Difference between revisions of "RF Tutorial Lesson 11: Designing a Microstrip MESFET Amplifier"

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{{projectinfo|Tutorial| Designing a Microstrip MESFET Amplifier |RF150.png|In this project, the basic concepts of RF.Spice A/D are demonstrated, and a simple voltage divider is modeled and examined.|
+
{{projectinfo|Tutorial| Designing a Microstrip MESFET Amplifier |RF150.png|In this project, you will build and test a distributed RF amplifier using your own S-parameter-based MESFET model.|
  
*[[CubeCAD]]
+
*RF Amplifier
*Visualization
+
*S-Parameter-Based MESFET Model
*[[EM.Tempo#Lumped Sources | Lumped Sources]]
+
*Maximum Gain Design
*[[EM.Tempo#Scattering Parameters and Port Characteristics | S-Parameters]] 
+
*Microstrip Line Segment
*[[EM.Tempo#Far Field Calculations in FDTD | Far Fields]]
+
*AC Frequency Sweep
*[[Advanced Meshing in EM.Tempo]]
+
*Power Gain
|All versions|{{download|http://www.emagtech.com/content/project-file-download-repository|EM.Tempo Lesson 1|[[EM.Cube]] 14.8}} }}
+
|All versions|{{download|http://www.emagtech.com/downloads/ProjectRepo/RFLesson11.zip RF Lesson 11}} }}
  
=== What You Will Learn ===
+
== 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.   
 
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.   
Line 29: Line 29:
 
|}
 
|}
  
Create a text file as indicated in the tables above. Open [[RF.Spice]]'s Device Editor and select "Create New RF Device from S-Parameter Test File..." from its RF Menu. Follow the program's prompts step by step and create your new RF BJT and MESFET devices.
+
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:
  
 
{| class="wikitable"  
 
{| class="wikitable"  
Line 40: Line 40:
  
 
== Building & Testing a Distributed MESFET Amplifier with Microstrip Components==
 
== Building & Testing a Distributed MESFET Amplifier with Microstrip Components==
 
[[File:RF150.png|thumb|640px|The MESFET Amplifier with microstrip matching networks at its input and output.]]
 
  
 
The following is a list of parts needed for this part of the tutorial lesson:
 
The following is a list of parts needed for this part of the tutorial lesson:
Line 61: Line 59:
 
! scope="row"| NP1
 
! scope="row"| NP1
 
| N-Type RF MESFET
 
| N-Type RF MESFET
| Imported Model: MyRFBJT
+
| Imported Model: MyMESFET
 
|-
 
|-
! scope="row"| XTL1
+
! scope="row"| XMS1
| Generic T-Line
+
| Microstrip Line
| Z0 = 50, eeff = 1, len = 75
+
| w = 1.15, h = 0.5, er = 3.4. len = 8.19
 
|-
 
|-
! scope="row"| XTL2
+
! scope="row"| XMS2
| Generic T-Line
+
| Microstrip Line
| Z0 = 50, eeff = 1, len = 18
+
| w = 1.15, h = 0.5, er = 3.4. len = 4.57
 
|-
 
|-
! scope="row"| XTLO1
+
! scope="row"| XMS3
| Generic Open Stub
+
| Microstrip Line
| Z0 = 50, eeff = 1, len = 59.55
+
| w = 1.15, h = 0.5, er = 3.4. len = 2.05
 
|-
 
|-
! scope="row"| XTLO2
+
! scope="row"| XMS4
| Generic Open Stub
+
| Microstrip Line
| Z0 = 50, eeff = 1, len = 59.55
+
| w = 1.15, h = 0.5, er = 3.4. len = 19.76
 
|-
 
|-
 
! scope="row"| RS, RL
 
! scope="row"| RS, RL
Line 84: Line 82:
 
|}
 
|}
  
 
+
<table>
----
+
<tr>
 
+
<td>
AC Voltage Source: VS (keyboard shortcut: Alt+V)
+
[[File:RFTUT11_5.png|thumb|500px|The property dialog of the MESFET device.]]
 
