How Common Emitter Transistor Characteristics Impact Circuits
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.
Apparatus
First, you need to have NPN transistor
A 3-volt battery and a 30-volt battery
Two high resistance rheostats are a must for this experiment.
Also, you need to have 0-3 volt voltmeter along with 0-30 volt voltmeter
One 0-50 μA micro-ammeter and one 0-50 mA milli-ammeter
Liewksie, you need to have two one way keys.
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
First, you have to put everything in the order as asked by the teacher who is administrating the process.
Make sure that all your connections are well placed and tight in their positions.
Now, check the least count and the zero error of your voltmeters and ammeters.
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
To read the base voltage and the base current, you need to apply a forward bias voltage to the base junction.
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.
You have to repeat the given steps 4-5 times by making the collector voltage reach up to 10V.
Once you get all the readings to make sure, turn everything back to zero.
Taking Readings for Output Characteristics
First, you need to adjust the collector voltage to zero and base current to 10 milliamperes. After that, write down the collector current.
Now to note down, more readings make the collector voltage go up from 10V, 20V to 30V.
Repeat the steps with the base current 20, 30, and 40 milliamperes.
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.
FAQs on Key Characteristics of Common Emitter Configuration in NPN & PNP Transistors
1. What is meant by a common-emitter (CE) configuration of a transistor?
In a common-emitter (CE) configuration, the emitter terminal of the transistor is made common to both the input and output circuits. The input signal is applied between the base and the emitter, while the output is taken from between the collector and the emitter. This setup is the most widely used transistor configuration in electronic circuits.
2. What are the input characteristics of an NPN transistor in a common-emitter configuration?
The input characteristics describe the relationship between the input current (base current, I_B) and the input voltage (base-emitter voltage, V_BE). This is represented by a curve plotting I_B against V_BE while keeping the output voltage (collector-emitter voltage, V_CE) constant. The curve resembles that of a forward-biased p-n junction diode, showing that the base current increases exponentially after the knee voltage (around 0.7V for silicon) is crossed.
3. What do the output characteristic curves for a common-emitter transistor show?
The output characteristic curves show the relationship between the output current (collector current, I_C) and the output voltage (collector-emitter voltage, V_CE) for a constant input current (base current, I_B). These curves are plotted for different fixed values of I_B and typically show three distinct regions of operation:
- Active Region: Where I_C is almost constant and proportional to I_B. This is the primary region for signal amplification.
- Saturation Region: Where V_CE is very low, and I_C is high. The transistor acts like a closed switch.
- Cut-off Region: Where the base current I_B is zero, causing the collector current I_C to be nearly zero. The transistor acts like an open switch.
4. How is the current amplification factor (β) defined in the common-emitter configuration?
The current amplification factor, denoted by the Greek letter beta (β), is a key parameter for a transistor in the common-emitter configuration. It is defined as the ratio of the change in collector current (ΔI_C) to the corresponding change in base current (ΔI_B) while the collector-emitter voltage (V_CE) is kept constant. It indicates the current gain of the transistor and is typically a large value (often 50 to 300).
5. Why is the common-emitter configuration the most widely used for transistor amplifiers?
The common-emitter (CE) configuration is preferred for most amplifier applications because it is the only configuration that provides both significant current gain and significant voltage gain. This results in a very high power gain, which is the product of voltage and current gain. Additionally, it has moderate input and output impedances, making it easier to match with other circuit stages.
6. What is the fundamental difference between the characteristics of an NPN and a PNP transistor in common-emitter mode?
While the shapes of the characteristic curves for NPN and PNP transistors are similar, the key difference lies in the polarities of the voltages and the directions of the currents. For an NPN transistor, the base-emitter and collector-emitter voltages (V_BE and V_CE) are positive, and current flows into the base and collector. For a PNP transistor, these voltages are negative, and conventional current flows out of the base and collector terminals.
7. Why is the input resistance low and the output resistance high in a CE configuration?
The input resistance is low because the input circuit involves the base-emitter junction, which is forward-biased during normal operation. A forward-biased junction offers very little opposition to current flow. Conversely, the output resistance is high because the output circuit involves the collector-base junction, which is reverse-biased. A reverse-biased junction offers very high opposition to current flow, resulting in high output resistance.
8. What happens to the collector current if the base current is zero in a common-emitter setup?
If the base current (I_B) is zero, the transistor enters the cut-off region. In this state, the transistor is effectively 'off'. Ideally, the collector current (I_C) should also be zero. However, in a real transistor, a very small current called the collector-emitter leakage current (I_CEO) still flows from the collector to the emitter. For most practical purposes as per the CBSE 2025-26 syllabus, when I_B is zero, I_C is considered to be zero.
