common-emitor-transistor

Draw Common Emitter Transistor Characteristics Using LTspice

Initially, before going into the details of common emitter transistor characteristics, we will check the basics first. We know, a transistor has three terminals.

  • Base
  • Emitter
  • Collector

You can see the symbol of an NPN transistor in the below image.

Symbol of an NPN transistor

There are three types of transistor configurations.

  • Common base configuration
  • Common Emitter configuration
  • Common Collector configuration.

In this post we are discussing about “Common emitter transistor configuration. In common emitter configuration, emitter is common to both input and output. i.e the input is applied between the base and the emitter and the output is taken between the collector and the emitter.

Characteristics of common emitter configuration

There are two types of transistor characteristics .

  • Input characteristics
  • Output characteristics.

Input characteristics of a transistor can be represented as a graph between the input voltage  and input current for a constant output voltage. The below diagram shows the circuit diagram of a common emitter transistor configuration to plot the input characteristics.



In the above example, we are using transistor “BC 547B”. “Vbe” is the base emitter voltage applied between base and the emitter . “Vce” is the collector emitter voltage applied between collector and emitter. Here, the emitter terminal is common to both input and output. So, it is called common emitter configuration. “R1” is the input resistance having a value of 620 ohm and “R2” is the output resistance  having a value of 420 ohm. “Ib”  is the base current and “Ic” is the collector current. Set “Vbe” to 0 Vt in the beginning and “Vce” to 1 Volt. Now perform DC sweep analysis In Ltspice to plot input characteristics.

Now, click on the simulate button and select simulation command. Then from the pop-up window select “DC Sweep”. Here you need to select the option ” First Source” since we are plotting the input characteristics. So, enter the values for the below parameters. Refer the below screen.

  • Name of first voltage source – Vbe
  • Type of Sweep – Linear
  • Start Value – 0
  • Stop Value -1
  • Increment- 0.01

 Now click OK and click “run”. Then the screen will be divided in to  two parts (circuit & graphic part) as seen in the below screen shot. 



In the above figure , x axis is “Vbe” with values varying from 0 to 1 v. For plotting input characteristic, measure the “Ib” and plot it on “y” axis. In order to do it, right click on the graphic plane and select “add trace” and select “Ib” from the list which is shown in figure.

Now, Click OK .Then we will get the input characteristic. You can see the plot of input characteristics in the below diagram.

Output characteristics

Output characteristics can be plotted on a graph between output voltage (Vce)  and output current (Ic) for a constant base current(Ib).The circuit diagram for plotting Output characteristics is shown below.



First, set both voltage source (Vbe & Vce) to zero. For plotting output characteristics, change the collector emitter voltage from 0 to 10v and change the base emitter voltage from 0 to 1v. Observe the collector current “Ic”. Now perform DC sweep analysis In Ltspice to plot output characteristics..

Now, click on the simulate button and select simulation command. Then from the pop-up window select “DC Sweep”. Here you need to select the option ” First Source” since we are plotting the output characteristics. So, enter the values for the below parameters. Refer the below screen.

  • Name of first voltage source – Vce
  • Type of Sweep – Linear
  • Start Value – 0
  • Stop Value -10
  • Increment- 1

The second voltage to be sweep is Vbe.

  • Name of second voltage source – Vbe
  • Type of Sweep – Linear
  • Start Value – 0
  • Stop Value -1
  • Increment- 0.01



Now click on “OK” button and click “Run”. Then the screen will be divided in to  two parts (circuit & graphic part) as seen in the below screen shot. In the below figure , x axis is “Vce” having value is vary from 0 to 10 v. For plotting output characteristic measure the “Ic” and plot it on y axis. In order to do this, right click on the graphic plane select “add trace” and select “Ic” from the list. Click OK and we will get the output characteristic. Refer the below screen shot.

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Happy reading…

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