Telecommunication Systems
Aim: Design the bias and matching networks for a single transistor low noise amplifier
Project Description - Design a low noise amplifier using an Infineon BFP640 RF transistor. The amplifier is to be used to amplify the L5 GPS signal and so the centre frequency is 1176.45MHz and bandwidth 30MHz. The requirements over the entire bandwidth are: a noise figure of less than 1.0dB, a gain of 20dB ±0.3dB and S22 should be less than -15dB ("minus 15dB"). The circuit will be used in a portable device powered by a 3.6 lithium battery.
State and justify the chosen bias point, including (where appropriate) reference to the transistor data sheet (specifications in tables, graphs etc), the given design specifications, and any assumptions you have made. (The chosen graph below may not be relevant - and more than one graph may be relevant - this is just to give you an idea).Design methodology
1. In view of the project specification, assumptions (if any) about the eventual use of the design, and the information contained in the BFP640 data sheet, determine/propose a suitable biasing point for the transistor, in terms of VCE and IC.
2. In view of the project specification and your knowledge and experience of the characteristics of different biasing circuits (for example, those explored in ADS Example Sheet 3), determine a suitable biasing configuration for the BFP640 transistor.
3. Use ADS to simulate your designed bias circuit (use models of "real" resistors). Use ADS to confirm that the bias conditions are as you designed them to be, and, using the correct S parameter model for the transistor, also use it to determine the S parameters of the transistor plus the biasing circuit.
4. Use ADS to determine the source and load stability circles, gain circles, and noise circles.
5. Choose an appropriate value of source reflection coefficient so that the gain and noise figure would be within specifications (assuming you correctly designed matching circuits using ideal components).
6. Conjugately match the load reflection coefficient so that S22 would be within specification (at the centre frequency assuming you correctly designed matching circuits using ideal components).
7. Design all possible matching circuits for your chosen configuration, using:
a. Paper Smith chart, and/or
b. Confirm using ADS (for example, using the procedure explored in ADS Example Sheet 2).
8. Choose what you believe are the most appropriate input and output matching circuits, and simulate your design using ideal components for the matching circuits. Compare the performance of your design with the project specifications (a simulation output data display, "AmpDesign", is provided to enable you to do this - you must include this in your report).
9. Replace all ideal components used in your circuit with models of "real" components. Compare the characteristics of your circuit with the previous case (using "ideal" matching components) and the project specifications - again, use the simulation output data display provided.
10. You may find that you need to reconsider the design decisions you made in previous steps in light of simulation results. If so, this would make for an interesting discussion in your report.
ADS design tools
1. Open the GetDCParameters design. This will simulate the DC characteristics of the transistor, so that IB and VBE can be determined, for your choice of VCE and IC, as with ADS Exercise Sheet 3. Note that the BFP transistor uses two different models to simulate its behaviour. One is a Spice model for the device, which is used to simulate the DC properties of the device. The other is an S parameter model, based on measured characteristics at different bias conditions and frequencies. To switch between the two models, or to check which model is selected currently, descend into the device hierarchy and follow the instructions on the schematic. To descend into the hierarchy right click the transistor component and then select the "push into hierarchy" menu item, or, select the component and then press the large downward arrow on the top toolbar. Each of the two models can be enabled/disabled by selecting the model and then clicking the second icon from the right on the top toolbar (next to the wand - it resembles a resistor in a box with a cross through it). Confirm that the Spice model (the lower of the two on the left) is selected. Once you have done this you will need to return to the top level; this is done by pressing the upward arrow on the toolbar.
2. Open the GetSParameter schematic. You will need to modify the design to include the biasing components. (Even at this stage I would recommend the use of models of real resistors that are available from the AmpDesignReal schematic sheet, because the "real" components have a significant capacitance to ground.) For the DC simulations it would be prudent to confirm that the bias point is set correctly. You will see on the GetSParameter schematic a DC simulation icon. Ensure that the Spice model is selected for the transistor. Simulate the circuit, but (for now) ignore the simulation output window that appears. Go back to the schematic and select "Simulate -> Annotate DC solution". The currents and voltages at all the nodes on the circuit will be displayed. Confirm that these are as you expect.
3. Once you are happy with the biasing components you will need to disable the DC simulation component and enable the SP component. (Right click the component and then select the "component-activate/deactivate" menu item.) Descend into the hierarchy of the BPF640 device and follow the instructions on the schematic to select the appropriate model. The S parameter files are held in the H63TCE_wrk\data\bfp640 directory. An explanation of how the file names relate to the bias conditions is included in the "BFP640 S parameter files" PDF available on Moodle. You need to select the correct S parameter file for your chosen bias conditions. State in your final report the S parameter file you used.
4. On running the simulation, a data display will automatically open, similar to that shown in figure 1. The red circle is the source mismatch stability curve. This will give an indication of where you need to choose your point. The purple circles are the constant available gain curves that arise from mismatching the source. These curves assume that the output is conjugately matched. The gain for each curve is given by the data matrix which is set by an equation on the bottom right of the data display window. The green curves are circles of constant noise.
The values of these circles are again set by a data matrix which is set by an equation. If the specified noise figure is below the minimum noise figure (NFmin) then no circle is drawn. This explains why there may be more parameters than circles. The action you need to take is to select the appropriate gain circle and then move the GS marker to the position you want. The position of the marker will be governed by its position relative to the noise circles and the stability circles. As you move the GS marker the input and output impedances - that you need to match - are automatically updated. The equations used to perform the necessary mathematics can be found by clicking the equations tab on the bottom of the data display. These do not need to be changed and if you do I cannot guarantee that they will work.
5. Now that you have the input and output impedances you can design the matching networks. I will require either the Smith Charts showing how you calculated the matching components or a step-by- step guide to how you calculated their values. (Just quoting the values will gain you no marks since you could have taken the values straight from an online calculator.) Blank Smith Charts can be found on Moodle, in the "Extra materials and resources" section. You could use ADS to confirm your matching circuit designs, using the methods used in ADS Example Sheet 2.
6. Once you have the component values of your matching circuits you will be in a position to test the performance of your circuit. Open the AmpDesign schematic and draw you circuit and then test. The AmpDesign data display will open. The graphs which are displayed will be required for your report.
7. Now you will need to modify the design so that all the ideal components - including DC Blocks and DC Feeds - are changed to more realistic devices. The models for these devices are derived from physical S parameter measurements and can be found on the AMpDesignReal schematic. Enter you modified circuit and run the simulations.
Attachment:- Assignment.rar