier? What would be the new values of Gamma_s and Gamma_L?