1. Power Amplifiers & Multivibrators

2. Power Amplifier
Designed to deliver a large amount of power to drive loads like loud speaker, CRT, servo motor etc.
Meant for raising the power level of the input signal to a large extent. Hence the input to the power amplifiers should be large.
To provide large input signal, voltage amplifier is used before a power amplifier.
Also called large signal amplifiers
Characterized by high efficiency, maximum power handling capacity and good impedance matching.

3. Voltage amplifier Vs Power Amplifer

4. Applications
Used in the last stage of a transmitter in a communication system
Used in radio and TV receivers
Used in electro-mechanical control system to drive motors
Used in computer hard disk driver stage
Used in motorized valves
Used in robotics and process control instruments.

5. Classification
Based on the choice of Q-point on transfer characteristics, Power amplifiers are classified as:
Linear Amplifier
Class A Power amplifier
Class B power amplifier
Class AB power amplifier
Class C power amplifier
Nonlinear Amplifier
Class D power amplifier
Class S power amplifier

6. Class A Amplifier
Q point is chosen in Linear/Active region, hence transistor is in ON state.
So the collector current does not become zero for any part of the cycle.

7. Class B Amplifier
Q point is chosen in the cut-off region, where the collector current is zero
During the positive half cycle of sinusoidal input, the transistor is brought out of cut-off as it gets a forward bias and it conducts.
During the negative half cycle, the transistor gets a negative bias and it is taken deep into cut-off and so the transistor does not conduct.
So the collector current flows only for one cycle.

8. Class AB Amplifier
Q point is chosen between that of class A and Class B. i.e above cut-off and below the active region.
Hence for a sinusoidal input, the transistor conducts for a full positive half cycle and for a part of the negative half cycle.
When the negative input becomes very high, the transistor is taken into cut-off and there is no output.
Thus the collector current flows for full positive half cycle and part of the negative half cycle.

9. Class C Amplifier Q point is chosen much below cut-off.
For a sinusoidal input, even during the positive half cycle, the output current flows only when the input signal exceeds the minimum current required to take the transistor into conduction.
During the negative half cycle, the transistor is driven deeper into cut-off and hence there is no output.
Hence collector current flows for less than half cycle.

10. Class D & Class S Amplifiers
Operates for pulsed inputs and hence the transistor is ON for a short interval and OFF for a long interval.
Hence the drop across the transistor is negligible and so maximum power is delivered to the load which increases the efficiency to 100%.
Does not respond to variations in input signal (eg. AM, FM signals)
Class S
Designed to amplify the modulated signals (eg. AM, FM Signals)

11. Comparison of Linear Amplifiers

12. Performance analysis of power amplifiers
Efficiency
Power dissipation
Distortion

13. Efficiency A power amplifier converts DC power into AC power.
Hence conversion efficiency is defined as the ability of the transistor to convert the DC power of the supply into AC power delivered to the load.
This is also called as theoretical efficiency or collector circuit efficiency.

14. Power Dissipation
It is the Ability to dissipate the heat developed within it.
As power amplifiers handle high currents, they get heated up during operation.
This heat has to be dissipated; if not the transistor will get damaged.
So the power dissipation capacity of the transistors should be increased by using suitable provisions like heat sinks.

15. Distortion
Defined as the change in output from the input in terms of its shape, frequency and phase.
Accordingly it is called as amplitude distortion, frequency distortion and phase distortion.
In the case of power amplifiers due to the large input signal, the transistor operation becomes non-linear and hence results in amplitude distortion.
For good performance, the distortion is expected to be low.

16. Class A Amplifier
A class-A amplifier has its operating point chosen in the linear or active region.
Hence for a given AC input, the transistor remains forward biased and conducts for both the half cycles. Hence output current flows for both the half cycles.
Since the output current flows for both the cycles, the power loss in the circuit is more and the power transferred to the load is less. This reduces the efficiency of the class-A amplifier.
Classified as:
Series fed class-A amplifier
Inductor coupled class-A amplifier
Transformer coupled class-A amplifier (In Syllabus)
For series fed amplifiers, the efficiency is 25%, for Inductor coupled amplifiers, the efficiency is 50% and for transformer coupled amplifiers, the efficiency is 50%

17. Transformer Coupled Class A Amplifier
Avoids power loss across collector load and improves the collector efficiency.
RB is the biasing resistor to maintain transistor in active region.
A transformer is connected at the collector through which the load resistor RL is connected.
Derivation

