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Current gain bjt amplifier examples

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WATCH RELATED VIDEO: Small Signal Analysis of BJT

Module 3.6


The Application Activity in this section involves a preamplifier circuit for a public address system. The complete system includes the preamplifier, a power amplifier, and a dc power supply. You will focus on the preamplifier in this section and then on the power amplifier in Section 7.

The things you learned about biasing a transistor in Section 5 are now applied in this section where bipolar junction transistor BJT circuits are used as small-signal amplifiers. The term small-signal refers to the use of signals that take up a relatively small percentage of an amplifier's operational range. Additionally, you will learn how to reduce an amplifier to an equivalent dc and ac circuit for easier analysis, and you will learn about multistage amplifiers.

The differential amplifier is also covered. The biasing of a transistor is purely a dc operation. The purpose of biasing is to establish a Q-point about which variations in current and voltage can occur in response to an ac input signal. In applications where small signal voltages must be amplified--such as from an antenna or a microphone-variations about the Q-point are relatively small.

Amplifiers designed to handle these small ac signals are often referred to as small-signal amplifiers. The American inventor Lee De Forest is one of several pioneers of radio development. De Forest experimented with receiving long distance radio signals and in patented an electronic device named the audion, which was the first amplifier.

De Forest's new three-electrode triode vacuum tube boosted radio waves as they were received and made possible what was then called "wireless telephony," which allowed the human voice, music, or any broadcast signal to be heard. Lowercase italic subscripts are used to indicate ac quantities of rms, peak, and peak-to-peak currents and voltages: for example, Ic, Ie, Ib, Vc, and Vce rms values are assumed unless otherwise stated.

Instantaneous quantities are represented by both lowercase letters and subscripts such as ic, ie, ib, and vce. In addition to currents and voltages, resistances often have different values when a circuit is analyzed from an ac viewpoint as opposed to a dc viewpoint. Lowercase subscripts are used to identify ac resistance values.

For example, Rc is the ac collector resistance, and RC is the dc collector resistance. You will see the need for this distinction later. A linear amplifier provides amplification of a signal without any distortion so that the out put signal is an exact amplified replica of the input signal.

A voltage-divider biased transistor with a sinusoidal ac source capacitively coupled to the base through C1 and a load capacitively coupled to the collector through C2 is shown in FIG. The coupling capacitors block dc and thus prevent the internal source resistance, Rs, and the load resistance, RL, from changing the dc bias voltages at the base and collector.

The capacitors ideally appear as shorts to the signal voltage. The sinusoidal source voltage causes the base voltage to vary sinusoidally above and below its dc bias level, VBQ. The resulting variation in base current produces a larger variation in collector current because of the current gain of the transistor. As the sinusoidal collector current increases, the collector voltage decreases. The collector current varies above and below its Q-point value, ICQ, in phase with the base current.

A transistor always produces a phase inversion between the base voltage and the collector voltage. The operation just described can be illustrated graphically on the ac load line, as shown in FIG. The sinusoidal voltage at the base produces a base current that varies above and below the Q-point on the ac load line, as shown by the arrows.

Ib and Ic are on different scales. Lines projected from the peaks of the base current, across to the IC axis, and down to the VCE axis, indicate the peak-to-peak variations of the collector current and collector-to emitter voltage, as shown.

The ac load line differs from the dc load line because the effective ac collector resistance is RL in parallel with RC and is less than the dc collector resistance RC alone. This difference between the dc and the ac load lines is covered in Section 7 in relation to power amplifiers. The ac load line operation of a certain amplifier extends 10 uA above and below the Q-point base current value of 50 uA as shown in FIG. Determine the resulting peak-to-peak values of collector current and collector-to-emitter voltage from the graph.

Projections on the graph of FIG. To visualize the operation of a transistor in an amplifier circuit, it is often useful to represent the device by a model circuit.

A transistor model circuit uses various internal transistor parameters to represent its operation. Transistor models are described in this section based on resistance or r parameters. Another system of parameters, called h parameters, is briefly described. The italic lowercase letter r with a prime denotes resistances internal to the transistor. TABLE 1 -- r parameters. The resulting simplified r-parameter equivalent circuit is shown in FIG.

