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{{projectinfo|Tutorial| Analyzing the Frequency Response of Multistage BJT Amplifiers |AnaTUT15 16AnaTUT16 17.png|In this project, you will analyze several use the Bode Plotter to simulate the frequency response of BJT amplifiers and examine their power efficiency and distortion characteristicswith different device models.|

|All versions|{{download|http://www.emagtech.com/contentdownloads/project-file-download-repository|ProjectRepo/AnalogLesson13.zip Analog Tutorial Lesson 13|[[RF.Spice A/D]] R15}} }}

In this tutorial you will build and test two-stage common emitter and class AB amplifiers using different BJT devices and compare examine their power efficienciesfrequency response. Since small-signal approximation does not hold well for power amplifiers, you You will learn how to import an external device model and use [[it in your circuit. You will also become familiar with RF.Spice]]'s Transient Test to analyze these nonlinear circuitsBode Plotter virtual instrument.

== Building the a Two-Stage BJT Amplifier & Examining Its DC Bias ==

The voltage source VS is an small-signal AC source with a peak amplitude of 1mV. Let's first take a look at the DC bias of the amplifier. You can run a DC Bias Test of your amplifier to find the operating point [[parameters]] of Q1 and Q2. Or you may simply run a live simulation of your circuit and enable circuit [[animation]] using "Show Voltage Text". The figure below shows the DC voltage at the operating point:

[[File:AnaTUT16 6.png|thumb|750px| The operating point DC voltage voltages in the two-stage common-emitter BJT amplifier.]]

In Tutorial Lesson 3, you ran an AC Frequency Sweep to get the frequency response of your simple BJT amplifier. You can certainly do the same thing here, too. However, [[RF.Spice A/D]] provides a quick virtual instrument for this purpose. It is called the '''Bode Plotter''' and you can place one from the Instrument Panel on the right side of the screen. First, you have to set up your virtual instrument. Click the {{key|Setup}} button and open the '''Input/Output''' tab of the drop-down panel. Set your input between Nodes 0 and 1 and set your output between Nodes 10 and 0. Set the '''Signal Amplitude''' equal to 1mV and keep the default zero value of '''Signal Offset'''. In the '''Sweep''' tab of the drop-down panel, set the Start and Stop frequencies to 10Hz and 20MHz, respectively. Set the value of '''Step / Interval''' equal to 50 and choose the "Decade" option for '''Interval Type'''. You are set!! Click the {{key|Run Sweep}} button of the instrument, and the frequency response is immediately plotted. Note Bode Plotter, by default, plots the gain of your circuit define as V(out)/V(in):

[[File:AnaTUT16 7.png|thumb|750px360px| The Setting up the Bode Plotter virtual instrument: the Input/Output page.]] </td><td>[[File:AnaTUT16 8.png|thumb|360px| Setting up the Bode plot for Plotter virtual instrument: the two-stage common-emitter amplifier using two 2N2222 BJTsSweep page.]]

You are now set!! Click the {{key|Run Sweep}} button of the instrument, and the frequency response is immediately plotted. Note the Bode Plotter, by default, plots the gain of your circuit define as V(out)/V(in). As you can see from the figurebelow, your two-stage amplifier provide a high gain of 80dB, but the frequency response rolls off at about 100kHzbelow 10kHz. Your amplifier cannot be used for higher frequencies!  <table><tr><td>[[File:AnaTUT16 9.png|thumb|750px| The Bode plot of the two-stage common-emitter amplifier using two 2N2222 BJTs.]] </td></tr></table> == Importing a New BJT Model == In the previous part, you picked a commercial BJT part from RF.Spice's extensive parts database. In many cases, you may need to use a new device model that doesn't already exist in RF.Spice's database. You can define a new device model from the ground up, or you may import new device models from external text files. In [[RF.Spice A/D]], a device or part is the combination of a simulation or process model and a symbol. You can build a complete new device and store it to the parts database. You can also use an existing part and simply change the model behind it.  In this part of the tutorial lesson, you are going to import a new BJT model from a text file called "MyNewBJT.TXT". Open a blank text file using any text editor such as Windows Notepad and type in the following text ad save it to the file:  ----  .model MyNewBJT npn is = 2.0e-16 bf = 50 vaf = 100 rb = 5 rc = 1 cje = 0.4p  + vje = 0.8 mje = 0.4 cjc = 0.5p vjc = 0. 8 ccs = 1p   ----

[[File:AnaTUT16 815.png|thumb|750px550px| The common-emitter amplifierImporting the new BJT process model called "MyNewBJT" in RF.Spice's Device Manager.]]

[[File:AnaTUT16 918.png|thumb|750px600px| The common-emitter amplifierChanging the process model of a BJT device in its property dialog.]]

[[File:AnaTUT16 1016.png|thumb|750px480px| The common-emitter amplifierImporting the new BJT process model called "MyNewBJTModel".]]

[[File:AnaTUT16 1110.png|thumb|750px| The operating point DC voltage in the two-stage common-emitter BJT amplifierwith "MyNewBJT" process models.]]

[[File:AnaTUT16 1211.png|thumb|750px| The Bode plot of the two-stage common-emitter amplifierusing two "MyNewBJT" process models.]]

[[File:AnaTUT16 1312.png|thumb|750px| The schematic of a two-stage common-emitter BJT amplifierwith shunt series feedback.]]

[[File:AnaTUT16 1413.png|thumb|750px| The Bode plot of the two-stage common-emitter amplifierwith shunt series feedback.]]

In Keep in mind that you could have got the above figure, p(vcc) is the DC power same simulation results by running an AC Frequency Test of the DC power supply VCC, while p(rl) is the power delivered to the loadyour amplifier circuit. The ratio p(rl)/p(vcc) defines If you are not satisfied with the power efficiency small size of the amplifier. From Bode plot on the above plotsvirtual instrument, you can read make a graph of it. Open the peak-to-peak input '''Export''' tab of the setup panel of the instrument and output voltages, voltage gain and power efficiency: click its {| border="0"|-| valign="top"||-{key| class="wikitable"|-! scope="col"| RL! scope="col"| Vin(p-p)! scope="col"| Vout(p-p)! scope="col"| Voltage Gain! scope="col"| I_RL(p-p)! scope="col"| P_VCC! scope="col"| P_RL! scope="col"| Power Efficiency|-| 10k&Omega; | 1686mV | 200mV | 8Copy To Graph}} button.43 | 168.5&mu;A | 15new tab opens up in your project workspace with a large graph of the Bode plot.68mW | 71Note that you have to set the right scale type and axis limits for both the horizontal frequency axis and the vertical gain axis.28&mu;W| 0.46%|-|}

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