Lecture # 16 &17Lecture # 16 &17
Complementary symmetry Complementary symmetry & push-pull Amplifiers& push-pull Amplifiers
Prepared by Prepared by
Engr Sarfaraz Khan TurkEngr Sarfaraz Khan Turk
Lecturer at IBT LUMHS JamshoroLecturer at IBT LUMHS Jamshoro
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In class B, the transistor is biased just off. The AC signal turns the transistor on.
The transistor only conducts when it is turned on by one-half of the AC cycle.
In order to get a full AC cycle out of a class B amplifier, you need two transistors:
Class B AmplifierClass B Amplifier
There are two main types of types of class-B Amplifiers
1. The Basic or Standard Push-Pull Amplifiers.
2. The Complementary-symmetry(or quasi-complementary amplifiers.
The standard or Basic class-B The standard or Basic class-B Push-Pull AmplPush-Pull Amplifierifier
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The standard or Basic Push-Pull Amplifier contains two transistors of the same type with the emitter tied together
It uses a center-tapped transformer, or a transistor phase splitter on the input and the center-tapped transformer on the output.
This Amplifier type is shown in the figure 1 . This configuration is less commonly used in the modern
electronics due to more expense spend on input and output transformer which may occupy more area and produce distortion in the nebouring electronic devices.
Note the transistor types and the transformer. This is the standard Push-pull Amplifier configuration.
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The Standard or Basic Class-B push-pull Amplifier(contd)
• During the positive half-cycle of the AC input, transistor Q1 (npn) is conducting and Q2 (npn) is off.
• During the negative half-cycle of the AC input, transistor Q2 (npn) is conducting and Q1 (npn) is off.
Each transistor produces one-half of an AC cycle. The transformer combines the two outputs to form a full AC cycle.
This circuit is less commonly used in modern circuits
The Standard or Basic Class-The Standard or Basic Class-B push-pull AmplifierB push-pull Amplifier
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The two same npn transistors will be use in this configuration
Complementary-symmetry Complementary-symmetry class-B Push-Pull amplifiersclass-B Push-Pull amplifiers
A class –B complementary-symmetry Push-Pull Amplifier using complementary transistors (a pair of one npn and one pnp transistors, with matched characteristics).
A npn transistor that provides the positive half of the AC cycle. An pnp transistor that provides the negative half of the AC
cycle. No required input and output Transformers. This configuration is most widely used in the modern
electronics due to less expensive as compared to the standard push-pull amplifier .
Two disadvantages of this circuit are:1. Using two separate DC Power supplies for Vcc.2. Cross over distortion.
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Complementary-symmetry Complementary-symmetry class-B Push-Pull amplifiersclass-B Push-Pull amplifiers
Why push-pull amplifier use?Why push-pull amplifier use?Why the standard or Basic push-pull Amplifier less Why the standard or Basic push-pull Amplifier less commonly use in the modern electronics?commonly use in the modern electronics?
Why complementary symmetry amplifiers Why complementary symmetry amplifiers preferred over the push-pull Amplifiers?preferred over the push-pull Amplifiers?
Why pnp transistor use in the complementary Why pnp transistor use in the complementary symmetry push-pull Amplifier?symmetry push-pull Amplifier?
What is the current source and the current sink?What is the current source and the current sink?
Which one transistor works as a current source?Which one transistor works as a current source?
Which one transistor works as a current sink?Which one transistor works as a current sink?
Which one transistor works as a push?Which one transistor works as a push?
