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(Transformer Coupling)

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Presentation on theme: "(Transformer Coupling)"— Presentation transcript:

1 (Transformer Coupling)
POWER AMPLIFIERS (Transformer Coupling)

2 Class-A The theoretical maximum efficiency of a basic RC- coupled class-A amplifier is limited to 25%. In practical circuit, the efficiency is less than 25%. Used for output power of about 1 W only. Transformer coupling can increase the maximum efficiency to 50% Disadvantage of transformer coupling – expensive & bulky.

3 Transformer-coupled common-emitter amplifier
Neglecting transformer resistance and assuming RE is small;

4 For ideal transformer;

5 Turn ratio is designed for maximum symmetrical swing, hence;
The maximum average power delivered to load equals maximum average power delivered to the primary of the transformer (VCC and ICQ are maximum possible amplitudes of signal)

6 The average power supplied by the VCC source is;
The maximum possible efficiency is;

7 EXAMPLE

8 Solution DC analysis

9 Using Thevenin’s theorem;

10 Taking the VBB – B – E loop;
Substituting values;

11 Using reflected resistance, the circuit becomes as shown

12 The corresponding ac equivalent circuit;
Using hybrid- model, the circuit may be represented as follows;

13 The voltage gain; And the amplitude of the output voltage across the transformer primary is;

14 At the transformer secondary;
n1 : n2 = 3 : 1

15 The average power to the load;
The average power supplied by the source; The power conversion efficiency;

16 Transformer-coupled emitter-follower amplifier
The maximum power conversion efficiency = 50%

17 EXAMPLE Known: Calculate a for PL = 5 W Calculate 

18 Solution DC analysis

19 Replacing the Tx with the equiv. impedance
The AC equivalent circuit

20 The equivalent circuit (hybrid- model);
The voltage gain is;

21 The amplitude of the output voltage is;

22 For 5 W output power; Hence; The transformer turn ratio;

23 As calculated previously;
Neglecting power in R1 and R2, the power supplied by the source is; The power conversion efficiency is;


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