18. Push Pull Amplifiers
In a single power amplifier, due to the nonlinearity of transfer characteristics, the output is appreciably distorted.
Such distortions are greatly reduced by using push-pull configurations where two transistors are used and made to conduct alternatively.
These amplifiers are extensively used in transmitters, receivers, tape recorders, public addressing system etc.
The push-pull amplifier has a centre tapped transformer both at the input and output.
The input transformer splits the signal with equal amplitude and opposite phase and feeds it to the base of the transistors. Since the input to the transistor is out of phase, their outputs are also out of phase.
The output transformer combines these outputs to provide a sinusoidal output.
The push-pull amplifier is operated in class-A, class-B and class-AB mode.

19. Class B Push-Pull Amplifier

20. Class B Push-Pull Amplifier – Contd..
The input signal is fed through a centre-tapped transformer. The output voltage is collected across the transformer secondary.
In class-B amplifier as the transistor is biased at cut-off, the output current flows for only one half-cycle.
During the positive half cycle; transistor Q1 conducts and delivers the current to the load in one direction (push) and during the negative half cycle; transistor Q2 conducts and delivers the current to the load in the opposite direction (pull).
Combining the individual signal output results in an undistorted output signal across the load.
Derivation

21. Direct Coupled Class B Push Pull
In Direct coupled push pull class-B amplifier, the load RL is directly connected to the circuit.
The circuit shows an input transformer which acts as a phase splitter.

22. Direct Coupled Class B Push-Pull
The transformer provides the input to the transistors with equal amplitude and opposite phase. Hence the transistors conduct alternatively for every half cycle.
During the positive half cycle, the transistor Q1 is forward biased and transistor Q2 is reverse biased. Hence transistor Q1 conducts to reproduce the signal as such at its emitter.
During the negative half cycle, the transistor Q2 is forward biased and transistor Q1 is reverse biased. Hence transistor Q2 conducts to reproduce the signal with a phase of 180◦ at its collector.
When both these outputs are combined we get a sinusoidal output across the load.

23. Direct Coupled Class B Push-Pull
Advantages
No output transformer is required
Due to the elimination of transformer, the cost of the circuit reduces drastically
Circuit become compact
Disdvantages
Need of two power supplies (±VCC)
It is not possible to obtain maximum power transfer to the load
Load isolation is not available

24. Complementary Symmetry Push-Pull Class B Power Amplifier
A complementary symmetry push-pull class-B amplifier overcomes the need for a transformer both at the input and output.
The circuit makes use of the two transistors one npn and other pnp.
The transistors have to be symmetrical which means they have to be made of same semiconductor material and all the transistor parameters have to be the same.
Since the transistors are symmetrical and complementary of each other, the circuit is known as complementary symmetry push-pull amplifier.

25. Complementary Symmetry Push-Pull Class B Power Amplifier

26. Complementary Symmetry Push-Pull Class B Power Amplifier
Transistor Q1 is an npn type and transistor Q2 is a pnp type. Both the transistors are operated in CC configuration where the input is given to the base and output is observed at the emitter.
The collectors are connected to the supply voltage. Since the transistors are operated in CC configuration, the voltage gain Av ∼= 1 and hence the output at the emitter follows the input at the base. That is why this circuit is also known as emitter follower class-B push-pull amplifier.
The resistors R1 and R2 act as a voltage divider network to bias the transistor at cut-off.
The AC input is applied at the base of the transistors. During the positive half cycle, the npn transistor Q1 is forward biased and pnp transistor Q2 is reverse biased.

27. Complementary Symmetry Push-Pull Class B Power Amplifier
Hence transistor Q1 conducts above cut-in voltage to reproduce the positive half cycle at the output. Similarly, during the negative half cycle, the pnp transistor Q2 is forward biased and the npn transistor Q1 is reverse biased.
Hence pnp transistor Q2 conducts above cut-in voltage and reproduces the negative half cycle at the output.
Thus the AC input is reproduced at the output with cross-over distortion which can be over come by biasing the transistors slightly above the cut-off. The efficiency of the class-B complementary symmetry circuit is 78.5%.