Which one transistor works as a pull? Which one transistor works as a pull? 88
Current sources and loadsCurrent sources and loads
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when driving a reactive load we need to when driving a reactive load we need to supplysupply current at some times (the output acts current at some times (the output acts as a as a current sourcecurrent source))
at other times we need to at other times we need to absorbabsorb current (the current (the output acts as a output acts as a current sinkcurrent sink))
Current sources and current sinkCurrent sources and current sink the circuit above is a the circuit above is a goodgood current source but current source but
a a poorpoor current sink (stored charge must be current sink (stored charge must be removed by removed by RREE))
an alternative circuit using an alternative circuit using pnp pnp transistorstransistors (below) is a (below) is a goodgood current sink but a current sink but a poorpoor current sourcecurrent source
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Complementary-symmetry Complementary-symmetry class-B Push-Pull amplifiersclass-B Push-Pull amplifiersPush-pull amplifiersPush-pull amplifiers combining these combining these
circuits can produce circuits can produce an arrangement that an arrangement that is both a good current is both a good current source and a good source and a good current sinkcurrent sink
this is termed a this is termed a push-pull amplifierpush-pull amplifier
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Complementary-symmetry class-B Complementary-symmetry class-B Push-Pull amplifiersPush-Pull amplifiers
Since one part of the Since one part of the circuit (T1) pushes the circuit (T1) pushes the signal high during the signal high during the positive +ve half-cycle positive +ve half-cycle and the other part of the and the other part of the circuit (T2) pulls the circuit (T2) pulls the signal low during the signal low during the negative -ve half cycle, of negative -ve half cycle, of the input AC signal the the input AC signal the circuit is referred to as a circuit is referred to as a push-pull circuitpush-pull circuit
The Complementary-symmetry The Complementary-symmetry Push-Pull Amplifier StagesPush-Pull Amplifier Stages
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As Vin increases, Q1 is on and pushes a current into RL.(During positive +ve half cycle of the input of AC signal.
As Vin decreases, Q2 is on and pulls a current out of RL. .(During negative -ve half cycle of the input of AC signal.
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Input DC power Input DC power
The power supplied to the load by an amplifier is The power supplied to the load by an amplifier is drawn from the power supplydrawn from the power supply
The amount of this DC power is calculated usingThe amount of this DC power is calculated using
The DC current drawn from the source is the The DC current drawn from the source is the average value of the current delivered to the average value of the current delivered to the loadload
dcCCdci IVP )(
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Input DC powerInput DC power
The current drawn from a single DC supply has the The current drawn from a single DC supply has the form of a full wave rectified signal, while that drawn form of a full wave rectified signal, while that drawn from two power supplies has the form of half-wave from two power supplies has the form of half-wave rectified signal from each supplyrectified signal from each supply
On either case the average value for the current is On either case the average value for the current is given bygiven by
The input power can be written as The input power can be written as
pdc II 2
pCCdci IVP2
)(
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Output AC powerOutput AC power
The power delivered to the load can be calculated The power delivered to the load can be calculated using the following equationusing the following equation
The efficiency of the amplifier is given byThe efficiency of the amplifier is given by
Not that Not that
Therefore the efficiency can be re-expressed as Therefore the efficiency can be re-expressed as
L
pL
L
ppLaco R
V
R
VP
28)()(
)(
L
pLp R
VI )(
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Output AC powerOutput AC power
The maximum efficiency can be obtained if The maximum efficiency can be obtained if
The value of this maximum efficiency will beThe value of this maximum efficiency will be
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Power dissipated by the output Power dissipated by the output transistorstransistors
The power dissipated by the output transistors as The power dissipated by the output transistors as heat is given byheat is given by
The power in each transistor is given byThe power in each transistor is given by
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ExampleExample
Example 1:Example 1: For class B amplifier providing a 20-V For class B amplifier providing a 20-V peak signal to a 16-peak signal to a 16-ΩΩ speaker and a power supply speaker and a power supply of of VVCCCC=30 V=30 V, determine the input power , output , determine the input power , output
power and the efficiencypower and the efficiency
Solution:Solution:
The input power is given byThe input power is given by
The peak collector load current can be found fromThe peak collector load current can be found from
pCCdci IVP2
)(
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ExampleExample
Solution:Solution:
The input power isThe input power is
The output power is given byThe output power is given by
The efficiency is The efficiency is
WP dci 9.23)25.1(302
)(
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Maximum power dissipated by the Maximum power dissipated by the output transistorsoutput transistors
The maximum power dissipated by the two The maximum power dissipated by the two transistors occurs when the output voltage across transistors occurs when the output voltage across the load is given bythe load is given by
The maximum power dissipation is given byThe maximum power dissipation is given by