+
</td>
Two 50&Omega; Resistors: RS and RL
+
</tr>
 
+
</table>
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. s<sub>12</sub> = 0. Moreover, |s<sub>11</sub>| < 1 and |s<sub>22</sub>| < 1. Therefore, the MESFET is unconditionally stable. This reduces the input and output reflection coefficients to:   
 
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. s<sub>12</sub> = 0. Moreover, |s<sub>11</sub>| < 1 and |s<sub>22</sub>| < 1. Therefore, the MESFET is unconditionally stable. This reduces the input and output reflection coefficients to:   
Line 107: Line 95:
  
 
<math> \Gamma_{out} = s_{22} </math>
 
<math> \Gamma_{out} = s_{22} </math>
 
  
 
The conjugate matching conditions at the input and output of the unilateral transistor reduce to:
 
The conjugate matching conditions at the input and output of the unilateral transistor reduce to:
Line 114: Line 101:
  
 
<math> \Gamma_L = s_{22}^{\ast} </math>
 
<math> \Gamma_L = s_{22}^{\ast} </math>
 
  
 
Furthermore, you have:
 
Furthermore, you have:
Line 122: Line 108:
 
<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>
 
<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>
  
[[File:RF146.png|thumb|450px|Using RF.Spice's Device Editor for designing a 50&Omega microstrip line.]]
+
[[File:RF146.png|thumb|450px|Using RF.Spice's Device Manager for designing a 50&Omega microstrip line.]]
 
+
 
To achieve a gain of 11dB, you have 2.5dB more available gain. So you set G<sub>S</sub> = 2dB and G<sub>L</sub> = 1dB for a total gain of G<sub>T</sub> = 2dB + 8dB + 1dB = 11dB. The complex value of &Gamma;<sub>S</sub> is found on the constant circle G<sub>S</sub> = 2dB, and the complex value of &Gamma;<sub>L</sub> is found on the constant circle G<sub>L</sub> = 1dB, in both cases trying to minimize the distance from the center of the Smith Chart. This requirement yields:  
 
To achieve a gain of 11dB, you have 2.5dB more available gain. So you set G<sub>S</sub> = 2dB and G<sub>L</sub> = 1dB for a total gain of G<sub>T</sub> = 2dB + 8dB + 1dB = 11dB. The complex value of &Gamma;<sub>S</sub> is found on the constant circle G<sub>S</sub> = 2dB, and the complex value of &Gamma;<sub>L</sub> is found on the constant circle G<sub>L</sub> = 1dB, in both cases trying to minimize the distance from the center of the Smith Chart. This requirement yields:  
  
Line 130: Line 115:
 
&Gamma;<sub>L</sub> = 0.22 &ang;70&deg;         
 
&Gamma;<sub>L</sub> = 0.22 &ang;70&deg;         
  
[[File:RF154.png|thumb|500px|The MESFET Amplifier without the source and load sections for the purpose of network analysis.]]
+
For your MESFET amplifier, you will use the same input and output matching network types as in Tutorial Lesson 10, consisting of a 50&Omega; transmission line segment together with a shunt 50&Omega; Open Stub.  
  
For your MESFET amplifier, you will use the same input and output matching network types of the previous part consisting of a 50&Omega; transmission line segment together with a shunt 50&Omega; Open Stub. For this project, you will use a thin lossless dielectric substrate of thickness h = 0.5mm and relative permittivity &epsilon<sub>r</sub> = 3.4. A 50&Omega; 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 &lambda;<sub>g</sub> = 45.74mm.
+
For this project, you will use a thin lossless dielectric substrate of thickness h = 0.5mm and relative permittivity &epsilon<sub>r</sub> = 3.4. A 50&Omega; 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 &lambda;<sub>g</sub> = 45.74mm. The lengths of the microstrip segments are found to be:  
 
+
The lengths of the microstrip segments are found to be:  
+
  
 
{| class="wikitable"  
 
{| class="wikitable"  
Line 150: Line 133:
 
|}
 
|}
  
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 s<sub>12</sub> = 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 |s<sub>21</sub>| is almost 11dB as expected from the design. However, the value of the return loss |s<sub>11</sub>| 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.   
+
Place and connect all the parts as shown in the figure below. 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. Leave the positive pin of XMS2 and XMS4 open.
 +
 
