An amplifier is an electronic circuit which is used to increase the strength of a weak input signal in terms of voltage, current, or power. The process of increasing the strength of a weak signal is known as amplification. For example, in common emitter configuration, emitter is common to both input and output. Here we are discussing about the complete AC analysis of a common emitter amplifier using Lt spice .
Here we presume that you have the basic knowledge of electronic circuits and their designs . But, if you are new to Lt spice, then please refer our old tutorials first before reading further. In these tutorials we have explained how to draw the circuit diagram using Lt spice and how to give values to various components.
The below screenshot shows the circuit diagram of common emitter amplifier in Lt spice .
The major component of a common emitter amplifier is transistor. In this circuit BC547B transistor is used. R1 and R2 are the base resistors whose values are
R1 = 33 K and R2 = 6.8 K.
Rc is the collector resistance with 2.7 k ohm.Re is the emitter resistance whose value is 580 ohm. RL is the load resistance with a value of 10K ohm.
C1 and C2 R2 coupling capacitor having the value of 10 mf. C3 is an emitter capacitor with the value 47 microfarad.
VCC is the supply voltage which is equal to 15 V.
“Vin” is the input voltage applied to the base of the transistor through the coupling capacitor C1.
“Vout” is that output voltage which measured across the load resistance RL.
How to give values to “Vin”
In order to apply the input values, first right click on the voltage source. A popup menu will open with various options. Select the advanced setting from the above menu and you will see a window like the one displayed in the below screen shot. Key in the required values here.
Note : An an example, here we are using a sine wave with 40 mv peak to peak. Click on the sine wave and enter the values which is shown in figure.
Now, we can test our circuit design with various methods. First, we are using transient analysis function. It will give you a graphical visualization of both the input and output voltages.
Click on the simulate menu from the top menu bar and select “Edit simulation Cmd”. You will see a pop up window as shown in the below screenshot. Select the “transient” tab and provide the value for “stop time” (refer the below figure).Here we have taken 5 ms as stop time. Then, press OK.
Click on the simulate menu and select run. You will see the graphical window on your screen. In order to display both the input an output simultaneously in one plane, right click on the graphic plane and click on the “add plot” plane. Then two plane will appear. For displaying the input and out put, right click on the graphic plane and then click on the add traces. See the below screen.
Here we will plot the frequency response of the common emitter amplifier using AC analysis. AS mentioned in the above example, click on simulate menu and open the Edit Simulate Command window. Select the AC Analysis tab from the pop up window.Provide the values for the various parameters as shown in the below screen shot.
After running the program we will get the frequency response of our circuit as shown below.
From the above frequency response plot, we can calculate various parameters like gain, bandwidth etc. We are not discussing it here as those are out of scope for the current post.
However, we are planning to create a detailed post on gain calculation and bandwidth calculation in detail.
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