![Copyright
©
2017
by
S
Chand
And
Company
Limited.
All
rights
reserved.
CAUSES OF AMPLIFIER GAIN
VARIATIONS
The primary cause of gain variation in amplifiers is the presence
of capacitances, some of which are connected in series along
the signal path and some in parallel.
The different types of external capacitances present in an
amplifier circuit are
• Coupling and bypass capacitors–usually of large capacitance
value, [Fig. 60.37 (a)].
• Internal or inter-element capacitances of the transistor and
stray wiring capacitance [Fig. 60.37 (b)]
41](https://image.slidesharecdn.com/17117908-230227235138-c6691a37/85/17117908-ppt-41-320.jpg)
17117908.ppt
- 1. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TRANSFORMER-COUPLED CLASS-A AMPLIFIER Power Diagram • The power distribution for a transformer coupled class-A amplifier is shown in Fig. 60.23. Since load loss is zero, the triangle A of Fig. 60.17 is nonexistent. Moreover, the load line of Fig. 60.17 becomes a.c. load line in Fig. 60.23 Fig.60.23 1
- 2. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TRANSFORMER-COUPLED CLASS-A AMPLIFIER Consequently, = . Area of triangle B equals that of triangle C. Also, sum of A and B represents power delivered to the transistor. It can be proved that maximum possible overall efficiency and the maximum possible collector efficiency of a class-A amplifier using an output transformer are both 50%. In general, overall efficiency of a transformer-coupled class-A amplifier is given by CEQ V VCC 2
- 3. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TRANSFORMER-COUPLED CLASS-A AMPLIFIER Proof • The conditions for the development of maximum ac power are shown in Fig. 60.24. It must be kept in mind that here Fig. 60.24 3
- 5. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B AMPLIFIER Biasing condition for class-B operation has been shown both on current transfer characteristic and output characteristic for CE configuration in Fig. 60.25. As seen, the transistor has been biased to cut-off. It remains FR- biased for only half-cycle of the input signal. Hence, its conduction angle is only 180°. It is obvious that with zero signal, its collector current is zero. 5
- 6. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B AMPLIFIER Fig. 60.25 6
- 7. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B AMPLIFIER Since negative half-cycles are totally absent from the output, the signal distortion is high as compared to class-A amplifiers. Since input voltage is large (upto ), voltage amplification is reduced. Zero-signal input represents worst condition for class-A amplifiers but best condition for class-B amplifiers. In class-B amplifiers, transistor dissipates more power with increase in signal strength but opposite is the case in class-A amplifiers. CC V 7
- 8. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B AMPLIFIER Average current in class-B operation is less than in class-A, hence power dissipated is less. Consequently, maximum circuit (or overall) efficiency of a class-B amplifier is 78.5% when peak signal makes (min) = 0. In general The half-sine wave type of collector current contains very pronounced even harmonics (particularly the second one). These impulses can be rendered useful either by employing push-pull circuit or by using tuned amplifiers. CE V 8
- 9. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B AMPLIFIER Power Relations for Class-B Operation 9
- 10. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B AMPLIFIER Maximum Values : The operation for maximum signal input is shown in Fig. 60.26 Fig. 60.26 10
- 11. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B PUSH-PULL AMPLIFIER It employs two identical transistors operating as a single-stage of amplification. As shown in Fig. 60.27, the base of the two CE- connected transistors have been connected to the opposite ends of the secondary of the input transformer and collectors to the opposite ends of the primary of the output transformer . For getting a balanced circuit, the two emitters have been returned to the centre tap of secondary and VCC connected to the centre tap on the primary of . Since zero bias is required for cut-off, the two bases have been earthed. A push- pull amplifier is sometimes referred to as balanced amplifier. 1 T 2 T 1 T 2 T 11
- 13. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B PUSH-PULL AMPLIFIER Principle of Operation • In Fig. 60.28, the transfer characteristic of transistor B has been plotted upside down with respect to that of A in order to get the combined output. Fig. 60.28 13