 +
<table>
 +
<tr>
 +
<td>
 +
[[File:RF154.png|thumb|640px|The MESFET Amplifier without the source and load sections for the purpose of network analysis.]]
 +
</td>
 +
</tr>
 +
</table>
 +
 
 +
Run a Network Analysis Test of this circuit according to the table below:
  
 
{| border="0"
 
{| border="0"
 
|-
 
|-
| valign="bottom"|
+
| valign="top"|
[[File:RF151.png|thumb|900px|left|The graph of magnitude of s11, s21 and s22 parameters of the MESFET amplifier circuit.]]
+
 
|-
 
|-
 +
{| class="wikitable"
 +
|-
 +
! scope="row"| Start Frequency
 +
| 3G
 +
|-
 +
! scope="row"| Stop Frequency
 +
| 5G
 +
|-
 +
! scope="row"| Steps/Interval
 +
| 10Meg
 +
|-
 +
! scope="row"| Interval Type
 +
| Linear
 +
|-
 +
! scope="row"| Parameter Set
 +
| S
 +
|-
 +
! scope="row"| Graph Type
 +
| Smith or Cartesian (Amplitude Only) with Decibels checked
 
|}
 
|}
  
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 G<sub>P</sub> for the power gain of your amplifier. Use the same definition: G<sub>P</sub> = 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.
+
The figure below shows the results for S11, S21, S12 and S22 parameters. Note that since S<sub>12</sub> = 0, its dB-scale plot falls at a very large negative number. Therefore, you need to adjust the scale of the vertical axis. Or you can deselect S12 from the graph's legend and zoom to fit. The insertion gain |s<sub>21</sub>| is almost 11dB as expected from the design. However, the value of the return loss |s<sub>11</sub>| 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.   
 +
 
 +
<table>
 +
<tr>
 +
<td>
 +
[[File:RFTUT11_6.png|thumb|750px|left|The graph of magnitude of S11, S21 and S22 parameters of the MESFET amplifier circuit (S12 = 0).]]
 +
</td>
 +
</tr>
 +
</table>
 +
 
 +
== Running an AC Frequency Sweep to Compute Power Gain ==
 +
 
 +
Next, connect the AC voltage source and the source and load resistors and place two ammeters at the source and load ammeters in a similar manner as in the last part of Tutorial Lesson 10. The figure below shows the circuit with the source, load and ammeters:
 +
 
 +
<table>
 +
<tr>
 +
<td>
 +
[[File:RF150.png|thumb|640px|The MESFET amplifier with microstrip matching networks at its input and output.]]
 +
</td>
 +
</tr>
 +
</table>
 +
 
 +
Run an AC Frequency Sweep Test of your amplifier according to the table below:
  
 
{| border="0"
 
{| border="0"
 
|-
 
|-
| valign="bottom"|
+
| valign="top"|
[[File:RF153.png|thumb|900px|left|The graph of the power gain of the MESFET amplifier vs. frequency.]]
+
 
|-
 
|-
 +
{| class="wikitable"
 +
|-
 +
! scope="row"| Start Frequency
 +
| 3G
 +
|-
 +
! scope="row"| Stop Frequency
 +
| 5G
 +
|-
 +
! scope="row"| Steps/Interval
 +
| 10Meg
 +
|-
 +
! scope="row"| Interval Type
 +
| Linear
 +
|-
 +
! scope="row"| Preset Graph Plots
 +
| i(iam1), i(iam2)
 
