Help with AC Amplifier

Thread Starter

Raymonde

Joined May 5, 2010
2
Although I have some knowledge of digital logic, analog circuits are a bit of a mystery so I need help please. We have a digital phone system where the base ringer activates a small 8 ohm speaker using 1.8v AC. I want to light a couple of LEDs (red) when the speaker activates (without any other connections to the phone except to the speaker) so I'm thinking a simple emitter follower amplifier to get sufficient voltage to drive the LEDs. The LEDs would be about 6 feet from the phone and could house a 9v battery for the amp power source. Is an emitter follower a reasonable approach?
 

Audioguru

Joined Dec 20, 2007
11,248
An emitter-follower LOSES voltage but it increases current, it does not increase voltage.
A common-emitter transistor with the load from its collector to the positive supply (for an NPN transistor) amplifies voltage and also amplifies current.
The transistor will need a resistor in series with its base and the LED will need a series resistor. The resistors limit the current.
 
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