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tech_vaibhav_ee • Jun 7, 2008

voltage amplification

i am new to electronics ... at present i am reading about various electronic devices and how do they work...i have some doubts over the application of a transistor amplifier(common emitter npn)....
first i would tell u what i know....
there are two circuits , one between base and emitter and other between emitter and collector....the collector emitter circuit is given a large reverse bias voltage and base emitter is given a forward bias voltage...
as the base emitter voltage increase , current in the collector emitter circuit also increase proportionally..(i also know why does it happen...)
how is this circuit used as a voltage amplifier ,i mean on increasing the base emitter voltage only the current in collector emitter increase while the voltage across it remains constant, then how do the voltage gets amplified??
mkaras • Jun 7, 2008
Acting as a voltage amplifier it will be easy to analyze this by considering to put resistor in the collector to V++ supply connection. Put another small resistor from the emitter of the transistor to GND. Now also put a pair of resistors from the base with one to the V++ and one to GND. Select these resistors so that the base is biased at some nominal current that results in the collector current to that such that about one half of the V++ voltage is dropped across collector resistor. Using a small cap put from a signal source to the base resistor connections. You will note that the signal amplitude at the base is amplified roughly by the ratio of the collector resistor value divided by the emitter resistor value. Of course you have to pick the resistor values properly. Here is a simple example circuit I have in LTSpice to show an example:


The following graph shows the input and output waveforms. The input is a 1KHz sine wave with a peak to peak amplitude of 200 mV. The output, as you can see, is about 2.2V peak to peak for a gain of about 10. (~ 1K/100).

mkaras • Jun 7, 2008
Note that the single transistor common emitter amplifier circuit inverts the signal.

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