|}
 
|}
  
 +
<table>
 +
<tr>
 +
<td>
 +
[[File:RFTUT11_7.png|thumb|750px|The graph of variation of input source current and output load current as a function of frequency.]]
 +
</td>
 +
</tr>
 +
</table>
 +
 +
Also define a custom output plot called "Power_Gain" for your amplifier using the same definition: G<sub>P</sub> = 20*log10(abs(i(am2)/i(am1))).
 +
 +
{| border="0"
 +
|-
 +
| valign="top"|
 +
|-
 +
{| class="wikitable"
 +
|-
 +
! scope="row"| Start Frequency
 +
| 3G
 +
|-
 +
! scope="row"| Stop Frequency
 +
| 5G
 +
|-
 +
! scope="row"| Steps/Interval
 +
| 10Meg
 +
|-
 +
! scope="row"| Interval Type
 +
| Linear
 +
|-
 +
! scope="row"| Preset Graph Plots
 +
| Custom: Power_Gain
 +
|}
 +
 +
 +
The figure below shows the graph of power gain vs. frequency. Using the crosshairs you can read the value of the power gain at 4GHz to be 11.148dB, which agrees well with the value of insertion gain calculated in the previous part.
 +
 +
<table>
 +
<tr>
 +
<td>
 +
[[File:RFTUT11_8.png|thumb|750px|The graph of the power gain of the MESFET amplifier vs. frequency.]]
 +
</td>
 +
</tr>
 +
</table>
 
<p>&nbsp;</p>
 
<p>&nbsp;</p>
[[Image:Back_icon.png|40px]] '''[[RF.Spice_A/D#RF.Spice_A.2FD_Tutorial | Back to RF.Spice A/D Tutorial Gateway]]'''
+
[[Image:Back_icon.png|40px]] '''[[RF.Spice_A/D#RF.Spice_A.2FD_Tutorials | Back to RF.Spice A/D Tutorial Gateway]]'''

Latest revision as of 18:46, 8 November 2016

Tutorial Project: Designing a Microstrip MESFET Amplifier
RF150.png

Objective: In this project, you will build and test a distributed RF amplifier using your own S-parameter-based MESFET model.

Concepts/Features:

  • RF Amplifier
  • S-Parameter-Based MESFET Model
  • Maximum Gain Design
  • Microstrip Line Segment
  • AC Frequency Sweep
  • Power Gain

Minimum Version Required: All versions

'Download2x.png Download Link: RF Lesson 11

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 G14A.png

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: MyMESFET
XMS1 Microstrip Line w = 1.15, h = 0.5, er = 3.4. len = 8.19
XMS2 Microstrip Line w = 1.15, h = 0.5, er = 3.4. len = 4.57
XMS3 Microstrip Line w = 1.15, h = 0.5, er = 3.4. len = 2.05
XMS4 Microstrip Line w = 1.15, h = 0.5, er = 3.4. len = 19.76
RS, RL Resistor 50
The property dialog of the MESFET device.

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]

Using RF.Spice's Device Manager for designing a 50&Omega microstrip line.

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 as in Tutorial Lesson 10, 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 figure below. 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. Leave the positive pin of XMS2 and XMS4 open.

The MESFET Amplifier without the source and load sections for the purpose of network analysis.

Run a Network Analysis Test of this circuit according to the table below:

Start Frequency 3G
Stop Frequency 5G
Steps/Interval 10Meg
Interval Type Linear
Parameter Set S
Graph Type Smith or Cartesian (Amplitude Only) with Decibels checked

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. Or you can deselect S12 from the graph's legend and zoom to fit. 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.

The graph of magnitude of S11, S21 and S22 parameters of the MESFET amplifier circuit (S12 = 0).

Running an AC Frequency Sweep to Compute Power Gain

Next, connect the AC voltage source and the source and load resistors and place two ammeters at the source and load ammeters in a similar manner as in the last part of Tutorial Lesson 10. The figure below shows the circuit with the source, load and ammeters:

The MESFET amplifier with microstrip matching networks at its input and output.

Run an AC Frequency Sweep Test of your amplifier according to the table below:

Start Frequency 3G
Stop Frequency 5G
Steps/Interval 10Meg
Interval Type Linear
Preset Graph Plots i(iam1), i(iam2)
The graph of variation of input source current and output load current as a function of frequency.

Also define a custom output plot called "Power_Gain" for your amplifier using the same definition: GP = 20*log10(abs(i(am2)/i(am1))).

Start Frequency 3G
Stop Frequency 5G
Steps/Interval 10Meg
Interval Type Linear
Preset Graph Plots Custom: Power_Gain


The figure below shows the graph of power gain vs. frequency. Using the crosshairs you can read the value of the power gain at 4GHz to be 11.148dB, which agrees well with the value of insertion gain calculated in the previous part.

The graph of the power gain of the MESFET amplifier vs. frequency.

 

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