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Transistor amplifier question

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Newbie level 3
Aug 13, 2007
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What I can't figure out is how we can get bigger sign wave out of the small signal?
By using different formulas I came up with the following values to make an single NPN transistor common emitter amplifier. RB1= 10K, RB2=5.1K, Rc=2.2K and Re=470Ω. I used 82pF capacitors for all coupling capacitors. For example if we put 100mV or lower peak to peak in the base and Collector Vcc is +12V. There should be a 0.7V voltage drop at the base/emitter junction. That means transistor is already on. When I measured output at the collector it was DC and there was no sign wave. My question is Once transistor is on with the bias voltage of 0.65 or 0.7 volts how a small ac signal can effect the output which is already been effected by the bias voltage. I understand transistor as a switch but somebody can help me understand how to amplify small ac or rf signals. Thanks:cry:

The base current modulation gets amplified in both emitter and collector. See those equations also.


Is that means output will be on the DC level but ac wave shape? If, for example we have 12VDC on the collector supply ignoring Rc, then output will be AC signal +12V above the DC level. I am confused.[/b]

If ur thinking of how the signal is amplified,its due to power provided by fixed dc supplies. physical viewpoint.
From circuit point of view and refering to c/c relatin ur o/p with ur input terminals;u cn obtain the o/p signal for any i/p.

i hope this helps

Thanks, IBO and brmadhukar.
If transistor was turning on and off with each high and low of the ac voltage swing at the base, then I could see output changing according to the input. But if the transistor is already on because of the dc bias voltage, it will stay on even ac goes high or low at the base.....How ac is effecting the collector current which is already passing through the transistor? Thanks

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