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amplifier types


Guest bhuvaneshnick

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You can use an opamp that is a very good voltage amplifier. But it has low output current.
You can use an emitter-follower transistor that is a good current amplifier. But it has no voltage gain.

Power is voltage times current. A power amplifier can have good voltage gain and good current gain.

Many speakers are 8 ohms. 20W of power into an 8 ohm speaker is fairly loud.
Simple arithmetic shows a voltage of 12.7V RMS (18V peak) and a current of 1.57A RMS (2.22A peak) in an 8 ohm speaker producing 20W.

An opamp can easily provide an output of 12.7V RMS into a resistance of 1000 ohms but its maximum output current is only about 28mA RMS so its maximum output power is 12.7V x 28mA= 0.36W.
Since the maximum output current of an opamp is about 28mA RMS then its output into an 8 ohm speaker is only 0.006W.   

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Guest bhuvaneshnick

indeed great explaination.And one more doubt why does voltage amplifier need high input resistance and 0 output resistance current amplifier need 0 input resistance and high output resistance.explain in detially.thank you

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why does voltage amplifier need high input resistance and 0 output resistance current amplifier need 0 input resistance and high output resistance.

No.
A voltage amplifier does not "need" a high input resistance:
1) A common-base transistor amplifies voltage but has a low input resistance. A common-emitter transistor also amplifies voltage but has a fairly high input resistance.
2) An inverting opamp amplifies voltage and can have a very low input resistance. A non-inverting opamp also amplifies voltage but can have an extremely high input resistance.

A current amplifier can have a low or high input resistance. A current amplifier can also have a low or high output resistance.

But maybe you are asking about a voltage SOURCE and a current SOURCE, not amplifiers.
1) A voltage source has a very low output resistance.
2) A current source has a very high output resistance.     
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Guest bhuvaneshnick

i am getting to the point thanks.if we connect micropone as input and loudspeaker as output in common emitter confguration(without any resistors)does this connected configuration itself provide high input impedence and low output impedence.i am newbie sorry if my question is too basic

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No. A single transistor is not an audio power amplifier. Simple arithmetic shows why.

The output voltage from a microphone is about 0.01V. 2W into an 8 ohm speaker is 4V RMS. Then a voltage gain of 4V/0.01V= 400 is needed.

The output current from a microphone is about 3.7uA. 2W into an 8 ohm speaker is 0.5A RMS. A current gain of 0.5A/3.7uA= 135,100 is needed.

But a single transistor has a maximum voltage gain of about 100 and a maximum current gain of maybe 50.
Also, a common-emitter transistor needs two resistors to base its base, and it probably needs an emitter resistor. It is bad to use the speaker as the collector load for a single transistor because the DC current will offset its cone which causes damage and distortion.

A very simple power amplifier to drive a loudspeaker usually has an input common-emitter transistor, a common-emitter voltage amplifier transistor and a pair of NPN-PNP common-collector output transistors. That is 4 transistors, a few capacitors and many resistors. Or an audio amplifier IC can be used.

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Guest bhuvaneshnick

ok,i got that.then we are choosing the input resistance and output resistance for proper biasing.then why did you said common emitter configuration has high input impedence(making it high or low is in our hand right)

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Biasing has nothing to do with input resistance or output resistance.


Biasing allows a transistor to be a linear amplifier. If it is biased wrong then it rectifies the signal with horrible distortion because it is very non-linear.

A common emitter transistor has a much higher input resistance than a common-base transistor.

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