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ExampleExample
Example 2:Example 2: For class B amplifier using a supply of For class B amplifier using a supply of VVCCCC=30 V and driving a load of 16-=30 V and driving a load of 16-ΩΩ, determine , determine
the input power , output power and the efficiencythe input power , output power and the efficiency
Solution:Solution:
The maximum output power is given byThe maximum output power is given by
The maximum input power drawn from the supply The maximum input power drawn from the supply isis
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ExampleExample
Solution:Solution:
The efficiency is given byThe efficiency is given by
The maximum power dissipated by each transistor The maximum power dissipated by each transistor is is
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Class B Amplifier circuits Class B Amplifier circuits
A number of circuit arrangements can be used A number of circuit arrangements can be used to realize class B amplifierto realize class B amplifierWe will consider in this course two We will consider in this course two arrangements in particulararrangements in particular
1.1. The first arrangement uses a single input signal fed The first arrangement uses a single input signal fed to the input of two complementary transistors to the input of two complementary transistors (complementary symmetry circuits)(complementary symmetry circuits)
2.2. The second arrangement uses two out of phase The second arrangement uses two out of phase input signals of equal amplitudes feeded to the input input signals of equal amplitudes feeded to the input of two similar NPN or PNP transistors (quasi-of two similar NPN or PNP transistors (quasi-complementary push-pull amplifier)complementary push-pull amplifier)
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Complementary symmetry circuitsComplementary symmetry circuitsfirst arrangement first arrangement
This circuit uses This circuit uses both npn and pnp both npn and pnp transistor to transistor to construct class B construct class B amplifier as shown amplifier as shown to the leftto the leftOne disadvantage One disadvantage of this circuit is the of this circuit is the need for two need for two separate voltage separate voltage supplies supplies
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Complementary symmetry circuits Complementary symmetry circuits
another disadvantage of this circuit is the another disadvantage of this circuit is the resulting cross over distortionresulting cross over distortion
Cross over distortion can be eliminated the by Cross over distortion can be eliminated the by biasing the transistors in class AB operation biasing the transistors in class AB operation where the transistors are biased to be on for where the transistors are biased to be on for slightly more than half a cycleslightly more than half a cycle
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Class AB biasing to solve Class AB biasing to solve crossover distortioncrossover distortion
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Complementary symmetry circuits Complementary symmetry circuits
A more practical version A more practical version of a push-pull circuit of a push-pull circuit using complementary using complementary transistors is shown to transistors is shown to the rightthe rightThis circuit uses to This circuit uses to complementary complementary Darlington pair Darlington pair transistors to achieve transistors to achieve larger current driving larger current driving and lower output and lower output impedanceimpedance
Complementary-symmetry push-pull circuit using Darlingtion transistors
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Complementary symmetry circuits Complementary symmetry circuits Second arrangement Second arrangement
As stated previously the second arrangement As stated previously the second arrangement which uses two equal input signals of opposite which uses two equal input signals of opposite phase has to be preceded by a phase inverting phase has to be preceded by a phase inverting network as shown belownetwork as shown below
Phase splitter circuit
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Quasi-complementary push pull Quasi-complementary push pull amplifier second arrangement amplifier second arrangement In practical power amplifier circuits it is In practical power amplifier circuits it is preferable to uses npn for both transistorspreferable to uses npn for both transistorsSince the push pull connection requires Since the push pull connection requires complementary devices, a pnp high power complementary devices, a pnp high power transistor must be used.transistor must be used.This can be achieved by using the circuit shownThis can be achieved by using the circuit shown
Quasi-complementary push-pull transformer less power amplifier
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ExampleExample
Example:Example: For the circuit shown, calculate the input For the circuit shown, calculate the input power, output power and the power handled by power, output power and the power handled by each transistor and the efficiency if the input each transistor and the efficiency if the input signal is 12 signal is 12 VVrmsrms
Solution:Solution:The peak input voltage isThe peak input voltage is
The output power isThe output power is
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ExampleExampleSolution:Solution:The peak load current isThe peak load current is
The dc current can be found from the peak asThe dc current can be found from the peak as
The input power is given byThe input power is given by
The power dissipated by each transistor is given byThe power dissipated by each transistor is given by
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There is a slight problem associated with the push pull amplifier There is a slight problem associated with the push pull amplifier arrangement. Transistors require a 0.7V difference between the base arrangement. Transistors require a 0.7V difference between the base and the emitter in order for them to start conducting.and the emitter in order for them to start conducting.