This is the resistance "seen" looking into the emitter of a forward-biased transistor. These factors are shown with a transistor symbol in FIG. The numerator will be slightly larger for higher temperatures or transistors with a gradual instead of an abrupt junction. A manufacturer's datasheet typically specifies h hybrid parameters hi, hr, hf, and ho be cause they are relatively easy to measure. Each of the four h parameters carries a second subscript letter to designate the common-emitter e , common-base b , or common-collector c amplifier configuration, as listed in TABLE 3.

The term common refers to one of the three terminals E, B, or C that is referenced to ac ground for both input and output signals. The characteristics of each of these three BJT amplifier configurations are covered later in this section.

Relationships of h Parameters and r Parameters The ac current ratios, aac and bac, convert directly from h parameters as follows:. Because datasheets often provide only common-emitter h parameters, the following formulas show how to convert them to r parameters. We will use r parameters throughout the text because they are easier to apply and more practical.

As you have learned, a BJT can be represented in an ac model circuit. Three amplifier configurations are the common-emitter, the common-base, and the common-collector. The common-emitter CE configuration has the emitter as the common terminal, or ground, to an ac signal. CE amplifiers exhibit high voltage gain and high current gain. The input signal, Vin, is capacitively coupled to the base terminal, the output signal, Vout, is capacitively coupled from the collector to the load. The amplified output is out of phase with the input.

There is no signal at the emitter because the bypass capacitor effectively shorts the emitter to ground at the signal frequency. All amplifiers have a combination of both ac and dc operation, which must be considered, but keep in mind that the common-emitter designation refers to the ac operation. The output signal is out of phase with the input signal. As the input signal voltage changes, it causes the ac base current to change, resulting in a change in the collector current from its Q-point value.

If the base current increases, the collector current increases above its Q-point value, causing an increase in the voltage drop across RC.

This increase in the voltage across RC means that the voltage at the collector decreases from its Q-point. So, any change in input signal voltage results in an opposite change in collector signal voltage, which is a phase inversion. To analyze the amplifier in FIG.

To do this, a dc equivalent circuit is developed by removing the coupling and bypass capacitors because they appear open as far as the dc bias is concerned.

This also removes the load resistor and signal source. The dc equivalent circuit is shown in FIG. To analyze the ac signal operation of an amplifier, an ac equivalent circuit is developed as follows:. This is why a dc source is called an ac ground. The ac equivalent circuit for the common-emitter amplifier in FIG. Notice that both RC and R1 have one end connected to ac ground red be cause, in the actual circuit, they are connected to VCC which is, in effect, ac ground.

In ac analysis, the ac ground and the actual ground are treated as the same point electrically. The amplifier in FIG. Ground is the common point in the circuit. An ac voltage source, Vs, is shown connected to the input in FIG. If, however, the ac source has a nonzero internal resistance, then three factors must be taken into account in determining the actual signal voltage at the base. These are the source resistance Rs , the bias resistance R1 R2 , and the ac input resistance at the base of the transistor Rin base.

This is illustrated in FIG. A high value of input resistance is desirable so that the amplifier will not excessively load the signal source. This is opposite to the requirement for a stable Q-point, which requires smaller resistors. The conflicting requirement for high input resistance and stable biasing is but one of the many trade-offs that must be considered when choosing components for a circuit.

The total input resistance is expressed by the following formula:. As you can see in the figure, the source voltage, Vs, is divided down by Rs source resistance and Rin tot so that the signal voltage at the base of the transistor is found by the volt age-divider formula as follows:.

To develop an expression for the ac input resistance looking in at the base, use the simplified r-parameter model of the transistor. The input resistance looking in at the base is:. The output resistance of the common-emitter amplifier is the resistance looking in at the collector and is approximately equal to the collector resistor. The ac voltage gain expression for the common-emitter amplifier is developed using the model circuit in FIG.

The gain is the ratio of ac output voltage at the collector Vc to ac input voltage at the base Vb. To get the overall gain of the amplifier from the source voltage to collector, the attenuation of the input circuit must be included. Attenuation is the reduction in signal voltage as it passes through a circuit and corresponds to a gain of less than 1.

