BASIC ELECTRONICS - PowerPoint Presentation

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BASIC ELECTRONICSSlide2

Transistor Audio Power Amplifiers

Introduction

A transistor amplifier that raises the power level of the signals that have audio range is known as transistor audio power amplifier. In general, the last stage of a multistage amplifier the power stage. The power amplifier differs from all the previous stages in that here a concentrated effort is made to obtain maximum output power. A transistor that is suitable for power amplification is generally called a power transistor. It differs from other transistors mostly in size; it is considerably larger to provide for handling the great amount of power.

The main criterion for a power amplifier is the maximum a.c. power output. Now an amplifier convert s d.c. power from supply into a.c power output. Therefore, the ability of a power amplifier to convert d.c poer from supply into a.c. output power is a measure of its effectiveness. This is known as collector efficiency. For instance, if the d.c.power supplied by the battery is 10 W and a.c. output power is 2 W, then collector efficiency is 2/10=20%. The greater the collector efficiency, large is the a.c. power output and hence better is the power amplifier.Slide3

Classification of Power Amplifiers

Transistor power amplifiers handle large signals. Many of them are driven so hard by the input large signal that collector current is either cut off or is in the saturation region during a large portion of the input cycle. Therefore, such amplifiers are generally classified according to their mode of operation i.e. the portion of the input cycle during which the collector current is expected to flow. On this basis, they are classified as:

(i) Class A power amplifier

(ii) Class B power amplifier

(iii) Class C power amplifier

(i) In class A power amplifier, the collector current flows at all times during the full cycle of the signal. Obviously, for this to happen, the power amplifier must be biased in such a way that no part of the signal is cut off. As the output wave shape is exactly similar to the input wave shape, therefore, such amplifiers have least distortion. However, they have low collector efficiency (about 35%)Slide4

(ii) In class B power amplifier, the collector current flows during the positive half-cycle of the input signal only. Obviously, in class B operation, no biasing circuit is needed at all. Since negative half-cycle of the signal is cut off in class B amplifier, a severe distortion occurs. However, class B power amplifiers provided higher power output and collector efficiency (50-60%). Such amplifiers are mostly used for power amplification in push pull arrangement. In such an arrangement, two transistors are used in class B operation. One transistor amplifies the positive half –cycle of the signal while the other amplifies the negative half-cycle.

(iii) In class C power amplifier, the collector current flows for less than half-cycle of the input signal. In class C operation, the base is given some negative bias so that collector current does not flow just when the positive half-cycle of the signal starts. Such amplifiers are never used for general amplification. However, they are used as tuned amplifiers i.e. to amplify a narrow band of frequencies near the resonant frequency.

Collector efficiency

It is the ratio of a.c. output power to the d.c. input power of an amplifier i.e.

Collector Collector efficiency indicates how well a power amplifier can convert d.c. input power (i.e. power from collector supply) into a.c. output power. The greater the collector efficiency, the better is the power amplifier.Slide5

Important Points About Power Amplifiers

The following are some important points about power amplifier:

(

i) Power amplifiers are generally transformer coupled to permit impedance matching.

(ii) Power amplifiers usually used a push-pull circuit i.e. two transistors are used in class B operation for amplification. This permits higher collector efficiency.

(iii) If a single transistor is used for power amplification, it must be used in class A operation.

(iv) The most important consideration in power amplifiers is the collector efficiency.Slide6

TRANSISTOR TUNED AMPLIFIERS

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Inroduction

Sometimes it is desired that an amplifier should amplify either a single frequency or a narrow band of frequencies. For instance, radio and television transmission are carried on specific radio frequency assigned to the broadcasting stations. To achieve this, the simple resistive load in the collector is replaced by a tuned LC circuit whose impedance strongly depends upon frequency. Such a tuned circuit becomes very selective and amplifies very strongly signals of resonant frequency and a narrow band on either side. Therefore, the use of the tuned LC circuit in conjunction with a transistor makes possible the selection and efficient amplification of particular desired frequency. Such an amplifier is called a tuned amplifier.Slide8

Series tuned circuit.

It is essentially a series LC circuit. The most important characteristic of this circuit is that at some frequency (called resonant frequency), the inductive reactance and capacitive reactance become equal, resulting in minimum circuit impedance and maximum circuit current. Under such conditions, the circuit is said to be in series resonance. At series resonance:

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(

i) Resonant

frequency

Where L and C are in

Henery and Farad respectively. Slide10

(

ii) Circuit impedance = R, the circuit resistance. (

iii

) Circuit current

(iv) Q of coil Q of capacitor Q of circuit (v) Bandwidth of circuit Slide11

Tuned Amplifiers.

A tuned amplifier uses a tuned LC load in the collector instead of a resistive load. A tuned LC permits three principal advantages. First, it enables to select one frequency (i.e.

resonant frequency) out of a number of frequencies present. Second, a tuned amplifier is always operated in class C mode for higher collector efficiency. The tuned LC load nullifies the distortion introduced due to class C operation. Third, by adjusting the coupling between the load and tank circuit, we can achieve impedance matching. The results in the maximum transfer of power to the load.Slide12

Tuned amplifiers find wide application in electronics. For example, radio and television receives use tuned amplifiers to select one radio frequency from the many being broadcast. They are also used in other forms of communication such as a radar, sonar and telemetry.Slide13

BASIC ELECTRONICS

MODULATION

&

DEMODULATIONSlide14

Introduction

In radio transmission, it is necessary to send the audio signal (20

Hz

to 20

KHz) such as music, speech etc. from a broadcasting station over great distances to a receiver. This communication of audio signal does not employ any wire and is sometimes called wireless. The audio signal cannot be sent directly over the air to appreciable distance because radiation of electrical energy is practicable only at high frequencies e.g. above 30 KHz. This difficulty is overcome by superimposing the electrical audio signal on a very high frequency wave called carrier wave. The resultant waves are called modulated waves or radio waves and the process is called “modulation”. The modulated or radio waves are sent out from the broadcasting station and carry the audio signal to larger distances. At the radio receiver, the audio signal is extracted from the modulated wave by the process of “demodulation”. The audio signal is then amplified and fed to the speaker for sound reproduction. Slide15

Modulation

The process of changing some characteristics (

e.g.