As we can see from the two graphs the Output voltage does not perfectly reflect the input voltage. There is an area called cross over distortion where neither transistor is conducting due to them requiring 0.7V to switch them on.
Input Voltage
Output voltage
Crossover DistortionCrossover Distortion
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Crossover DistortionCrossover Distortion
If the transistors Q1 and Q2 do not turn on and off at exactly the same time, then there is a gap in the output voltage. For large Vin, the output follows the input with a fixed DC offset, however as Vin becomes small the output drops to zero and causes “Crossover Distortion.”
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Improved Push-Pull StageImproved Push-Pull Stage
With a battery of VWith a battery of VBB inserted between the bases of Q inserted between the bases of Q11 and Q and Q22, the dead , the dead
zone is eliminated.zone is eliminated.
VB=VBE1+|VBE2|
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Amplifier DistortionAmplifier Distortion
If the output of an amplifier is not a complete AC sine wave, then it is distorting the output. The amplifier is non-linear.
This distortion can be analyzed using Fourier analysis. In Fourier analysis, any distorted periodic waveform can be broken down into frequency components. These components are harmonics of the fundamental frequency.
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HarmonicsHarmonics
Harmonics are integer multiples of a fundamental frequency.
If the fundamental frequency is 5kHz:
1st harmonic 1 x 5kHz2nd harmonic 2 x 5kHz3rd harmonic 3 x 5kHz4th harmonic 4 x 5kHzetc.
Note that the 1st and 3rd harmonics are called odd harmonics and the 2nd and 4th are called even harmonics
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Harmonic DistortionHarmonic Distortion
According to Fourier analysis, if a signal is not purely sinusoidal, then it contains harmonics.
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Harmonic Distortion CalculationsHarmonic Distortion Calculations
The total harmonic distortion (THD) is determined by:
100A
A%Ddistortion harmonic nth %
1
nn
100DDDTHD % 23
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Harmonic distortion (D) can be calculated:
where A1 is the amplitude of the fundamental frequency An is the amplitude of the highest harmonic
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Power Transistor Derating CurvePower Transistor Derating Curve
Power transistors dissipate a lot of power in heat. This can be destructive to the amplifier as well as to surrounding components.
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For Student References (for Complementary symmetry & push-pull Amplifiers) Read:
Chapter 12 Power Amplifiers from the book Electronic devices, circuit and systems (Micheal M cirovic) topics 12.6 sub topic from (12.6.1 to 12.6.2)and 12.7 to 12.8.
Chapter 11 Power Amplifiers from the book introductory electronic devices and circuits by (Robert T .paynter)conventional flow version. topic 11.4 class B amplifiers complementary-symmetry or push pull amplifiers. chapter 8 Introduction to Amplifiers from (Robert T .paynter)topic 8.3 (see only class B amplifiers)Wikipedia & world wide web.
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