INPUT-OUTPUT IMPEDANCE OF 3 MODES OF BJT OPERATION

Audioguru

Joined Dec 20, 2007
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Simply learn about transistors. There are tutorials here and Google has many of them.
A Common-base transistor has a low input impedance.
A common-emitter transistor has a medium input impedance.
A common-collector transistor has a high input impedance.

The datasheet of the 2N3904 transistor has a graph of its typical input impedance when it is common-emitter at different currents. The tutorials show how to calculate the input impedance.
 
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