Need some help with transistors for amplification

Discussion in 'General Electronics Chat' started by w a n, Aug 25, 2009.

  1. w a n

    Thread Starter New Member

    Aug 25, 2009
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    I'm trying to build a little speaker the takes input from a 1/8" headphone jack. My components are the audio jack (to take input from an iPod), a AA battery holder to hold 2 AAs, an on/off switch, a pot to control volume, two speakers, and an NPN transistor for amplification. My original circuit basically just had the audio input hooked up to the collector of the transistor, the 3v (from the AA batteries) hooked up to the base of the transistor, and the speaker hooked up to the emmitter; thus the audio signal would be amplified by the two batteries and sent to the speaker. Obviously this wont work because I need two grounds (one for the battery and one for the audio in) but have only one source. How could I make a circuit that amplifies the signal and goes to the battery ground and audio ground?

    On another note, how could you determine the emmitter current from the base and collector current? say I have a collector current of 3 amps and a base current of 2 amps, would the emmitter current be 6? (3 x 2).
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    You need at least 3 transistors to make an extremely simple amplifier. One transistor makes a horrible amplifier that eats batteries.

    With only 3V for the supply that quickly drops to only 2V then the sound level will be very low.

    Here is a very simple amplifier that has 3 transistors and has poor performance:
     
  3. ogg

    New Member

    Sep 23, 2009
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    Hi, I have just built the same one transistor amp circuit actually, I'm using the battery's ground for the audio. Did you try this? It sounds awful, but it works: increase the voltage (I went up to 9V) and it will get louder.

    I built it to learn about transistors and amplification, not as a quality amplifier. As I understand it, the single transistor is the fundamental building block of amplification. I would really like to understand why it sounds so bad and why it would eat batteries. Broadly, I'd like to know what the issues are with single transistors that mean we need more compelx circuits to amplify sound effectively.

    In the three transistor circuit, could you say what the other components are doing that result in a better amplifier? Any help would be greatly appreciated, thanks in advance.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    In general, a good audio amplifier has a fairly high input impedance and a fairly low output impedance. A single BJT transistor typically can have one of these features but not both at the same time. For this reason, additional stages are added so that first stages provides the high input impedance while the final stage provides the low output impedance.

    hgmjr
     
  5. KitCarlson

    Member

    Sep 27, 2009
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    Perhaps investigate an emitter follower circuit. It is simple and efficient. It will not amplify the voltage, just the current. If you use a low impedance speaker you will have some volume. Amplified mini-speakers are available at reasonable cost (less than $20), some amplifiers are very power efficient and only require one AA battery. They might even have digital pwm amplifiers.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    A single transistor amp in class A common collector will sound beautiful. It just won't be very loud or very energy efficient.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Only when it is connected correctly.
    Wan had the collector, base and emitter pins of the transistor connected wrong. The transistor was probably saturated. He was feeding up to 3A or 6A through the speakers which offsets them and causes severe distortion.
     
  8. KitCarlson

    Member

    Sep 27, 2009
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    The RB:

    It seems to me the Class A is a common emitter circuit. Not common collector.

    The emitter follower is common collector known as a buffer amp, it provides impedance matching and current amplification.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Yep, sorry I meant common emitter.
     
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