28. Complementary Symmetry Push-Pull Class B Power Amplifier
Advantages
Avoids the need for both input and output transformers. Hence the circuit is cheap and compact.
Since either of the transistors conducts for every half cycle, the output impedance of the circuit is low. Hence it finds use in low impedance circuit. Example, the output stage of op-amp.
Disadvantages
Requires two biasing or supply voltage
If the transistors are not identical, even harmonics will not be cancelled. Hence the distortion will be high

29. Cross Over Distortion in Class B Push-Pull Amplifier
In a class-B push-pull amplifier, the transistors are biased at cut-off.
When an AC input is applied to a class-B push-pull amplifier and as long as input voltage is less than cut-in voltage, the transistors do not conduct and there is no output. When input voltage is greater than cut-in voltage, the transistors conduct to respond to the input in an amplified form at the output. This results in a distorted output.
These distortions that occur at the zero cross-overs when input voltage is less than cut-in voltage are known as cross-over distortions.

30. Class AB Push-Pull Amplifier
The distortions that occur in Class B Push-Pull power amplifier can be overcome if a small amount of current is allowed to flow through the transistor under zero input conditions.
In other words, the transistors have to be biased slightly above cut-off, so that the transistors are already in the conducting state.
Now if an AC input is applied the transistors conduct for the full cycle and produce an undistorted output waveform.
This mode of operation where the transistors are biased above cut-off is known as class-AB power amplifier

31. Class AB Push – Pull Amplifier
In a class-AB push-pull amplifier, a voltage divider network R1 and R2 is included to provide the base bias for both the transistors so that they conduct under no signal condition

32. Class AB Push-Pull Contd…
The bias provided to the transistor keeps them just above cut-off so that the transistors conduct for more than half a cycle and less than full cycle. That is why, they are said to operate in class-AB mode.
Since they conduct under zero signal condition, they overcome the problem of cross-over distortion.
The disadvantages of a class-AB amplifier in comparison with a class-B amplifiers are
Wastage of standby power across the transistors
Low conversion efficiency compared to a class-B amplifier

33. Capacitor coupled class AB Push-Pull
Due to the absence of input coupling transformer, the cost of the circuit reduces.
The effective area occupied by this configuration is relatively less than the transformer coupled class-AB amplifier.
This configuration suffers from input coupling problem for maximum signal transfer.

34. Class C Power Amplifier
For a Class-C amplifier, the transistor is biased deep at cut-off so that it conducts only for a short interval of time.

35. Class C Power Amplifier
The output voltage across the load contains many harmonic frequencies. L and C act as a parallel resonance circuit and the LC-tuned circuit removes the harmonic frequencies other than the fundamental frequency.
A negative bias (VBB) is connected at the base of the transistor in order to drive the Q-point deep into cut-off.
The radio frequency choke (RFC) inductance presents a high impedance to the high-frequency input and thereby prevents the DC component from shorting the ac input signal.
A Class-C amplifier is primarily used for high-power and high frequency application like a radio-frequency transmitter.
The principle advantage of Class-C amplifier over other amplifiers is that, it has very high efficiency. Derivation

36. Multivibrators
Multivibrators are two-stage switching circuits in which the output of first stage is fed as input to second stage and vice versa.
The outputs of the two stages are complementary.
Types of Multivibrator
Astable Multivibrator
Bistable Multivibrator
Monostable Multivibrator

37. Astable Multivibrator
Astable or free running Multivibrator generates a square wave without any external triggering pulse.
It has no stable states or it has two quasi stable states. It switches back and forth from one state to another, remaining in a state for a particular time which is decided by a capacitive circuit

38. Astable Multivibrator Circuit

39. Astable Multivibrator Components
The collector of each stage is connected to the base of the other stage through a capacitor.
Due to this capacitive coupling between the two devices, the devices are taken to cut-off and saturation alternatively at regular time intervals decided by the RC time constant of the circuit.
Since the circuit keeps changing states on its own accord it is also known as free-running Multivibrator.

40. Astable Multivibrator Working

41. Astable Multivibrator Working
Q1 is ON and Q2 is OFF

42. Calculation of Time Constant
The time taken for one full cycle i.e., the duration for a device to change state and come back to the original state is defined as T, which is T1+T2.
This change of state occurs when VB > Vγ and VB < Vγ.
Derivation

43. Disadvantages of Astable Multivibrator
The square waveform has a slight distortion i.e., it is not straight since C1 and C2 charge through RC1 and RC2 respectively. This problem is overcome by connecting a diode and a resistor
Both the transistors may go into saturation simultaneously. The reverse transition is not possible and the Multivibrator is said to be in the blocked state. This condition occurs only if the bias is applied slowly rather than applying instantly. This can be overcome by:
1) Using External Trigger circuit (Gated Astable MV Ckt.) 2) Emitter coupled Astable MV