1. Lecture # 16 &17 Complementary symmetry & push-pull Amplifiers
Prepared by
Engr Sarfaraz Khan Turk
Lecturer at IBT LUMHS Jamshoro

2. Class B Amplifier
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:
There are two main types of types of class-B Amplifiers
The Basic or Standard Push-Pull Amplifiers.
2. The Complementary-symmetry(or quasi-complementary amplifiers.

3. The standard or Basic class-B Push-Pull Amplifier
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.

4. 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

5. The Standard or Basic Class-B push-pull Amplifier
The two same npn transistors will be use in this configuration

6. Complementary-symmetry class-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:
Using two separate DC Power supplies for Vcc.
Cross over distortion.

7. Complementary-symmetry class-B Push-Pull amplifiers

8. Why push-pull amplifier use?
Why the standard or Basic push-pull Amplifier less commonly use in the modern electronics?
Why complementary symmetry amplifiers preferred over the push-pull Amplifiers?
Why pnp transistor use in the complementary symmetry push-pull Amplifier?
What is the current source and the current sink?
Which one transistor works as a current source?
Which one transistor works as a current sink?
Which one transistor works as a push?
Which one transistor works as a pull?

9. Current sources and loads
when driving a reactive load we need to supply current at some times (the output acts as a current source)
at other times we need to absorb current (the output acts as a current sink)

10. Current sources and current sink
the circuit above is a good current source but a poor current sink (stored charge must be removed by RE)
an alternative circuit using pnp transistors (below) is a good current sink but a poor current source

11. Complementary-symmetry class-B Push-Pull amplifiers
combining these circuits can produce an arrangement that is both a good current source and a good current sink
this is termed a push-pull amplifier

12. Complementary-symmetry class-B Push-Pull amplifiers
Since one part of the circuit (T1) pushes the signal high during the positive +ve half-cycle and the other part of the circuit (T2) pulls the signal low during the negative -ve half cycle, of the input AC signal the circuit is referred to as a push-pull circuit

13. The Complementary-symmetry Push-Pull Amplifier Stages
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.

14. Input DC power
The power supplied to the load by an amplifier is drawn from the power supply
The amount of this DC power is calculated using
The DC current drawn from the source is the average value of the current delivered to the load

15. Input DC power
The current drawn from a single DC supply has the form of a full wave rectified signal, while that drawn from two power supplies has the form of half-wave rectified signal from each supply
On either case the average value for the current is given by
The input power can be written as

16. Output AC power
The power delivered to the load can be calculated using the following equation
The efficiency of the amplifier is given by
Not that
Therefore the efficiency can be re-expressed as

17. Output AC power The maximum efficiency can be obtained if
The value of this maximum efficiency will be

18. Power dissipated by the output transistors
The power dissipated by the output transistors as heat is given by
The power in each transistor is given by

19. Example
Example 1: For class B amplifier providing a 20-V peak signal to a 16-Ω speaker and a power supply of VCC=30 V, determine the input power , output power and the efficiency
Solution:
The input power is given by
The peak collector load current can be found from

20. Example Solution: The input power is The output power is given by
The efficiency is

21. Maximum power dissipated by the output transistors
The maximum power dissipated by the two transistors occurs when the output voltage across the load is given by
The maximum power dissipation is given by

22. Example
Example 2: For class B amplifier using a supply of VCC=30 V and driving a load of 16-Ω, determine the input power , output power and the efficiency
Solution:
The maximum output power is given by
The maximum input power drawn from the supply is

23. Example Solution: The efficiency is given by
The maximum power dissipated by each transistor is

24. Class B Amplifier circuits
A number of circuit arrangements can be used to realize class B amplifier
We will consider in this course two arrangements in particular
The first arrangement uses a single input signal fed to the input of two complementary transistors (complementary symmetry circuits)
The second arrangement uses two out of phase input signals of equal amplitudes feeded to the input of two similar NPN or PNP transistors (quasi-complementary push-pull amplifier)

25. Complementary symmetry circuits first arrangement
This circuit uses both npn and pnp transistor to construct class B amplifier as shown to the left
One disadvantage of this circuit is the need for two separate voltage supplies

26. Complementary symmetry circuits
another disadvantage of this circuit is the resulting cross over distortion
Cross over distortion can be eliminated the by biasing the transistors in class AB operation where the transistors are biased to be on for slightly more than half a cycle

27. Class AB biasing to solve crossover distortion

28. Complementary symmetry circuits
A more practical version of a push-pull circuit using complementary transistors is shown to the right
This circuit uses to complementary Darlington pair transistors to achieve larger current driving and lower output impedance
Complementary-symmetry push-pull circuit using Darlingtion transistors

29. Complementary symmetry circuits Second arrangement
As stated previously the second arrangement which uses two equal input signals of opposite phase has to be preceded by a phase inverting network as shown below
Phase splitter circuit

30. Quasi-complementary push pull amplifier second arrangement
In practical power amplifier circuits it is preferable to uses npn for both transistors
Since the push pull connection requires complementary devices, a pnp high power transistor must be used.
This can be achieved by using the circuit shown
Quasi-complementary push-pull transformer less power amplifier

31. Example
Example: For the circuit shown, calculate the input power, output power and the power handled by each transistor and the efficiency if the input signal is 12 Vrms
Solution:
The peak input voltage is
The output power is

32. Example Solution: The peak load current is
The dc current can be found from the peak as
The input power is given by
The power dissipated by each transistor is given by

33. Crossover Distortion
There is a slight problem associated with the push pull amplifier arrangement. Transistors require a 0.7V difference between the base 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

34. Crossover 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.”

35. Improved Push-Pull Stage
VB=VBE1+|VBE2|
With a battery of VB inserted between the bases of Q1 and Q2, the dead zone is eliminated.

36. Amplifier 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.

37. Harmonics Harmonics are integer multiples of a fundamental frequency.
If the fundamental frequency is 5kHz:
1st harmonic 1 x 5kHz
2nd harmonic 2 x 5kHz
3rd harmonic 3 x 5kHz
4th harmonic 4 x 5kHz
etc.
Note that the 1st and 3rd harmonics are called odd harmonics and the 2nd and 4th are called even harmonics

38. Harmonic Distortion
According to Fourier analysis, if a signal is not purely sinusoidal, then it contains harmonics.

39. Harmonic Distortion Calculations
Harmonic distortion (D) can be calculated:
where
A1 is the amplitude of the fundamental frequency
An is the amplitude of the highest harmonic
The total harmonic distortion (THD) is determined by:

41. Power 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.

42. 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 ( to )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.