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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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