For example, if the signal amplitude is reduced by half, the attenuation is 2, which can be expressed as a gain of 0. Suppose a source produces a 10 mV input signal and the source resistance combined with the load resistance results in a 2 mV output signal.

That is, the input signal is reduced by a factor of 5. Assume that the amplifier in FIG.


Electronic devices: BJT Amplifiers [part 1]

For a transistor to act as an amplifier, it should be properly biased. We will discuss the need for proper biasing in the next chapter. Here, let us focus how a transistor works as an amplifier. A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal.

Since the current inside the transistor comprises both electrons and DC current gain which is the ratio of collector current to base current.

BJT Amplifier


Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. I'm asking because my answer does not agree with that of the book, and I can't see the fault in my calculation:. The left and right side schematics below are entirely equivalent to each other within numerical truncation errors :. It is the Thevenin voltage that precedes the Thevenin resistance to the base. In actual fact, it depends upon the collector current in active mode, anyway.

How to Design Common Emitter Amplifier

current gain bjt amplifier examples

This voltage phase shift can be explained as follows:. The ac emitter resistance of a transistor is a dynamic value like zener impedance that is used only in ac calculations. For a small-signal amplifier, the value of. The process used to determine the value of is demonstrated in Example 9.

Figure below. It is called the common-emitter configuration because ignoring the power supply battery both the signal source and the load share the emitter lead as a common connection point shown in Figure below.

Electronic Devices - BJT Amplifiers


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. The basic amplifier, figure 9. The transistor, as we have seen in the previous chapter, is a three-terminal device.

Basic Electronics | BJT Amplifiers

Solved examples with detailed answer description, explanation are given and it would be easy to understand. Here you can find objective type Electronic Devices BJT Amplifiers questions and answers for interview and entrance examination. Multiple choice and true or false type questions are also provided. You can easily solve all kind of Electronic Devices questions based on BJT Amplifiers by practicing the objective type exercises given below, also get shortcut methods to solve Electronic Devices BJT Amplifiers problems. Refer to this figure. Find the value of I E. All Rights Reserved. Contact us: info.

uses a diode and a current source to duplicate the behavior of the transistor. frequency transistor applications. Current gain from voltage gain.

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There are different types of transistor amplifiers operated by using an AC signal input. This is interchanged between the positive value and negative value, hence this is the one way of presenting the common emitter amplifier circuit to function between two peak values. This process is known as the biasing amplifier and it is an important amplifier design to establish the exact operating point of a transistor amplifier which is ready to receive the signals hence it can reduce any distortion to the output signal. In this article, we will discuss common emitter amplifier analysis. The Amplifier is an electronic circuit that is used to increase the strength of a weak input signal in terms of voltage, current, or power. The process of increasing the strength of a weak signal is known as Amplification.

The quiescent collector current I C , of a transistor is increased by changing resistances. As a result.

Lesson 2 - Bipolar Transistor Lesson Plan introduction Introduction Transistor structures and symbols Transistor currents Current gain Transistor connections. Title: Bipolar Transistor Objectives: - Know the structure and symbols of bipolar transistors - Able to calculate the current gain - Understand how the currents in a transistor are split Figure Introduction Amplifier Principle Fig. The thin and lightly doped base gives almost all these electrons enough time to diffuse into the collector. These electrons flow through the collector, through R C , and into the positive terminal of the V CC voltage source.

The Application Activity in this section involves a preamplifier circuit for a public address system. The complete system includes the preamplifier, a power amplifier, and a dc power supply. You will focus on the preamplifier in this section and then on the power amplifier in Section 7. The things you learned about biasing a transistor in Section 5 are now applied in this section where bipolar junction transistor BJT circuits are used as small-signal amplifiers.




Comments: 4
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  1. Brlety

    I think you will allow the mistake. I offer to discuss it.

  2. Malvyn

    Instead of criticism write the variants is better.

  3. Vitaxe

    It is possible to speak on this subject for a long time.

  4. Gersham

    I apologize, but it doesn't come my way. Can the variants still exist?