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Transformer Coupling

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Introduction Figure shown the transformer coupling to a load resistance such as a Loudspeaker in a radio. With a transformer – the small load resistance can be stepped up to a higher impedance level This improve the voltage gain – also since no signal power is wasted in a collector resistor, all the ac power is delivered to the final load resistor Very popular at audio frequencies (20 Hz kHz)

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**AMPLIFIER AMPLIFIER OBJECTIVE**

CC RB1 Vout VCC Vin(ac) Rgen RB2 RC RL RE AMPLIFIER OBJECTIVE PROVIDE V, I , POWER to the LOAD most efficiently

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AMPLIFIER POWER LOSS CC RB1 Vout VCC Vin(ac) Rgen RB2 RC RL RE

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**TRANSFORMERS V & I transformation**

Nprim : Nsec Vprim Vsec Iprim Isec I & V ONLY ac

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**TRANSFORMERS Z & R transformation**

Nprim : Nsec Vprim Vsec Iprim Isec Zsec Zprim I & V ONLY ac

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**TRANSFORMER COUPLED AMPLIFIER**

Vout VCC n :1 Vin(ac) RL CC RB1 Vout VCC n :1 Vin(ac) Rgen RB2 RL

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**TRANSFORMER COUPLED AMPLIFIER**

CC RB1 Vout VCC n :1 Vin(ac) Rgen RB2 RL VCC RB2 RB1 RprimDC DC EQUIVALENT CIRCUIT IDEALLY Rprim,DC = 0

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**TRANSFORMER COUPLED AMPLIFIER**

CC RB1 Vout VCC n :1 Vin(ac) Rgen RB2 RL CC RB1 VCC Vin(ac) Rgen RB2 Rsec,ac ac EQUIVALENT CIRCUIT Rprim,ac = n2 RL

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**TRANSFORMER COUPLED AMPLIFIER DC LOAD LINE**

‘ideal’ DC load line ‘Q’ point determined by BIAS CIRCUIT VCEQ = VCC Q

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**TRANSFORMER COUPLED AMPLIFIER ac LOAD LINE**

OPERATION maximum output voltage ‘ideal’ DC load line Q ac load line ‘Q’ point determined by BIAS CIRCUIT VCEQ = VCC 2VCC

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**TRANSFORMER COUPLED AMPLIFIERS**

Advantages: electrical isolation: SAFETY NO XC voltage drop : input higher efficiency : no RC power loss zero or 180 degree phase shift : DOT NOTATION impedance matching : better power transfer voltage / current transformation : can be used as a ‘gain’ stage Disadvantages: COST WEIGHT frequency response SIZE : reduces as frequency increases

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**EXAMPLE: Determine the amplifier Q-point**

CC RE RB2 RB1 Vout VCC 20 V RL k :1 Vin(ac) Rgen Determine the amplifier Q-point Operating Q-point for the max. o/p i/p impedance, o/p load power

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SOLUTION: Q-point ii) Operating Q-point for the max. o/p

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iii) i/p impedance, CC RE RB2 RB1 k :1 Vin(ac) Rgen Rin Vout RL Rinb

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iii) o/p load power,

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Electrical, Electronic and Digital Principles (EEDP)

Electrical, Electronic and Digital Principles (EEDP)

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