Changes
/* Changing the Tuning of the Colpitts Oscillator */
{{projectinfo|Tutorial| Designing Low and High Frequency Oscillator Circuits BJT Colpitts Oscillators |TUT12-8.png|In this project, the basic concepts of RF.Spice A/D are demonstrated, you will build and analyze a simple voltage divider is modeled and examinedColpitts oscillator with an LC tank circuit.|
*[[CubeCAD]]Bipolar Junction Transistor*VisualizationParallel LC Resonant Circuit *[[EM.Tempo#Lumped Sources | Lumped Sources]]Oscillation Frequency*[[EM.Tempo#Scattering Parameters and Port Characteristics | S-Parameters]] *[[EM.Tempo#Far Field Calculations in FDTD | Far Fields]] *[[Advanced Meshing in EM.Tempo]] Barkhausen Criterion|All versions|{{download|http://www.emagtech.com/contentdownloads/project-file-download-repository|EMProjectRepo/AnalogLesson10.Tempo zip Analog Lesson 1|[[EM.Cube]] 14.810}} }}
=== What You Will Learn ===
== Designing a BJT Colpitts Oscillator ==
The following is a list of parts needed for this part of the tutorial lesson:
|-
! scope="row"| C1
| Capacitor
| 100n
|-
! scope="row"| C2 - C3
| Capacitor
| 1n
|-
! scope="row"| C4
| Capacitor
| 27n
|-
! scope="row"| L1
| Inductor
| 1n27u
|-
! scope="row"| Q1
| NPN BJT 2N2222
| beta = 150
|}
To place the transistor part 2N222, open the "'''Parts Bin'''" by selecting the "'''Add Part'''" tab of the "'''Toolbox'''" on the left side panel. By default, the "'''Function'''" tab of the Part Bin is active. This means that parts are sorted based on their function. Open the "'''Active Components'''" menu and select the "'''Transistor...'''" item and from its submenu select the '''NPN...''' item as shown below. All the NPN BJT devices are listed in the Parts Bin. Scroll down the list and find and select “'''2N2222'''”. Either double-click the part's name or click the {{key|Place Part}} button at the bottom of the Parts Bin to place the device on your schematic.
<table><tr><td>[[File:TUT12----6A.png|thumb|650px| Selecting a BJT part in Parts Bin.]] </td></tr></table>
<math>f_o = \frac {1}{2 \pi \sqrt{ L_1 \left( \frac{C_3 C_4}{C_3 + C_4} \right) } }</math>
[[File:TUT12-7.png|thumb|300px400px| The operating point results obtained through DC Bias Test of the Colpitts Oscillator.]]
Note that C1 and C2 are bypass and coupling capacitors, respectively. In this Colpitts oscillator circuit, the feedback factor β(s) is C3/(C3 + C4) ≅ C3/C4. The voltage gain A is given by:
<math>A = g_m R = \frac{I_C}{V_T} R_3 = \frac{kI_C}{qT} R_3</math>
Therefore, the Barkausen Barkhausen criterion can be written as:
<math>A \beta(s) = \left( \frac{C_3}{C_3 + C_4} \right). \left( \frac{I_C}{V_T} \right). R_3 \ge 1</math>
Before running a Transient Test of your Colpitts oscillator, first run a DC Bias Test to find the DC operating point of the BJT. The results of the DC bias test are shown in the opposite figure. According to this table, the quiescent collector current I<sub>C</sub> is 2.985mA. The Barkhausen criterion in this case is thus satisfied:
<math>A \beta(s) = \left( \frac{1nF}{16nF} \right). \left( \frac{2.985e985\times 10^{-3}}{26\times 10^{26e-3}} \right). 2e3 2\times 10^3 = 14.351 \ge 1</math>
Now run a Transient Test of your oscillator circuit with start and stop times set at 0 and 100μs, respectively, and a Step Ceiling equal to 1ns. The output voltage graph is shown in the figure below. The output signal oscillates between 2V and 14V. A signal period of almost 1μs can be measured, which corresponds to an oscillation frequency of 1MHz.
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[[File:TUT12-8.png|thumb|800px720px|The output voltage of the Colpitts Oscillator.]]
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<td>
[[File:TUT12-9.png|thumb|800px720px|The output voltage of the modified Colpitts Oscillator with C3 = 0.25nF and L1 = 6.75μH.]]
</td>
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<td>
[[File:TUT12-10.png|thumb|800px720px|The output voltage of the modified Colpitts Oscillator with C3 = 62.5pF and L1 = 6.75μH.]]
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</table>
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