Characteristics of Common Emitter of NPN or PNP Transistors

Common-Emitter NPN Transistor

In order to study the characteristics of the common-emitter, we need to perform an experiment and take out its readings. For this experiment, you need to have these following materials as the apparatus. There is no theoretical way from which you can find the characteristics of the common-emitter of NPN transistors. So today, we will be helping you.


  1. First, you need to have NPN transistor

  2. A 3-volt battery and a 30-volt battery 

  3. Two high resistance rheostats are a must for this experiment. 

  4. Also, you need to have 0-3 volt voltmeter along with 0-30 volt voltmeter 

  5. One 0-50 μA micro-ammeter and one 0-50 mA milli-ammeter 

  6. Liewksie, you need to have two one way keys.

  7. Lastly, connecting wires to connect everything. 

First, let’s discuss what it meant to be a common emitter configuration. When the given emitter is connected to the collector and base, it is called an emitter in a common configuration. 

The input of the circuit is connected to the emitter and the base. The output of the circuit is taken from the collector and emitter. As a result, the emitter you have in the experiment is common in both input and output of the system, and hence it was the common configuration thus, it is named a common emitter. 

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We will be working to find the common base transistor characteristics experiment readings.

Common-emitter NPN Transistor Characteristics Experiment

The input resistance that we have in the given experiment is minimal because of the forward biasing of the current in the collector junction that provides us with resistance. 

\[R_{i} = \frac{\Delta V_{b}}{\Delta I_{b}}\]

The output that we get from the collector junction has a high value due to the presence of rever biasing. 

\[R_{o} = \frac{\Delta V_{c}}{\Delta I_{c}}\]

Talking about current gain in the common base configuration is the change in the current of the collector divided by the change in emitter current while keeping the base to collector voltage constant. 

\[\beta = \frac{\Delta I_{c}}{\Delta I_{b}}\]

Procedure to Conduct the Experiment 

  1. First, you have to put everything in the order as asked by the teacher who is administrating the process. 

  2. Make sure that all your connections are well placed and tight in their positions.

  3. Now, check the least count and the zero error of your voltmeters and ammeters.

  4. Lastly, before you start inserting the keys to take out reading, it is better to check the voltmeter readings as they need to be zero. 

Taking Readings for Input Characteristics 

  1. To read the base voltage and the base current, you need to apply a forward bias voltage to the base junction.

  2. The base voltage should be increased until you see the base current growing exponentially. Here you need to take out the reading of each base current for base voltage. 

  3. You have to repeat the given steps 4-5 times by making the collector voltage reach up to 10V.

  4. Once you get all the readings to make sure, turn everything back to zero. 

Taking Readings for Output Characteristics

  1. First, you need to adjust the collector voltage to zero and base current to 10 milliamperes. After that, write down the collector current. 

  2. Now to note down, more readings make the collector voltage go up from 10V, 20V to 30V.

  3. Repeat the steps with the base current 20, 30, and 40 milliamperes.

  4. Finally, record all the observations and write them down in your practical file. 

How to Identify PNP or NPN Transistor?

NPN transistors are more intuitive in comparison to PNP transistors in terms of voltage and current behaviour.

NPN is used in many cases to drive out the straightforward interface to digital output signals. 

When compared to PNP, NPN has some serious advantage due to the charge that carries in the PNP transistor are holes as it is formed via p-type doping.

In addition to this, NPN is far easier and cheaper to built then PNP transistors.

The full form of PNP is positive-negative-positive, and NPN is negative-positive-negative. 

Likewise, the NPN's charge carrier is electrons; thus, the switching time of NPN is faster.

The collector terminal is used to transmit positive voltage in NPN.

Lastly, in NPN, the current flows from collector to emitter as a result of positive supply, which was given by the base. Whereas in PNP, the flow of current starts from the emitter and ends at the collector. 

FAQ (Frequently Asked Questions)

1. How can I differentiate between NPN and PNP sensors?

When you are performing experiments, it is necessary that you know whether the sensor’s output needs to be in PNP and NPN. to confirm if the sensor is NPN or PNP, you need to force the sensor to the output and at that time check the meter display. The meter read numbers between 10 to 30, thus the given sensor is a PNP type transistor, which is also known as sourcing. 

On the other hand, if the display meter remains at point zero, your sensor is in NPN type, also known as sinking.

2. How does a single NPN transistor gets connected to a relay mechanism?

First, you need to know whether the transistor is current sinking (NPN) or current sourcing (PNP).

If your transistor is NPN, you have to connect it between a + power supply (Vcc) and the output (Vout).

On the other hand, if your system has got a PNP transistor, the relay needs to be connected between the sensor output and the ground.