power amplifier and voltage amp

Discussion in 'General Electronics Chat' started by ect_09, Aug 2, 2014.

  1. ect_09

    Thread Starter Member

    May 6, 2012
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    Hello,
    please tellme the difference between power amp. and voltage amplifer..

    what is the application of power amplifer.
    what is the application of voltage amplifier..

    Regards
     
  2. Kermit2

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    Feb 5, 2010
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  3. wmodavis

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    Oct 23, 2010
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    Power amp amplifies power.
    Voltage amplifier amplifies voltage.

    Application? See above.
     
  4. Lestraveled

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    May 19, 2014
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    Gee, everyone is a little on the snarky side today.

    I do not like any of the answers/descriptions I have seen so far on the web (or here). The answers are vague and misleading.

    Both a Voltage amplifier and a Power amplifier behave in the same way, they take an input voltage, amplify it, and output a voltage.
    Vin X gain = Vout.

    The only difference between a Voltage amp and a Power amp is the Power amp will output significantly more current than a voltage amp.

    Saying that a Power amp amplifies power is a misleading statement. They are both, voltage in - voltage out devices.
     
    ect_09 likes this.
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Not necessarily.

    @ect 09
    Was this homework or a school project?

    A full answer to your question could be quite complicated so tell us more about where you are coming from please.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    One example - an audio amplifier takes the tiny signals from a phonograph needle, a microphone or a musical instrument and outputs a low impedance, high power version of that signal to drive a loudspeaker. Both the voltage and the current have been amplified.

    One example - An AC transformer you plug into the wall outlet is a voltage amplifier with a gain less than unity. For instance voltage is dropped 10:1 from 120VAC to 12VAC. Current may be increased but certainly power is not. Such a transformer can be run in reverse to amplify the voltage of the input signal.
     
  7. Lestraveled

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    I disagree. You are confusing transformation with amplification. An amplifier samples a signal and adds to it from an external power source. Your transformer example just changes the voltage ratio without adding anything. This is the same as an antenna that has gain and calling it an amplifier.

    It is a misuse of terminology.
     
  8. Lestraveled

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    I borrowed this from Wikipedia:

    An electronic amplifier, amplifier, or (informally) amp is an electronic device that increases the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier modulates the output of the power supply.
     
  9. studiot

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    This is much more like it, but goes to far in my opinion, in that not all amplifiers increase the power since the amplification factor may be less than one.

    The important idea to take forwards is that power amplifiers are used to drive non electronic loads, ie convert electrical energy to mechanical or heat energy.

    The power for this is drawn from the supply and applied to the load in suitable format, controlled by the input electrical signal.

    Which brings me to my second point.

    We normally consider an electrical signal to be a voltage signal and a voltage amplifier conditions this signal in some way. It may make it bigger, smaller or change its frequency characteristics.

    But the bottom line is that the final output from a voltage amplifier is still a voltage signal to be input to another electrical device or stage, whereas the final output from a power amplifier is to be converted to another form of energy.

    So the signal in the following stage or device is (or should be) simply a function of the voltage amplifier, but the signal in the load of a power amplifier is also a function of the transducer characteristics of the load.

    Of course there will always be exceptions and grey areas but that is the main difference.
     
  10. MrChips

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    Oct 2, 2009
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    I would put it simply as follows:

    A voltage amplifier outputs a voltage with limited current.

    A power amplifier is designed to output higher currents.
     
  11. studiot

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    But higher than what?
     
  12. wayneh

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    +1 That's a clear distinction to recognize a power amplifier. But what's a pre-amp then? Compared to the input, both voltage and current have been increased, and yet the load is not intended to generate heat or motion.

    Recognizing what a power amplifier is raises the question of what is a voltage "amplifier", as opposed to signal conditioning. Does your amplifier cease to be an amplifier if the voltage gain is adjusted to <1? Must the output power of an "amplifier" be provided from an external source and never the signal?

    In my transformer example, what would you call it if it was a black box, 1V entering with a 10V output? What if a similar black box takes in power from another source but exhibits the same input and output. Is it now more of an amplifier than the first black box?
     
  13. MrChips

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    I just knew someone would ask that question.

    So using an LM675 as an example of a power opamp, it can deliver 3A into a load.

    Compare that with LM358 which can source 20mA. So there is a wide gap between the two.

    I would say anything above 200mA would be a power amp.

    You rarely find opamps with output currents in the range 20mA-200mA
     
  14. Lestraveled

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    LM13080, opamp, .25 amp output, 8 pin dip.......power amp??

    My definition of a power amp is: if it has heat sinks, it's a power amp.
     
  15. MrChips

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    National Semiconductor calls LM13080 a power opamp. I am not going to argue with them.
     
  16. Lestraveled

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    I agree, I don't think this topic deserves any kind of argument.
     
  17. studiot

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    Differentiating between amplifier types acccording to what they are driving is clear cut as you say.

    But as I also think this was a homework question, or why else would the answer

    A power amp amplifies power and a voltage amp amplifies voltage.

    be unsatisfactory?

    As to the amplification factor I have a laboratory (voltage) amplifier that offers switchable factors from 0.001 to 10,000.

    Signal conditioning and pre amps? Well most conditioning amps contain at least one voltage amplifier at the heart.
    For instance how about the statement 'an active filter is basically a voltage amplifier with frequency selective feedback'?

    External power source?

    Well there are signal powered amps, particularly in RF and (I think) in neurotransmitters. But we can also add further qualifiying adjectives such as active and passive.

    Yes indeed, so what would you use each to drive?
     
  18. to3metalcan

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    Jul 20, 2014
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    Maybe it's about intent? By which I mean, the load of the amp? Voltage amps usually do incidentally raise the current, but the *goal* of a VAS is clearly to reproduce a signal at a higher voltage, with output impedance as a *secondary* concern.

    Power amps always, in my experience, are meant to do something "in the real world," ie, driving a transducer of some sort.

    For example: The NJM4556 is an 8-pin DIP dual op-amp with relatively high current...it could be used as a voltage amp, but it could also be used to drive headphones. Same device...different purpose.
     
  19. MrChips

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    The NJM4556 can drive 70mA into 150Ω load.

    This hardly classifies it as a power amp.
     
  20. to3metalcan

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    Jul 20, 2014
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    And yet they are startlingly loud headphone drivers. Is that not a power application? I have a mixer upstairs...the signal from the voltage gain stage won't drive headphones on its own, but the 4556 in parallel unity-gain buffer-mode will drive the heck out of them (and no, it's not the highest-powered device...I picked it because it's irritatingly on the threshold. :) )
     
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