In the last part of this tutorial lesson, you will run an AC Sweep Test of your BJT amplifier circuit to examine the input and output voltages and find the voltage gain and power gain of your amplifier. To measure the power gain, we define input and output powers as the source and load powers:
<math> P_{in} = P_S = \frac{1}{2} R_S |I_S|^2 </math>
<math> P_{out} = P_L = \frac{1}{2} R_L |I_L|^2 </math>
<math> G_P = \frac{P_L}{P_S} = \left| \frac{I_L}{I_S} \right| ^2 </math>
Note that R<sub>S</sub> = R<sub>L</sub> = 50Ω. To compute the source and load currents I<sub>S</sub> and I<sub>L</sub>, you can place two ammeters (keyboard shortcut: Alt+Y), one between the AC voltage source and the source resistor, and the other just before the load resistor as shown in the opposite figure. Set the start and stop frequencies of the sweep to 500MHz and 1500MHz with a linear frequency step size of 10MHz. Add that the voltage amplitudes vm(in) and vm(out) to the graph plot list in the Edit Plot List dialog. Run the AC sweep, and you will get input and output voltage graphs like the ones shown below. Enable the tracking crosshairs and read the values at 1GHz. Note that when you have multiple plots, the tracking crosshairs read the value of the highlighted (selected) plot. The input and output voltage readings are 2.941V and 535mV, respectively. The input voltage value shows that the default 1V voltage of the source VS has been almost equally divided between the source resistor and the amplifier circuit.
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Before closing this tutorial lesson, you will calculate the voltage and power gains of your BJT amplifier. For this purpose, you will define "Custom Output Plots". In the AC Sweep Test Panel, click the "Preset Graph Plots..." button to open the Edit Plot List dialog. Define two new custom output plots with the names "Voltage_Gain" and "Power_Gain" as shown in the figures below. Note that a dB-scale definition has been used. Remove the voltage amplitudes vm(in) and vm(out) from the graph plot list in the Edit Plot List dialog. Run the AC sweep test to plot the voltage and power gain as a function of frequency as shown below. Using the tracking crosshairs read the values of the two gains at 1GHz. The power gain at this frequency is 15.11dB and the voltage gain is almost equal to it. Note that voltage gain rises to a peak around 800MHz, while the power gain rises monotonically. However, both input and output voltages of the amplifier start to decrease with increasing frequency. At 1.5GHz, the output voltage is about 1.5V.
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