Difference between revisions of "RF Tutorial Lesson 11: Designing a Microstrip MESFET Amplifier"
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− | + | Open [[RF.Spice]]'s Device Manager and select "Create New RF Device from S-Parameter Test File..." from its File Menu. Follow the program's prompts step by step and create your new MESFET devices according to the table below: | |
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Revision as of 23:36, 27 September 2015
What You Will Learn
In this tutorial you will learn how to import an RF FET model from a text file and will build a distributed RF amplifier using a unilateral MESFET along with physical microstrip components for the input and output matching networks.
Importing High Frequency MESFET Model
In Tutorial Lesson 10, you learned how to create a new RF BJT device. For this tutorial lesson, you need to create an RF MESFET device. The measured data for the MESFET device are given below:
f(GHz) | s11 | s21 | s12 | s22 |
---|---|---|---|---|
3.0 | 0.80 ∠ -90 ° | 2.80 ∠ 100 ° | 0 | 0.66 ∠ -50 ° |
4.0 | 0.75 ∠ -120 ° | 2.50 ∠ 80 ° | 0 | 0.60 ∠ -70 ° |
5.0 | 0.71 ∠ -140 ° | 2.30 ∠ 60 ° | 0 | 0.58 ∠ -85 ° |
Open RF.Spice's Device Manager and select "Create New RF Device from S-Parameter Test File..." from its File Menu. Follow the program's prompts step by step and create your new MESFET devices according to the table below:
File Name | Model Name | Symbol Name | Symbol |
---|---|---|---|
MyMESFET.txt | MyMESFET | mesfet_n |
Building & Testing a Distributed MESFET Amplifier with Microstrip Components
The following is a list of parts needed for this part of the tutorial lesson:
Part Name | Part Type | Part Value |
---|---|---|
VS | AC Voltage Source | 1V |
NP1 | N-Type RF MESFET | Imported Model: MyRFBJT |
XTL1 | Generic T-Line | Z0 = 50, eeff = 1, len = 75 |
XTL2 | Generic T-Line | Z0 = 50, eeff = 1, len = 18 |
XTLO1 | Generic Open Stub | Z0 = 50, eeff = 1, len = 59.55 |
XTLO2 | Generic Open Stub | Z0 = 50, eeff = 1, len = 59.55 |
RS, RL | Resistor | 50 |
AC Voltage Source: VS (keyboard shortcut: Alt+V)
Two 50Ω Resistors: RS and RL
N-Type MESFET: MyMESFET (imported model)
Four Microtrip Line Segments: XMS1, XMS2, XMS3 and XMS4 (keyboard shortcut: Alt+T)
Two Ammeters: AM1 and AM2 (keyboard shortcut: Alt+Y)
Two Net Markers: IN and OUT (keyboard shortcut: Alt+N)
The goal of this part is to design a distributed MESFET amplifier with a gain of 11dB at f = 4GHz. From the S-parameter data of the MESFET, we know that it is a unilateral transistor, i.e. s12 = 0. Moreover, |s11| < 1 and |s22| < 1. Therefore, the MESFET is unconditionally stable. This reduces the input and output reflection coefficients to:
[math] \Gamma_{in} = s_{11} [/math]
[math] \Gamma_{out} = s_{22} [/math]
The conjugate matching conditions at the input and output of the unilateral transistor reduce to:
[math] \Gamma_S = s_{11}^{\ast} [/math]
[math] \Gamma_L = s_{22}^{\ast} [/math]
Furthermore, you have:
[math] G_0 = |s_{21}|^2 = 6.25 = 8.0dB [/math]
[math] G_{Tmax} = G_S . G_0 . G_L = \frac{1}{1- |s_{11}| ^2} . |S_{21}|^2 . \frac{1}{1- |s_{22}| ^2} = 13.5dB [/math]
To achieve a gain of 11dB, you have 2.5dB more available gain. So you set GS = 2dB and GL = 1dB for a total gain of GT = 2dB + 8dB + 1dB = 11dB. The complex value of ΓS is found on the constant circle GS = 2dB, and the complex value of ΓL is found on the constant circle GL = 1dB, in both cases trying to minimize the distance from the center of the Smith Chart. This requirement yields:
ΓS = 0.33 ∠120°
ΓL = 0.22 ∠70°
For your MESFET amplifier, you will use the same input and output matching network types of the previous part consisting of a 50Ω transmission line segment together with a shunt 50Ω Open Stub. For this project, you will use a thin lossless dielectric substrate of thickness h = 0.5mm and relative permittivity &epsilonr = 3.4. A 50Ω microstrip line on this substrate has a width of 1.15mm. At the design frequency of f = 4GHz, the guide wavelength of this microstrip line is λg = 45.74mm.
The lengths of the microstrip segments are found to be:
Microstrip Component | Z0 | Electrical Length | Physical Width | Physical Length |
---|---|---|---|---|
XMS1 | 50Ω | 0.179λg | 1.15mm | 8.19mm |
XMS2 | 50Ω | 0.100λg | 1.15mm | 4.57mm |
XMS3 | 50Ω | 0.045λg | 1.15mm | 2.05mm |
XMS4 | 50Ω | 0.432λg | 1.15mm | 19.76mm |
Place and connect all the parts as shown in the above figure. For the shunt stubs, connect the microstrip segments XMS2 and XMS4 in a parallel fashion between the source and load resistors and the ground, respectively. First, remove the AC voltage source and the source and load resistors to perform network analysis. Run a Network Analysis Test of this circuit with start and stop frequencies set at 3GHz and 5GHz, respectively, with a linear frequency step size of 10MHz. Plot the S-parameters on an amplitude-only Cartesian graph. The figure below shows the results for s11, s21, s12 and s22 parameters. Note that since s12 = 0, its dB-scale plot falls at a very large negative number. Therefore, you need to adjust the scale of the vertical axis. The insertion gain |s21| is almost 11dB as expected from the design. However, the value of the return loss |s11| is only -5dB and certainly not very good. This is due to the fact that you had to deliberately introduce a mismatch in the input and output matching networks to achieve the specified gain of 11dB.
Next, connect the AC voltage source and the source and load resistors and place the source and load ammeters in a similar manner as in the last part of this tutorial lesson. Similarly, define a custom output plot called GP for the power gain of your amplifier. Use the same definition: GP = 20*log10(abs(i(am2)/i(am1))). Run an AC Frequency Sweep Test of your amplifier from 3GHz to 5GHz with linear frequency steps of 10MHz. The figure below shows the graph of power gain vs. frequency.