Biasing amplifier transistors

Transistors are developed in the year by the physicists of America named John Bardeen. Before the use of transistors, vacuum tubes used for the purpose to control electronic signals. But the complexity of designing vacuum tubes, more amount of power consumption paved the way for the inception of transistors in modern electronics. The transistors are one of the most frequently used semiconductors utilized in the applications of switching and amplification. To make the transistor function convincingly it must possess certain conditions to operate.

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Content:

• 4.10 Transistor Biasing Calculations
• Biasing a Bipolar Transistor in Common Emitter Configuration
• What is the biasing condition of junctions in bipolar junction transistor to work as an amplifier?
• Types of Bias
• Bipolar Junction Transistor Fixed Base Biasing Electronics Engineering Notes 1st Year
• Transistor as an Amplifier: Working & Circuit | NPN Transistor Amplifier
• Module 1.2
• Section 5 5 Biasing of BJT Amplifier circuit
• Biasing That Transistor: The Common Emitter Amplifier
• PCB Design & Analysis

WATCH RELATED VIDEO: Common Emitter Amplifier Biasing Calculations

4.10 Transistor Biasing Calculations

In the previous discussions, we assumed that for every portion of the input signal there was an output from the amplifier. This is not always the case with amplifiers. It may be desirable to have the transistor conducting for only a portion of the input signal. The portion of the input for which there is an output determines the class of operation of the amplifier. There are four basic classes of amplifier operations. Class A amplifiers are biased so that variations in input signal polarities occur within the limits of cutoff and saturation.

In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff. Saturation occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow.

Biasing an amplifier in this manner places the DC operating point between cutoff and saturation and allows collector current to flow during the complete cycle degrees of the input signal, thus providing an output which is a replica of the input.

The basic transistor amplifier discussed previously is an example of a class A amplifier. Although the output from this amplifier is degrees out of phase with the input, the output current still flows for the complete duration of the input. The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples.

For a comparison of output signals for the different amplifier classes of operation, refer to figure below during the following discussion. Amplifiers designed for class AB operation are biased so that collector current is zero cutoff for a portion of one alternation of the input signal.

This is accomplished by making the forward-bias voltage less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased during one alternation for the amount of time that the input signal voltage opposes and exceeds the value of forward-bias voltage.

Therefore, collector current will flow for more than degrees but less than degrees of the input signal, as shown in the figure above view B.

As compared to the class A amplifier, the DC operating point for the class AB amplifier is closer to cutoff. The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion. Amplifiers biased so that collector current is cut off during one-half of the input signal are classified class B.

The DC operating point for this class of amplifier is set up so that base current is zero with no input signal. When a signal is applied, one half cycle will forward bias the base-emitter junction and I C will flow. The other half cycle will reverse bias the base-emitter junction and I C will be cut off. Thus, for class B operation, collector current will flow for approximately degrees half of the input signal, as shown in the figure above view C. The class B operated amplifier is used extensively for audio amplifiers that require high-power outputs.

It is also used as the driver- and power-amplifier stages of transmitters. In class C operation, collector current flows for less than one half cycle of the input signal, as shown in figure above view D. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the DC operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow.

The class C operated amplifier is used as a radio-frequency amplifier in transmitters. From the previous discussion, you can conclude that two primary items determine the class of operation of an amplifier - 1 the amount of bias and 2 the amplitude of the input signal.

With a given input signal and bias level, you can change the operation of an amplifier from class A to class B just by removing forward bias. Also, a class A amplifier can be changed to class AB by increasing the input signal amplitude. However, if an input signal amplitude is increased to the point that the transistor goes into saturation and cutoff, it is then called an overdriven amplifier. You should be familiar with two terms used in conjunction with amplifiers - fidelity and efficiency.

Fidelity is the faithful reproduction of a signal. In other words, if the output of an amplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. The opposite of fidelity is a term we mentioned earlier - distortion. Therefore, a circuit that has high fidelity has low distortion.

In conclusion, a class A amplifier has a high degree of fidelity. A class AB amplifier has less fidelity, and class B and class C amplifiers have low or "poor" fidelity.

The efficiency of an amplifier refers to the ratio of output-signal power compared to the total input power. An amplifier has two input power sources: one from the signal, and one from the power supply. Since every device takes power to operate, an amplifier that operates for degrees of the input signal uses more power than if operated for degrees of the input signal.

By using more power, an amplifier has less power available for the output signal; thus the efficiency of the amplifier is low. This is the case with the class A amplifier. It operates for degrees of the input signal and requires a relatively large input from the power supply. Even with no input signal, the class A amplifier still uses power from the power supply.

Therefore, the output from the class A amplifier is relatively small compared to the total input power. This results in low efficiency, which is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity. Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation of the input, which results in less total input power than the class A amplifier. This leads to better efficiency.

Class B amplifiers are biased with little or no collector current at the DC operating point. With no input signal, there is little wasted power. Therefore, the efficiency of class B amplifiers is higher still. The efficiency of class C is the highest of the four classes of amplifier operations. Now that we have analyzed the basic transistor amplifier in terms of circuit configuration, class of operation, and bias, let's apply what has been covered to this amplifier.

