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Amplifier Conundrum!

Discussion in 'General Electronics Discussion' started by Flurng, Mar 20, 2013.

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  1. Flurng


    Dec 29, 2012
    Well, I've got me yet another stumper. ( 'leastwise, it's got ME stumped, anyway! ) Here's the situation;

    Many moons ago, I threw together a simple audio signal generator, based on a typical astable multivibrator circuit, which I built with that lil' sweetheart of a transistor, the 2N2222. I built this to generate a square wave signal to test audio amplifier circuits, and it works just great.

    So, I then whipped up a dirt-simple amplifier stage, using ( you guessed it... ) my little honey, the 2N2222, and fed it the signal from my generator. Again, all systems are go. This circuit powers a set of mini-headphones just fine, thank you. The parts list for my amp stage include; the transistor, a 33k load resistor, a 4.7k bias resistor, a 22uf capacitor, and a whole lot o' NUTHIN' else.

    So, I break out my 'scope to take a peek under the hood & see what's a-goin' on in there.

    NOW it gets freaky!

    That's cause ol' Scopey is telling me that, even though the transistor is rated at a voltage gain of around 100, my amp stage has got a gain of one-point-not-much, just BARELY over unity, yet I clearly hear a strong signal in my phones, which I DON'T get straight from the generator itself, so it's obviously amplifying somehow.

    Well, this had me all befuddled, until I realized that my amp circuit is PRECISELY 1/2 of the multivibrator circuit feeding it! Now it starts to make sense - by the time my square wave gets to my output amp stage, it's already ( effectively ) been through THOUSANDS of identical stages, back and forth between the two halves of the multivibrator. And if THEY have maxxed out the signal voltage, how can another, IDENTICAL amplification stage hope to add any more voltage?

    O.k. - so I've done pounded that much into my thick head. But, if I've got no voltage gain happenin', then what else is there? Well, it only took me a nanosecond ( give or take a few hours ) to figure out I must be amplifying CURRENT. But both circuits are connected to the same power source; a 9-volt battery, so where's this magical "current gain" coming from? And just WHO shot that "single bullet", anyway?

    Whoops, sorry - got carried away for a second. But, honestly, what's the story here? I'm just not gettin' the picture. Anyone got some thoughts on the matter? I'd love to hear 'em!
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Show us the circuit diagram.

    It's quiet possible your amplifier is amplifying current, not voltage and thus you don't see it on a scope.

    It's also possible that the input of the amplifier has such a large amplitude (compared to the supply voltage) that it really can't be [voltage] amplified any more.
  3. Flurng


    Dec 29, 2012
    I kinda figured.....

    As you suggested, I've uploaded a (crude) schematic of the circuit, so's y'all can see for yourselves what kind o' quick & dirty engineering I'm dealing with.

    Regarding your comment, I kinda figured that was the case - I'm still not certain this is the correct conclusion, but unless you see something I don't, it's the one I'm goin' with. In any case, the circuit's working, so I'm not looking to troubleshoot it, just hoping to improve my understanding of it.

    Thanks, by the way, for responding so promptly! Wow - that's gotta be a new record! After posting in so many forums and hearing nothing but crickets, it's nice to find a place with such an active membership! Right on!

    Attached Files:

    Last edited: Mar 21, 2013
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Your astable MV swings approximately between 0V and 9V, the operating voltage.
    The supply voltage of your "amplifier" is 9V, too. So the amplifier cannotz but swing between 0V and 9V. What do you expect?
    Even at a gain of >1 an amplifier cannot generate more output than the power supply allows (well, 2* if a push-pull stage is used).
    Since both the collector resistor of the MV and of the amplifier are 4.7kOhm, you cannot even draw more curtrent from your output stage than from the MV. The only advantage is that any current drawn from the output will have a negligible influence on the frequency of the astable MV. The output transistor decouples both.

    If you need a higher output voltage, you have to supply the output stage with a higher supply voltage.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    No, That's pretty much it. The "amplifier" actually produces no better output than the multivibrator itself, however it does buffer the output of the miltivibrator so that if you place a heavy load on the output it will not affect the frequency of the signal (which is what would happen without the extra stage).
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Oh, some of your designations are odd. 22kmF technically means 22F, and that's a HUGE value of capacitance.

    I'm not sure if you mean 22uF or 0.22uF, or even 0.022uF, but I'm sure it's not 22F.
  7. duke37


    Jan 9, 2011
    9V is a bit much for the multivibrator. You may generate more than the permitted reverse emitter/base voltage. The normal solution is to put a diode in series with the base to stop reverse current.
  8. Flurng


    Dec 29, 2012
    Howdy, Gang! First off, to all those who responded, thanx so much for the valuable input!

    To *Steve*; You're right - the capacitor value is a bit mysterious - all I can read off it is "22", and then some microscopic letters & hieroglyphics & such. I don't know WHY they don't just adopt some kind of color code, like they have for resistors! That would eliminate SO much guess work!

    To Mr. Harold Kapp; Thanx for the insight regarding the current through the collector resistors. I sort of guessed that the current throughout the whole system was limited to the supply current at best ( much like the voltage is limited to 9 volts max ), but that's why I'm mystified that I get an audible output from the amplifier stage, but NOT from the MV itself!

    To "duke37"; Interesting idea about putting a diode on the base lead - I'll give that a try. I might also try sending the supply voltage through a voltage divider & see what results that gives!

    Thanx, once again everybody, for the lively discussion! You've all given me lots to think about & I'll try & post further updates on the state of my circuit as I discover new results! ( translation; blow s**t up! ) Keep those ideas comin'! My ears are always open!
  9. BobK


    Jan 5, 2010
    A voltage divider is not a method for reducing a supply voltage, unless you are drawing a small constant current, which your multivirator is not. To reduce the supply voltage you would want a voltage regulator.

  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I'd say that 0.022uF is most likely.

    Resistor divider (as mentioned above) is a poor option. The diode in series with the base is a good one. You''ll know when you have it around the right way because if it's the wrong way the circuit won't work. (The cathode -- the end typically with a band -- goes toward the base).
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