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basic transistor ckts can produce audio?

Discussion in 'Audio' started by max_torch, Mar 19, 2014.

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  1. max_torch


    Feb 9, 2014
    Can the basic transistor configs for small-signal amplification: voltage divider, collector feedback, self-bias, ever be made loud enough for driving audio for a speaker(not earbuds)?
    Or should you only try to do it with power amplifier configurations: class A, class B, etc..?
    Or should you always have a small-signal amp function as pre-amp for a power amp?
    and how do you add more amplification stages without distorting the signal?
    No op amps please the discussion is limited to transistors only
  2. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Depends on what you call loud. But yes it could be done. Not all amplifiers need to use a power amplifier for you to be able to here this through a speaker.Depends on the power you are after. Remember this, the SPL (Sound pressure Level) we hear needs to be approx.10 times the original power (10db) @ 1m for us to perceive this as a doubling in volume.

    Answered above.

    Yes, most low level audio sources couldn't drive the power amp directly so a pre-amp is used. This is also generally where any filtering, tone control circuits appear, but frequency cross overs normally appear after the amplifier before the speakers unless they are active types.

    Why do you think adding more stages will produce distortion. You will get a bit more noise but if designed correctly this shouldn't be an issue. You wouldn't want to add too many stages as this can effect the dynamics of the music.

  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Well, a Class A power amplifier is actually the same configuration as a small-signal amplifier. You can use a voltage divider for the base, and/or collector feedback, self-bias (JFET or toob), and emitter degeneration. But to drive a speaker (typically 8 ohms), you need a pretty low output impedance, which means a high current. If you use a collector load resistor, it has to have a pretty low value, and with around half the supply voltage across it, it will dissipate a lot of heat and waste a lot of power. Ditto with the output device. A transformer in the collector improves things somewhat, but it must be designed so it doesn't saturate with the DC bias.

    Another factor to consider is that bipolar power transistors have a relatively low current gain, and it drops at higher collector currents. Typical current gains of 10~20 are normal, compared to current gains in the hundreds for a small-signal transistor in a small-signal circuit. So for proper operation, a Class A power amp may need a more complicated bias circuit than a small-signal amplifier does.

    Some audiophiles are prepared to live with these disadvantages because of the magical powers of Class A - specifically, no crossover distortion. But normal people consider it impractical for any significant amount of output power - the borderline is around half a watt I suppose - and so we use Class AB or Class B.
    Yes, you have to drive a bipolar Class A output stage with significant current, because of its low gain.
    I'm not sure what you mean. I would drive a bipolar Class A output stage from an emitter follower, probably, which would in turn be driven from a voltage gain stage of some kind, if necessary. Every transistor adds some distortion, but the distortion in a Class A output stage will be the main factor, unless feedback is used.

    Google Class A transistor output stage for more information.
  4. max_torch


    Feb 9, 2014
    useful pieces of advice and info from all. I understood some of the things you said, but I think I would have to make circuits on a breadboard to really know for sure. I'll try making some circuits and I'll try the things you guys said here. Maybe after I try some experiments I'll post my finding here
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