AMPLIFIER

                                  AMPLIFIER

WHAT IS AMPLIFIER?

AMPLIFIER, RAISING THE STRENGTH OF A WEAK SIGNAL WITHOUT CHANGE IN ITS SHAPE IS CALLED AMPLIFICATION AND AN ELECTRONIC CIRCUIT DOES AMPLIFICATION IS KNOWN AS AMPLIFIER.

CLASSIFICATION OF AN AMPLIFIER

CLASSIFICATION OF AMPLIFIER CAN BE DONE DEPENDING UPON VARIOUS PARAMETERS LIKE:

#BASED ON TRANSISTOR CONFIGURATION

#BASED ON ACTIVE DEVICE

#BASED ON INPUT SIGNAL

#BASED ON OUTPUT

#BASED ON NUMBER OF STAGES

#BASED ON FREQUENCY RESPONSE

#BASED ON BANDWIDTH

BASED ON ACTIVE DEVICE

AMPLIFIERS ARE CLASSIFIED INTO

#BJT AMPLIFIER

#FET AMPLIFIER

#BJT AMPLIFIER

1. BJT IS A BIPOLAR DEVICE IT'S OPERATION DEPENDS UPON BOTH MAJORITY AS WELL AS MINORITY CHARGE CARRIERS.

2. BJT HAS LOW INPUT IMPEDANCE

3. BJT IS A CURRENT CONTROLLED DEVICES.

4. BJT IS NOISIER

5. BJT OCCUPIES MORE SPACE

6. BJT IS LESS COSTLY

#FET

1. FET IS A UNIPOLAR DEVICE. IT OPERATION DEPENDS ONLY UPON MAJORITY CHARGE CARRIERS.

2. FET HAS HIGH INPUT IMPEDANCE

3. FET IS VOLTAGE CONTROLLED DEVICE.

4. FET HAS BETTER THERMAL STABILITY

5. FET ARE MORE COSTLY

#BASED ON MODE OF OPERATION/ OPERATING POINT/ Q POINT AMPLIFIERS ARE CLASSIFIED AS:-

THIS CLASSIFICATION DEPENDS UPON PORTION OF INPUT SIGNAL CYCLE DURING WHICH COLLECTOR CURRENT IS EXPECTED TO FLOW:-

CLASS A AMPLIFIER

#COLLECTOR CURRENT FLOWS FOR ENTIRE A.C CYCLE

#ITS EFFICIENCY IS LOW (25%)

#IN AUDIO VOLTAGE AMPLIFIERS, CLASS A BIASING IS USED BECAUSE DISTORTION IS DOMINATING CONSIDERATION FOR DESIGNING AUDIO AMPLIFIERS.

#CLASS A AMPLIFIERS WORK ON LINEAR PORTION OF CHARACTERISTIC CURVE TO MAINTAIN GOOD FIDELITY OF OUTPUT WITH INPUT SIGNAL

CLASS B AMPLIFIER

#COLLECTOR CURRENT FLOWS ONLY DURING POSITIVE HALF CYCLE OF INPUT A.C SIGNAL

#IT EFFICIENCY IS HIGH (78%)

#IT USED TO PRODUCE SEVERE DISTORTION AS ONLY HALF CYCLE IS REPRODUCED.

#HOWEVER WHEN USED AS PUSH-PULL AMPLIFIER USING TEO TRANSISTORS, FULL CYCLE CAN BE REPRODUCED SO THEY CAN BE USED AS AUDIO POWER AMPLIFIERS

CLASS C AMPLIFIER

#COLLECTOR CURRENT FLOWS FOR LESS THAN HALF CYCLE OF A.C SIGNAL

#THEY ARE MORE EFFICIENT (ABOVE 90%)

#THEY ARE NOT USED AS AUDIO AMPLIFIERS

#THEY ARE USED AS RF AMPLIFIERS AND OSCILLATORS

BASED ON INPUT SIGNAL, AMPLIFIERS ARE CLASSIFIED AS:-

-SMALL SIGNAL AMPLIFIER
-LARGE SIGNAL AMPLIFIER

FOR SMALL SIGNALS, BEHAVIOUR OF TRANSISTOR IS LINEAR AROUND AN OPERATING POINT BUT WHEN SIGNAL IS LARGE IS AMPLITUDE, THE BEHAVIOUR OF TRANSISTOR BECOMES NON-LINEAR.

