Transistors
- Bias Compensation
- Methods of Transistor Biasing
- Transistor Biasing
- Transistor as an Amplifier
- Operating Point
- Transistor Load Line Analysis
- Transistor Regions of Operation
- Transistor Configurations
- Transistors - Overview
Amplifiers
- Noise in Amplifiers
- Emitter Follower & Darlington Amplifier
- Negative Feedback Amplifiers
- Feedback Amplifiers
- Types of Tuned Amplifiers
- Tuned Amplifiers
- Class AB and C Power Amplifiers
- Class B Power Amplifier
- Push-Pull Class A Power Amplifier
- Transformer Coupled Class A Power Amplifier
- Class A Power Amplifiers
- Classification of Power Amplifiers
- Power Amplifiers
- Direct Coupled Amplifier
- Transformer Coupled Amplifier
- RC Coupling Amplifier
- Multi-Stage Transistor Amplifier
- Based on Configurations
- Classification of Amplifiers
- Basic Amplifier
Amplifiers Useful Resources
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Push-Pull Class A Power Amppfier
So far, we have seen two types of class A power amppfiers. The main problems that should be dealt with are low power output and efficiency. It is possible to obtain greater power output and efficiency than that of the Class A amppfier by using a combinational transistor pair called as Push-Pull configuration.
In this circuit, we use two complementary transistors in the output stage with one transistor being an NPN or N-channel type while the other transistor is a PNP or P-channel (the complement) type connected in order to operate them pke PUSH a transistor to ON and PULL another transistor to OFF at the same time. This push-pull configuration can be made in class A, class B, class C or class AB amppfiers.
Construction of Push-Pull Class A Power Amppfier
The construction of the class A power amppfier circuit in push-pull configuration is shown as in the figure below. This arrangement mainly reduces the harmonic distortion introduced by the non-pnearity of the transfer characteristics of a single transistor amppfier.
![Non Linearity](/amppfiers/images/non_pnearity.jpg)
In Push-pull arrangement, the two identical transistors T1 and T2 have their emitter terminals shorted. The input signal is appped to the transistors through the transformer Tr1 which provides opposite polarity signals to both the transistor bases. The collectors of both the transistors are connected to the primary of output transformer Tr2. Both the transformers are center tapped. The VCC supply is provided to the collectors of both the transistors through the primary of the output transformer.
The resistors R1 and R2 provide the biasing arrangement. The load is generally a loudspeaker which is connected across the secondary of the output transformer. The turns ratio of the output transformer is chosen in such a way that the load is well matched with the output impedance of the transistor. So maximum power is depvered to the load by the amppfier.
Circuit Operation
The output is collected from the output transformer Tr2. The primary of this transformer Tr2 has practically no dc component through it. The transistors T1 and T2 have their collectors connected to the primary of transformer Tr2 so that their currents are equal in magnitude and flow in opposite directions through the primary of transformer Tr2.
When the a.c. input signal is appped, the base of transistor T1 is more positive while the base of transistor T2 is less positive. Hence the collector current ic1 of transistor T1 increases while the collector current ic2 of transistor T2 decreases. These currents flow in opposite directions in two halves of the primary of output transformer. Moreover, the flux produced by these currents will also be in opposite directions.
Hence, the voltage across the load will be induced voltage whose magnitude will be proportional to the difference of collector currents i.e.
$$(i_{c1} - i_{c2})$$
Similarly, for the negative input signal, the collector current ic2 will be more than ic1. In this case, the voltage developed across the load will again be due to the difference
$$(i_{c1} - i_{c2})$$
As $i_{c2} > i_{c1}$
The polarity of voltage induced across load will be reversed.
$$i_{c1} - i_{c2} = i_{c1} + (-i_{c2})$$
To have a better understanding, let us consider the below figure.
![Addition Wave](/amppfiers/images/addition_wave.jpg)
The overall operation results in an a.c. voltage induced in the secondary of output transformer and hence a.c. power is depvered to that load.
It is understood that, during any given half cycle of input signal, one transistor is being driven (or pushed) deep into conduction while the other being non-conducting (pulled out). Hence the name Push-pull amppfier. The harmonic distortion in Push-pull amppfier is minimized such that all the even harmonics are epminated.
Advantages
The advantages of class A Push-pull amppfier are as follows
High a.c. output is obtained.
The output is free from even harmonics.
The effect of ripple voltages are balanced out. These are present in the power supply due to inadequate filtering.
Disadvantages
The disadvantages of class A Push-pull amppfier are as follows
The transistors are to be identical, to produce equal amppfication.
Center-tapping is required for the transformers.
The transformers are bulky and costly.