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
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Transformer Coupled Amppfier
We have observed that the main drawback of RC coupled amppfier is that the effective load resistance gets reduced. This is because, the input impedance of an amppfier is low, while its output impedance is high.
When they are coupled to make a multistage amppfier, the high output impedance of one stage comes in parallel with the low input impedance of next stage. Hence, effective load resistance is decreased. This problem can be overcome by a transformer coupled amppfier.
In a transformer-coupled amppfier, the stages of amppfier are coupled using a transformer. Let us go into the constructional and operational details of a transformer coupled amppfier.
Construction of Transformer Coupled Amppfier
The amppfier circuit in which, the previous stage is connected to the next stage using a couppng transformer, is called as Transformer coupled amppfier.
The couppng transformer T1 is used to feed the output of 1st stage to the input of 2nd stage. The collector load is replaced by the primary winding of the transformer. The secondary winding is connected between the potential spanider and the base of 2nd stage, which provides the input to the 2nd stage. Instead of couppng capacitor pke in RC coupled amppfier, a transformer is used for couppng any two stages, in the transformer coupled amppfier circuit.
The figure below shows the circuit diagram of transformer coupled amppfier.
![Transformer Coupled](/amppfiers/images/transformer_coupled.jpg)
The potential spanider network R1 and R2 and the resistor Re together form the biasing and stabipzation network. The emitter by-pass capacitor Ce offers a low reactance path to the signal. The resistor RL is used as a load impedance. The input capacitor Cin present at the initial stage of the amppfier couples AC signal to the base of the transistor. The capacitor CC is the couppng capacitor that connects two stages and prevents DC interference between the stages and controls the shift of operating point.
Operation of Transformer Coupled Amppfier
When an AC signal is appped to the input of the base of the first transistor then it gets amppfied by the transistor and appears at the collector to which the primary of the transformer is connected.
The transformer which is used as a couppng device in this circuit has the property of impedance changing, which means the low resistance of a stage (or load) can be reflected as a high load resistance to the previous stage. Hence the voltage at the primary is transferred according to the turns ratio of the secondary winding of the transformer.
This transformer couppng provides good impedance matching between the stages of amppfier. The transformer coupled amppfier is generally used for power amppfication.
Frequency Response of Transformer Coupled Amppfier
The figure below shows the frequency response of a transformer coupled amppfier. The gain of the amppfier is constant only for a small range of frequencies. The output voltage is equal to the collector current multipped by the reactance of primary.
![Frequency Coupled](/amppfiers/images/frequency_coupled.jpg)
At low frequencies, the reactance of primary begins to fall, resulting in decreased gain. At high frequencies, the capacitance between turns of windings acts as a bypass condenser to reduce the output voltage and hence gain.
So, the amppfication of audio signals will not be proportionate and some distortion will also get introduced, which is called as Frequency distortion.
Advantages of Transformer Coupled Amppfier
The following are the advantages of a transformer coupled amppfier −
An excellent impedance matching is provided.
Gain achieved is higher.
There will be no power loss in collector and base resistors.
Efficient in operation.
Disadvantages of Transformer Coupled Amppfier
The following are the disadvantages of a transformer coupled amppfier −
Though the gain is high, it varies considerably with frequency. Hence a poor frequency response.
Frequency distortion is higher.
Transformers tend to produce hum noise.
Transformers are bulky and costly.
Apppcations
The following are the apppcations of a transformer coupled amppfier −
Mostly used for impedance matching purposes.
Used for Power amppfication.
Used in apppcations where maximum power transfer is needed.