Need of Transistor Biasing, Types, Methods, Applications, Examples
What is Transistor Biasing? Definition
Providing an external DC power supply to a transistor with proper polarity to operate it in a certain mode is called Transistor Biasing. Biasing provides the proper polarity, voltage, current, and resistance to the transistor for its operation. We know that the transistor is the most usable semiconductor device whose main function is switching or amplifying electronic signals. Generally, we have to provide the signal to its input to be switched or amplified. But just providing the input signal only cannot give us a satisfactory output. So we must provide an external DC power supply to the transistor.
Remember that just applying the DC power supply in any manner cannot be called biasing. When it is applied in a proper method and it makes the transistor ready to function properly, it can be called Biasing.
Biasing ensures the proper polarity, voltage, and current. When a Transistor is used for switching, polarity is most important for biasing but when a transistor is used for amplification, polarity, the magnitude of voltage, and current all are very important for the basing process.
Why do we need Biasing of Transistors?
In simple words, biasing is needed to activate the transistor for the operation. Let's think in a very simple way without any theory or calculation. For example, we know that a transistor can be used for amplification. What is amplification? Amplification is to increase the power of a weak signal and it can be achieved by adding extra power to that weak signal. Similarly, when we provide a weak signal to the input of the transistor to obtain a high-power strong output signal we must provide external power to the transistor. The transistor will add this external power to the input signal and make it into a strong output signal. This external power is provided by the DC power source.
Now, coming to the depth of the biasing. Biasing is not only used to simply provide external power to the transistor. A transistor generally operates in three regions,
- Active Region
- Saturation Region
- Cutoff Region
A Bipolar Junction Transistor(BJT) provides the switching function in the cutoff region and saturation region and it provides the amplification function in the active region only.
On the other hand, a MOSFET(Metal Oxide Semiconductor Field Effect Transistor) provides the switching function in the cutoff region and saturation region and it provides the amplification function in the saturation region.
As you may understand, a transistor provides a particular function when it is in a particular region. So the biasing is needed to turn a transistor in a specific region by varying voltage, current, resistance, polarity, etc. For example, if a bipolar junction transistor is in the saturation region and you want to amplify a signal, it will not give you a satisfactory output. You first turn the BJT into an active region by proper biasing then you can amplify a signal properly.
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Transistor Biasing Types and Methods
Some common methods and types of transistor biasing are,
- Emitter Bias
- Collector Feedback Bias
- Emitter Feedback Bias
- Dual Feedback Bias
- Fixed Base Bias or Fixed Resistance Bias
- Fixed Emitter Resistor Bias
- Voltage Divider Bias
In short, if we see the difference between these bias methods, Fixed Base Bias (or Fixed Resistance Bias) is the simplest method where a resistor is connected to the base but it is very unstable because changes in temperature or transistor gain can easily disturb it, whereas Collector Feedback Bias connects a resistor from collector to base giving some automatic correction (feedback), so it is more stable than fixed bias but still not very strong.
The Emitter Bias uses both positive and negative supply or an emitter resistor to improve stability, making it better than the previous ones. The Fixed Emitter Resistor Bias adds a resistor in the emitter in a fixed bias circuit, which gives some stability due to negative feedback, so it is better than simple fixed bias. The Emitter Feedback Bias is similar but focuses more on the emitter resistor providing feedback to stabilize current, so it handles temperature changes better. The Dual Feedback Bias uses feedback from both collector and emitter.
And finally the Voltage Divider Bias is the most commonly used and most stable method, where two resistors form a voltage divider to set base voltage and an emitter resistor adds extra stability, so it keeps the operating point almost constant even if temperature or transistor properties change.
Examples of Transistor Biasing
- To operate an NPN transistor we need to keep the emitter to base junction in forward bias and collector to base junction in reverse bias.
- In transistor radios, the battery should be enough or above the cut-in voltage to turn on and more voltage means more sound and proper signaling until the saturation region ends.
- LED control circuits using transistors need proper polarity and minimum threshold voltage for operating.
Applications of Different Types of Biasing
- Fixed Resistance Biasing is used for switching applications and automatic gain control of transistor amplifiers.
- Fixed Biasing is also used in linear circuits.
- Collector feedback biasing is used in voltage amplifiers and high-input impedance amplifiers.
- Emitter Resistance biasing in stabilizing circuit.
- Voltage Divider Biasing is used in comparator circuits.
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