What You Will Learn
In this tutorial you will learn how to import RF BJT models from text files and will build a distributed RF amplifier using a bilateral RF BJT and generic transmission line components.
Importing High Frequency Transistor Models
RF.Spice's Parts Database contains a sizable collection of RF diodes and BJTs. As you saw in Tutorial Lesson 8, each S-Parameter BJT model corresponds to a certain DC operating point. Therefore, you will find a large number of RF BJT models associated with the same device but measured at different values of VCE and IC. The manufacturer data sheets of RF transistors usually contains a table of measured S-parameter data in the following format:
# GHz s ma r 50
freq1 |s11| ∠s11 |s21| ∠s21 |s12| ∠s12 |s22| ∠s22
freq2 |s11| ∠s11 |s21| ∠s21 |s12| ∠s12 |s22| ∠s22
freq3 |s11| ∠s11 |s21| ∠s21 |s12| ∠s12 |s22| ∠s22
...
You can import text files with a ".TXT" file extension containing S-parameter data of the above format to RF.Spice A/D. Better yet, you can create new active RF devices and add them to expand your parts database. To create a new device, you need to add the following two lines to the header of your text file:
.model <model_name>
.symbol <symbol_name>
The first line creates a unique model name in the database, and the second line picks the right symbol, which is usually either "npn_bjt_2port", or "jfet_n" or "mosfet_n" or "mesfet_n", or their p-type counterparts. For this tutorial lesson, you need to create an RF BJT.
The measured data for the RF BJT device are given below:
f(GHz) | s11 | s21 | s12 | s22 |
---|---|---|---|---|
3.0 | 0.80 ∠ -89 ° | 2.86 ∠ 99 ° | 0.03 ∠ 56 ° | 0.76 ∠ -41 ° |
4.0 | 0.72 ∠ -116 ° | 2.60 ∠ 76 ° | 0.03 ∠ 57 ° | 0.73 ∠ -54 ° |
5.0 | 0.66 ∠ -142 ° | 2.39 ∠ 54 ° | 0.03 ∠ 62 ° | 0.72 ∠ -68 ° |
Your MyRFBJT.txt file should look like the following:
.model MyRFBJT
.symbol bjt_npn_2port
# GHz s ma r 50
3.0 0.80 -89 2.86 99 0.03 56 0.76 -41
4.0 0.72 -116 2.60 76 0.03 57 0.73 -54
5.0 0.66 -142 2.39 54 0.03 62 0.72 -68
Create a text file as indicated in the table below. Open RF.Spice's Device Manager and select "Create New RF Device from S-Parameter Text File..." from its File Menu. Follow the program's prompts step by step and create your new RF BJT device.
File Name | Model Name | Symbol Name | Symbol |
---|---|---|---|
MyRFBJT.txt | MyRFBJT | bjt_npn_2port |
Amplifier Design for Maximum Gain
In this part of the tutorial lesson, you will design an RF amplifier using your MyRFBJT device for maximum gain through conjugate matching. The figure below shows the block diagram of a general transistor amplifier.
The input and output reflection coefficients are given by:
[math] \Gamma_{in} = \frac{Z_{in} - Z_0}{Z_{in} + Z_0} = s_{11} + \frac{ s_{12}s_{21} \Gamma_L }{1 - s_{22} \Gamma_L} [/math]
[math] \Gamma_{out} = \frac{Z_{out} - Z_0}{Z_{out} + Z_0} = s_{22} + \frac{ s_{12}s_{21} \Gamma_S }{1 - s_{11} \Gamma_S} [/math]
where ΓS and ΓL are the source and load reflection coefficients as defined in Tutorial Lesson 8.
The maximum power transfer will be achieved under conjugate matching conditions at the input and output of the transistor:
[math] \Gamma_S^{\ast} = s_{11} + \frac{ s_{12}s_{21} \Gamma_L }{1 - s_{22} \Gamma_L} [/math]
[math] \Gamma_L^{\ast} = s_{22} + \frac{ s_{12}s_{21} \Gamma_S }{1 - s_{11} \Gamma_S} [/math]
The above equations can be solved for ΓS and ΓL as follows:
[math] \Gamma_S = \frac{ B_1 \pm \sqrt{B_1^2-4|C_1|^2} }{2C_1} [/math]
[math] \Gamma_L = \frac{ B_2 \pm \sqrt{B_2^2-4|C_2|^2} }{2C_2} [/math]
where
[math] B_1 = 1 + |s_{11}|^2 - |s_{22}|^2 - |\Delta |^2 [/math]
[math] B_2 = 1 + |s_{22}|^2 - |s_{11}|^2 - |\Delta |^2 [/math]
[math] C_1 = s_{11} - \Delta s_{22}^{\ast} [/math]
[math] C_2 = s_{22} - \Delta s_{11}^{\ast} [/math]
[math] \Delta = s_{11}s_{22} - s_{12}s_{21} [/math]
The maximum transducer gain of the amplifier under the conjugate match conditions is given by:
[math] G_{Tmax} = G_S . G_0 . G_L = \frac{1}{1-|\Gamma_S|^2} . |S_{21}|^2 . \frac{1-|\Gamma_L|^2}{\left| 1-s_{22} \Gamma_L \right|^2} [/math]
Recalling the definition of the K-parameter from the previous tutorial lesson:
[math] K = \frac {1 - |s_{11}|^2 - |s_{22}|^2 + |\Delta |^2 } { 2|s_{12}s_{21}| } [/math]
one can show that for an unconditionally stable transistor (K > 1), the maximum transducer gain of the amplifier can be written as:
[math] G_{Tmax} = \frac{|S_{21}|}{|S_{12}|} \left( K - \sqrt{K^2-1} \right) [/math]
Building a Distributed RF BJT Amplifier
The following is a list of parts needed for this part of the tutorial lesson: