Transformer Coupled Amplifier

 Disadvantages of RC Coupled Amplifier

• The main disadvantage of RC Coupled amplifier is that it consists of low voltage gain and low power gain.
• The input impedance of the amplifier is low whereas the output impedance is high. 
• When the output impedance of first stage is comes with in parallel with low impedance of input stage of second stage in the multistage amplifier circuit, the effective load is decreased. 
• This will result in low voltage gain and low power gain.

Transformer Coupled Amplifier

• If the effect load resistance of each stage could be increased, the voltage gain and power gain could be increased. 
• This is possible in the Transformer Coupled Amplifier. 
• The low resistance of each stage can be reflected as high resistance by using impedance changing properties of transformer.

• Figure Shows two stage transformer coupled amplifier in which transformer primary winding is connected to the collector circuit whereas the secondary winding is connected to the input of the next stage.
• When AC signal is applied to the base of the first transistor, the base current flows. 
• The amplified signal is available at the collector circuit / primary of the transformer. 
• The voltage developed across collector circuit of the first stage is transferred to the input stage of the next transistor by transformer action.

Frequency Response

• The frequency response of the transformer coupled amplifier is poor. The voltage gain is constant over small range of frequencies. 
• The output voltage is equal to product of current passes through transformer primary winding and impedance of transformer primary winding.

Low frequency

• The impedance of transformer primary winding falls due to low impedance ( X_L = 2πFL ) therefore the gain decreases.

High frequency

• The impedance of the transformer winding increases due to high impedance ( X_L = 2πFL ) therefore the gain increases but the capacitance between winding turns acts as bypass capacitor to reduce the output voltage resulting overall voltage gain reduces.

Conclusion

• We finally conclude that the voltage gain is not constant over the range of frequencies therefore it produces frequency distortion. 
• The constant gain over audio frequency range is only possible by proper designed transformer.

Advantages

• An impedance matching is achieved in the transformer coupled amplifier. 
• This is possible by making inductive reactive of the primary winding is equal to output impedance of the transformer and inductive reactance of the secondary winding is equal to input impedance of the next stage.
• The transformer coupled amplifier provided higher voltage gain if proper impedance matching is done.

Disadvantages

• The frequency response of the transformer coupled amplifier is better than RC coupled amplifier but its cost is increased by 10 to 20 times that than of RC Coupled Amplifier.
• Poor frequency response i.e. voltage gain varies over wide range of frequencies
• Bulky and expensive at audio frequencies
• Higher frequency distortion because low frequency signal is less amplified as compared to high frequency signal
• The transformer coupling produces magnetic hum ( noise ) in the output.

Application

• It is used for power amplification ( particular in loud speaker ) when the load is very small. It is used for impedance matching. 
• The last stage of the multistage amplifier is power stage. 
• The maximum power transfer from the last stage of the amplifier to the load is possible when the output impedance of the last stage is equal to the impedance of the load. 
• Generally the impedance of the output device is in few ohms whereas the output impedance of the transformer is in terms of hundred ohms.
• The step down transformer of proper turn ratio is used for impedance matching. 
• The impedance of the secondary of the transformer is made equal to load impedance whereas the impedance of the primary winding is equal to output impedance of the last stage of the amplifier.
• Let us consider that the load resistance is equal to R_L. The reflected load resistance from secondary to primary is R_L’.

        R_L’ = ( N_P / N_S )² × R_L

              = ( I_S / I_P )² × R_L

Where

        N_P = Primary winding turns

        N_S = Secondary winding turns

        I_S = Secondary current

        I_P = Primary current

        R_L’ = Reflected load resistance

Conclusion

Maximum Power transfer

The maximum power transfer from last stage of the amplifier to the load when

Output impedance of amplifier = Input impedance of transformer

Load Impedance = Output impedance of transformer

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