IN THE PRESENCE OF SMALL SIGNALS, THE OUTPUT IS A REPLICA OF INPUT EXCEPT FOR A CHANGE IN AMPLITUDE.

A LARGE SIGNAL HOWEVER OPERATES DEVICE IN A NON-LINEAR FASHION BY CLIPPING THE OUTPUT OR INTRODUCING VARIOUS KINDS OF DISTORTIONS.

BASED ON OUTPUT, AMPLIFIERS ARE CLASSIFIED AS:-

-VOLTAGE AMPLIFIERS

-POWER AMPLIFIERS

INPUT SIGNAL IS GENERALLY SMALL RINGING FROM FEW 4V TO FEW MEGAVOLT. THESE SIGNALS IF FED DIRECTLY, CAN'T DRIVE LOUDSPEAKER OR PUBLIC ADDRESS SYSTEM.

THEREFORE SIGNAL IS FIRST FED TO VOLTAGE AMPLIFIER SO THAT VOLTAGE LEVEL OF THIS SIGNAL IS FIRST RAISED TO SUFFICIENT HIGH VALUE.

THE VOLTAGE IS TENDS TO USE BY POWER AMPLIFIERS. THIS POWER AMPLIFIER THEN PROVIDES SUFFICIENT POWER GAIN TO DERIVE AN OUTPUT DEVICE AND HENCE POWER AMPLIFIER IS CAPABLE OF DELIVERING POWER TO LOUDSPEAKER.

OHM'S LAW

V=IR

WHERE V IS EQUAL TO VOLTAGE

I IS EQUAL TO CURRENT 

AND R IS EQUAL TO RESISTANCE

#IF THE RESISTANCE IS HELD CONSTANT AND VOLTAGE IS INCREASED, THERE WILL BE A CORRESPONDING INCREASE IN CURRENT. THIS IS THE OPERATING MODE OF A VOLTAGE AMPLIFIER.

#IF THE VOLTAGE IS HELD CONSTANT AND CURRENT IS INCREASED, THERE IS A CORRESPONDING DECREASE IN RESISTANCE. THIS IS THE OPERATING MODE OF A POWER AMPLIFIER.

P=VI

WHERE P IS EQUAL TO POWER

V IS EQUAL TO VOLATAGE

AND I IS EQUAL TO CURRENT

WHEN VOLTAGE IS HELD CONSTANT AND CURRENT IS INCREASED, POWER IS INCREASED AS WELL.

BASED ON FREQUENCY RESPONSE AMPLIFIERS ARE CLASSIFIED AS:-

-AUDIO AMPLIFIERS

-RF AMPLIFIERS

-VIDEO AMPLIFIERS

#FREQUENCY RESPONSE

BAND OF FREQUENCIES OR FREQUENCY RANGE THAT THE AMPLIFIER WAS DESIGNED TO AMPLIFY.

BASED ON NUMBER OF STAGES, AMPLIFIERS ARE CLASSIFIED AS:-

-SINGLE STAGE AMPLIFIERS

-MULTISTAGE AMPLIFIERS

SINGLE STAGE AMPLIFIER CONSISTS OF ONLY ONE TRANSISTOR WHILE MULTISTAGE AMPLIFIER CONSISTS OF MORE THAN ONE TRANSISTOR IN A CIRCUIT.

AS PERFORMANCE OBTAINED FROM A SINGLE STAGE AMPLIFIER IS OFTEN NOT SUFFICIENT FOR MANY PRACTICAL APPLICATIONS, HENCE SEVERAL STAGES MAY BE COMBINED, FORMING A MULTISTAGE AMPLIFIERS. THESE STAGES ARE CONNECTED TO FORM INPUT OF THE SECOND STAGE, WHERE OUTPUT BECAME INPUT OF THE THIRD STAGE.