- 14. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B PUSH-PULL AMPLIFIER During the positive half-cycle of the signal, A is turned ON because its base is driven positive. It draws collector current in the upward direction from . Meanwhile, transistor B remains OFF because its base has negative voltage. Hence, = 0. Obviously, one positive half- cycle of the output signal appears across secondary load of T. During negative half-cycle of the input signal, B conducts whereas A remains OFF. Hence, is taken by B but = 0. Now, negative output half-cycle is produced across because is pulled down through secondary of . C-1 I CC V C-2 I L R C-2 I C-1 I L R C-2 I 2 T 2 T 14
- 15. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B PUSH-PULL AMPLIFIER It is obvious that in the absence of input signal, neither A nor B draws any collector current. Hence, there is no drain on VCC battery. Advantages • It has high efficiency, theoretical limit being 78.5%. It is primarily due to the fact that there is no power drawn by the circuit under zero-signal condition. • Since in push-pull arrangement, 180° phase difference exists between even-order harmonics produced by each transistor, they cancel out thereby giving an almost distortion-free output. This automatic cancellation of all even-order harmonics from the output current makes class-B push-pull amplifiers highly desirable for communication sound equipment. 15
- 16. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-B PUSH-PULL AMPLIFIER This double-ended class-B amplifier provides practically four times the power supplied by a single-ended amplifier provided signal load resistance remains the same. The dc components of the two collector currents through the two halves of the primary of flow in opposite directions so that net dc magnetisation of the transformer core is almost nil. Hence, distortions due to the effect of dc magnetic saturation of the transformer core are eliminated. Moreover, smaller size core can be used due to very small net core flux. Uses • Class-B push-pull amplifiers are extensively used for audio work in portable record players, as stereo amplifiers and in high-fidelity radio receivers. 2 T 16
- 17. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CROSSOVER DISTORTION Crossover distortion occurs as a result of one transistor cutting off before the other begins conducting. The effect is illustrated in Fig. 60.29. The distortion so introduced is called crossover distortion because it occurs during the time operation crosses over from one transistor to the other in the push-pull amplifier. Crossover distortion can be eliminated by applying slight forward bias to each emitter diode. It, in effect, means locating the Q-point of each transistor slightly above cut-off as shown in Fig. 60.30 so that each one operates for more than one half- cycle. Strictly speaking, it results in class-AB operation because each transistor may operate for about 200° (instead of 180°). 17
- 20. Copyright © 2017 by S Chand And Company Limited. All rights reserved. POWER EFFICIENCY OF PUSH- PULL AMPLIFIERS Consider the circuit shown in Fig. 60.31. Fig. 60.31 20
- 21. Copyright © 2017 by S Chand And Company Limited. All rights reserved. POWER EFFICIENCY OF PUSH- PULL AMPLIFIERS 21
- 22. Copyright © 2017 by S Chand And Company Limited. All rights reserved. POWER EFFICIENCY OF PUSH- PULL AMPLIFIERS 22
- 23. Copyright © 2017 by S Chand And Company Limited. All rights reserved. COMPLEMENTARY SYMMETRY PUSH- PULL CLASS-B AMPLIFIER The push-pull amplifier suffers from two disadvantages : • It requires a bulky and expensive output transformer • It requires two out-of- phase input signals which necessitates an input centre-tapped transformer or phase inverter. It makes the driver circuitry quite complicated. The complementary symmetry amplifier eliminates these two disadvantages while retaining the advantages of push-pull configuration. An elementary complementary symmetry class-B push-pull amplifier is diagrammed in Fig. 60.32. The two transistors are complementary to each other and operate as emitter-follower amplifiers. The input is capacitively-coupled whereas output is direct-coupled. 23
- 24. Copyright © 2017 by S Chand And Company Limited. All rights reserved. COMPLEMENTARY SYMMETRY PUSH- PULL CLASS-B AMPLIFIER Fig. 60.32 24