A reproduction of this amplifier is shown below for your convenience. This illustration is not just the basic transistor amplifier shown earlier but a class A amplifier configured as a common emitter using fixed bias.

From this, you should be able to conclude the following:. Because of its fixed bias, the amplifier is thermally unstable. Because of its class A operation, the amplifier has low efficiency but good fidelity. Because it is configured as a common emitter, the amplifier has good voltage, current, and power gain. In conclusion, the circuit configuration, class of operation, and type of bias are all clues to the function and possible application of the amplifier.

Bipolar Junction Transistors Amplifier Classes of Operation In the previous discussions, we assumed that for every portion of the input signal there was an output from the amplifier. Class A Amplifier Operation Class A amplifiers are biased so that variations in input signal polarities occur within the limits of cutoff and saturation.

A comparison of output signals for the different amplifier classes of operation. All Rights Reserved.

Biasing a Bipolar Transistor in Common Emitter Configuration

This circuit is simple to assemble and troubleshoot. There are fewer components used in this circuit than more elaborate biasing schemes require. The individual components can be swapped and replaced one at a time to see the effects on the performance of the circuit. In this way you can collect enough information to get intuition about how the circuit functions and how to tailor it so meet the specific needs of your application.

What is the biasing condition of junctions in bipolar junction transistor to work as an amplifier?

If you open up the perennial favourite electronics textbook The Art Of Electronics and turn to the section on transistors, you will see a little cartoon. If you apply a little more base current, he pushes up the collector a bit. If you wind back the base current, he drops it back. Of course the base-emitter junction is a diode and it is not a simple potentiometer that sits between collector and emitter. Fortunately it is possible to work with transistors without such an in-depth understanding of their operation, but before selecting the components surrounding a device it is still necessary to go a little way beyond transistor man. Imagine for a moment a simple transistor circuit involving a single NPN transistor with its emitter grounded, its collector tied to the positive supply by a resistor, and a potentiometer between ground and supply allowing any voltage to be supplied to the base. Because the emitter is grounded, even if sometimes via a resistor, this transistor configuration is referred to as a Common Emitter amplifier. In this circuit if you were to start with the potentiometer at the grounded end then the transistor would be turned off, and no current would flow. In an NPN transistor, the connection between base and collector is a PN junction, so as you might expect it shares its properties with the PN junction in a diode. A silicon diode starts to conduct when the voltage across it reaches about 0.

Types of Bias

Home » Bipolar Transistor Biasing. Going back to the basics is never a bad idea. Many electronics engineers are fluent with complex systems—such as microcontrollers, embedded OSes, or FPGAs—but seem to have more difficulties with single transistors. What a shame! A transistor can be a more adequate and cost-effective solution than an IC in many projects.

Bipolar Junction Transistor Fixed Base Biasing Electronics Engineering Notes 1st Year

Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor. Establishing the correct operating point requires the proper selection of bias resistors and load resistors to provide the appropriate input current and collector voltage conditions. When a bipolar transistor is biased so that the Q-point is near the middle of its operating range, that is approximately halfway between cut-off and saturation, it is said to be operating as a Class-A amplifier. This mode of operation allows the output current to increase and decrease around the amplifiers Q-point without distortion as the input signal swings through a complete cycle. In other words, the output current flows for the full o of the input cycle.

Transistor as an Amplifier: Working & Circuit | NPN Transistor Amplifier

Since it was invented, the transistor from 'transfer resistor' has come a long way. Early transistors were made from germanium, which was 'doped' with other materials to give the desired properties required for a semiconductor. The NPN transistors that were available at that time were low power and did not work as well as their PNP counterparts. When silicon was first used, the opposite was the case, and for quite some time the only really high power devices available were all of silicon NPN construction. Germanium is rarely used any more, although some examples are still available. A transistor can be represented as two diodes, with a junction in the middle.

Module 1.2

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Section 5 5 Biasing of BJT Amplifier circuit

RELATED VIDEO: Biasing an Audio Transistor

The term amplifier as used in this chapter means a circuit or stage using a single active device rather than a complete system such as an integrated circuit operational amplifier. An amplifier is a device for increasing the power of a signal. This is accomplished by taking energy from a power supply and controlling the output to duplicate the shape of the input signal but with a larger voltage or current amplitude. In this sense, an amplifier may be thought of as modulating the voltage or current of the power supply to produce its output.

Biasing That Transistor: The Common Emitter Amplifier

You can now explain with confidence what p-doping, n-doping, and depletion layers mean. Now you will put that knowledge to use. You have the transistor in your hand. You stare at it, knowing the power it contains and what it has done for the world. Here you will use your transistor to amplify some spikes.

PCB Design & Analysis

The Web This site. Transistors in amplifiers commonly use one of three basic modes of connection. Whether collector, base or emitter is chosen as being common to both input and output has a marked effect on how a transistor amplifier operates.