Amplitude, frequency or phase) of a carrier wave in accordance with the intensity of signal is known as ‘modulation’. Modulation means to “change”. In modulation, some characteristic of carrier is changed in accordance with the intensity of the signal. The resultant wave is called the modulated or radio wave and contains the audio signal. Therefore, modulation permits the transmission to occur at high frequency while it simultaneously allows carrying of the audio signal. Depending upon which characteristic of carrier is changed, the modulation can be of three types:

(i) Amplitude modulation (ii) Frequency modulation (iii) Phase modulation In amplitude modulation, the amplitude of the carrier is changed in accordance with the intensity of signal while the frequency of carrier remains unchanged. The audio signal is contained in the amplitude variation of the resultant AM wave. In frequency of the carrier is changed in accordance with the intensity of the signal. The audio signal is contained in the frequency variations of the

FM wave. In india, amplitude modulation is used in radio broadcasting. However, in TV transmission, frequency modulation is used for sound and amplitude modulation for picture signal. Slide16

Modulation Factor

An important consideration in amplitude modulation is to describe the extent to which the amplitude of the carrier wave is changed by the signal. This is described by a factor called modulation factor

m i.e.

Amplitude change of carrier Modulation factor, m

= ––––––––––––––––––––––

Normal carrier amplitude

The value of modulation factor depends upon the amplitude of the carrier and the signal. For example, if both carrier and signal have the same amplitude, say A, then modulation factor is A/A = 1 or 100%. The modulation factor indicates the strength and quality of the transmitted signal. The greater the modulation factor, the stronger and clearer will be the signal. However, modulation should not be more than 100% otherwise distortion will occur.Slide17

Demodulation

The process of recovering signal from the modulated wave is known as “demodulation”. It is essentially the reverse of modulation and is, therefore, rightly called demodulation or “detection”. Demodulation is done in the radio receiver by a circuit called “demodulator”. One of such circuits is a diode demodulator. Here the modulated wave is first rectified

i.e.

negative half of the modulated wave is eliminated. The rectified modulated wave contains the audio signal and the carrier. The carrier is then removed by a filter circuit and the recovered audio signal is amplified and fed to the speaker for sound reproduction.Slide18

Superhetrodyne receiver

It was first designed by Major Edwin H. Armstrong during the First World War. At present, all modern receivers utilise the superhetrodyne circuit. In this type of radio receiver, the selected radio frequency is converted to a fixed lower frequency, called

intermediate frequency (IF).

This is achieved by a special electronic circuit called the mixer circuit.

There is a local oscillator in the radio receiver itself. This oscillator produces high frequency waves. The selected radio wave is mixed with the high frequency wave in the mixer circuit. In this process, beats are produced and the mixer produces a frequency equal to the difference between local oscillator and radio wave frequency. The circuit is so designed that oscillator always produces a frequency 455 KHz above the selected radio frequency. Therefore, the mixer will always produce an intermediate frequency of 455 KHz regardless of the station to which the receiver is tuned. Slide19

ii. The obtaining of fixed IF (i.e. 455 KHz) is known as superhetrodyne

principle. As an example, if 600 KHz station is tuned, then local

oscillator will produce a frequency of 1055 KHz. Consequently, the

output from the mixer will have a frequency of 455 KHz.It is worthwhile to give a passing reference to the utility of getting fixed intermediate frequency in a superhetrodyne circuit. At this fixed intermediate frequency, the amplifier circuits operate with maximum stability, selectivity and sensitivity. As the conversion of incoming radio frequency to the intermediate frequency is achieved by hetrodyning or beating the local oscillator against radio frequency, therefore, this circuit is called superhetrodyne circuit. Slide20

Characteristics of a radio receiver

The ultimate goal of a radio receiver is to extract the signal from the desired radio wave and build up this signal to such a level so that it can operate the load (

e.g.

Speaker, earphones). In checking the specifications of a radio receiver, four characteristics are of general importance viz sensitivity, selectivity, fidelity and noise figure.

(i) Sensitivity. It is a measure of the level of the input signal to produce “the standard output”. Clearly, sensitivity is a function of the amount of amplification or number of stages used in the receiver. At first thought it might seem that a receiver could be designed for any degree of sensitivity merely by increasing the number of stages. Although the gain would be increased, it may not be usable. When the noise level generated within the receiver is stronger than the signal to be received, increased sensitivity is wasted. The receiver output will contain more noise than signal.Slide21

(ii)

Selectivity. It is measure of the ability of a radio receiver to select the desired radio wave while rejecting all others. This function is performed by the tuned circuits ahead of the detector stage. The selectivity of a radio receiver depends upon the number of tuned circuits and the

Q

of these circuits. (iii) Fidelity. It is a measure of how faithfully the receiver reproduces the original signal. A receiver is said have high fidelity if it amplifies all the frequencies equally well.(iv) Noise figure. Although a receiver can be designed for any gain, the usable gain is limited by the noise output from the receiver. In a radio receiver is tuned between stations. An ideal receiver-one that generates no noise—would have a noise figure of unity or 0 db.