- 25. Copyright © 2017 by S Chand And Company Limited. All rights reserved. COMPLEMENTARY SYMMETRY PUSH- PULL CLASS-B AMPLIFIER With no input signal, neither transistor conducts and, therefore, current through is zero. When input signal is positive-going, transistor A is biased into conduction whereas B is driven into cut-off. When the signal is negative-going. A is turned OFF while B conducts. Obviously, this circuit is a push-pull amplifier because turning one transistor ON turns the other OFF. The circuit possesses the essential characteristics of an emitter follower i.e. unity voltage gain, no phase inversion and input impedance much higher than output impedance. The circuit requires two dc supply batteries. Only one dc battery would be enough if ‘totem-pole’ circuit configuration is used. L R 25
- 26. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CLASS-C AMPLIFIER In such amplifiers, the active device i.e. the transistor is biased much beyond cut-off. Hence, • Output current flows only during a part of the possible half- cycle of the input signal, • There is no output current flow during any part of the negative half-cycle of the input signal, • Output signal has hardly any resemblance with the input signal. It consists of short pulses only, • Class-C amplifiers have high circuit efficiency of about 85 to 90%. 26
- 27. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TUNED AMPLIFIER The gain of a transistor amplifier depends directly on the value of its load impedance. Such a high impedance can be obtained by using a high-Q tuned or resonant LC circuit as load (Fig. 60.33). The frequency response curve of the amplifier assumes the same shape as the resonance curve of the tuned circuit. Non- linear distortion is eliminated because of high selectivity of the load impedance. Hence, output is nearly sinusodial. With the removal of distortion, high amplifier efficiency can be achieved by operating the transistor in its nonlinear region 27
- 28. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TUNED AMPLIFIER Fig.60.33 28
- 29. Copyright © 2017 by S Chand And Company Limited. All rights reserved. DISTORTION IN AMPLIFIERS Non-linear distortion : This occurs when transistor operates in the non-linear region of its characteristic. • When we visualize the signal in time domain, it is called amplitude distortion or waveform distortion; • When we think of the signal in the frequency domain, it is called harmonic distortion. This distortion occurs when input signal is of one frequency, say, a pure sine wave. • Intermodulation (IM) distortion—when input signal has more than one frequency (like speech ) 29
- 30. Copyright © 2017 by S Chand And Company Limited. All rights reserved. DISTORTION IN AMPLIFIERS Fig. 60.34 gives time-domain view of this distortion when it is called amplitude distortion. Fig. 60.34 30
- 31. Copyright © 2017 by S Chand And Company Limited. All rights reserved. DISTORTION IN AMPLIFIERS The frequency-domain view of non-linear distortion is shown in Fig. 60.35. Fig. 60.35 31
- 32. Copyright © 2017 by S Chand And Company Limited. All rights reserved. NOISE In general , it may be defined as any kind of unwanted signal not derived from or related to the input signal. Just as distortion is the limiting factor in the amplification of large signals, noise is the limiting factor in the case of small signals. 32
- 33. Copyright © 2017 by S Chand And Company Limited. All rights reserved. OTHER EXPRESSIONS FOR POWER GAIN 33
- 34. Copyright © 2017 by S Chand And Company Limited. All rights reserved. OTHER EXPRESSIONS FOR POWER GAIN 34
- 35. Copyright © 2017 by S Chand And Company Limited. All rights reserved. VOLTAGE AND CURRENT LEVELS 35
- 36. Copyright © 2017 by S Chand And Company Limited. All rights reserved. VARIATIONS IN AMPLIFIER GAIN WITH FREQUENCY If the input voltage of an amplifier is kept constant but its frequency is varied, it is found that the amplifier gain • Remains practically constant over a sizable range of mid- frequencies, • Decreases at low as well as high frequencies. A typical frequency- versus-gain curve is shown in Fig. 60.43. While analyzing this curve, three value of frequency are important • Mid-frequency range • Lower cut-off frequency • Upper cut-off frequency 36
- 37. Copyright © 2017 by S Chand And Company Limited. All rights reserved. VARIATIONS IN AMPLIFIER GAIN WITH FREQUENCY Fig. 60.36 37
