[SOLVED] Why BJT for amplification?

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psurya1994

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Why do we have those complex mechanisms for amplification? Why don't we use the transformer directly?
 

You can only use a transformer to step-down and step-up AC signals. For DC signals, using BJTs and other similar devices make the most sense.

I'm sure there are other valid reasons which people more knowledgeable here could share.
 
A transformer is large, heavy and expensive. In some applications, using a transformer would be fine.
 

A transformer can not give any power gain. If the output voltage is higher than the input voltage, then the output current will be lower than the input current.

With amplifier circuits we can increase the voltage and the current.
 
Amplification is to do with increasing gain. (Voltage, Current and Power).

BJTs depending on the configuration e.g common-emitter, allows you to increase both voltage, current gain, and power gain at the same time.

A transformer on the other hand does not give you overall power gain due to the turns ratio.

If you step up the voltage your current steps down by the same amount
 
A transformer will transform the effective impedances as well as the voltage. For example, if you have a 1000 Ohm load on a transformer secondary with a 100:1 step-up ratio, the effective impedance at the primary will be 1000/(100^2) = 0.1 Ohm.
 
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