- 38. Copyright © 2017 by S Chand And Company Limited. All rights reserved. VARIATIONS IN AMPLIFIER GAIN WITH FREQUENCY Where voltage gain of the amplifier decreases to 0.707 times the mid-frequency gain i.e. to in terms of power, where power amplification of the amplifier decreases to half its value at mid-frequencies 38
- 39. Copyright © 2017 by S Chand And Company Limited. All rights reserved. VARIATIONS IN AMPLIFIER GAIN WITH FREQUENCY In terms of decibels, where power gain falls by 3 dB. That is why the two cut-off frequencies are referred to as • –3 dB frequencies or • Down 3 dB frequencies or • 3 dB loss frequencies The points A and B in Fig. 60.43 are called 3-dB points or (sometime) as minus 3 dB points. The cut-off frequencies are also called roll-off frequencies because at these frequencies the amplifier gain starts rolling down from its midband value or maximum value. 39
- 40. Copyright © 2017 by S Chand And Company Limited. All rights reserved. VARIATIONS IN AMPLIFIER GAIN WITH FREQUENCY The frequency span between these two cut-off frequencies is called the pass band or bandwidth (BW) of the amplifier 40
- 41. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CAUSES OF AMPLIFIER GAIN VARIATIONS The primary cause of gain variation in amplifiers is the presence of capacitances, some of which are connected in series along the signal path and some in parallel. The different types of external capacitances present in an amplifier circuit are • Coupling and bypass capacitors–usually of large capacitance value, [Fig. 60.37 (a)]. • Internal or inter-element capacitances of the transistor and stray wiring capacitance [Fig. 60.37 (b)] 41
- 42. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CAUSES OF AMPLIFIER GAIN VARIATIONS Fig. 60.37 42
- 43. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CAUSES OF AMPLIFIER GAIN VARIATIONS In summary • Series-connected coupling and bypass capacitors cause decrease in amplifier gain at low frequencies. • Parallel-connected internal transistor capacitances and stray wiring capacitances cause decrease in amplifier gain at high frequencies. 43
- 44. Copyright © 2017 by S Chand And Company Limited. All rights reserved. MILLER EFFECT According to this effect, when viewed from input base terminal of the CE-connected transistor, the capacitance appears as (1 + ) i.e. it is amplified by a factor of (1 + ). In fact, Miller effect takes into account the feedback from the collector to base and vice-versa due to When is applied to the transistor’s base in Fig. 60.44 (b), the change in collector voltage is bc C bc C V A V A bc C i V i V 44
- 46. Copyright © 2017 by S Chand And Company Limited. All rights reserved. CUT-OFF FREQUENCIES OF CASCADED AMPLIFIERS Cascading of stages gives higher amplification but narrow bandwidth because the product of the two remains almost constant. The approximate values of composite lower and upper cut-off frequencies of the cascaded amplifier having n identical stages are 46
- 47. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TRANSISTOR CUT-OFF FREQUENCIES Even if no external stray capacitances were present in an amplifier, there would still be an upper limit on its frequency response due to • Internal or inter element capacitances of the transistor and • Transit time of charge carriers across the transistor junctions and through the semiconductor material. This limitation is expressed in terms of • Alpha cut-off frequency ( ) • Beta cut-off frequency ( ) α f β f 47
- 48. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TRANSISTOR CUT-OFF FREQUENCIES Alpha Cut-off Frequency : The alpha cut-off frequency is that high frequency at which the α of a CB-connected transistor becomes 0.707 of its low-frequency value (usually 1 kHz). Beta Cut-off Frequency : It is that high frequency at which the β of a CE-connected transistor drops to 0.707 of its low frequency (1 kHz) value. The fT of a Transistor : It is another high-frequency characteristic of a transistor. It is that high frequency of a CE- connected transistor where its β drops to unity i.e. β = 1. This frequency is much larger than but less than . α f β f β f 48
- 49. Copyright © 2017 by S Chand And Company Limited. All rights reserved. TRANSISTOR CUT-OFF FREQUENCIES Relation Between , and Gain-Bandwidth Product :As its name indicates, it is the product of the gain and bandwidth of an amplifier. It is very useful in comparing the performance capability of various circuits . For any amplifier, gain-bandwidth product (GBP) is constant and is equal to α f β f T f T f 49