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Topic: What is this amplifier doing, exactly? (Read 8674 times) previous topic - next topic

oric_dan

Quote
flows for appreciably less than half of each cycle

So what is the class if current flows for half of each cycle, as here?

As I recall, Class C RF amps rely on the low-duty "ping" to stimulate the LC tank,
and set it to oscillating.

Nick Gammon


Yes, if the input stays high, then the speaker burns up. Can you 100% guarantee the
Arduino I/O pin will always be low 100% of the time when not using the amp?


OK, so the input capacitor means only transitions drive the speaker, and if I happen to load a sketch that drives the pin high full-time, then the speaker is OK. I get it. Thanks.
Please post technical questions on the forum, not by personal message. Thanks!

More info:
http://www.gammon.com.au/electronics


So as a simple "square wave" amplifier (or indeed digital switch) are there any problems with the above circuit?


Hi Nick,

Have you tried something like this?  This will eliminate the possibility of DC on the speaker and has a similar small parts count.  The transistor is a 2N2222.
Facts just don't care if you ignore them.

oric_dan

Oof, 1/2 Amp through the transistor when input held high. Nice 2.5W glow worm.

MisterResistor


Quote
As I recall, Class C RF amps rely on the low-duty "ping" to stimulate the LC tank,
and set it to oscillating.



Right! low-duty "ping" or a spike like pulse. Most RADAR amplifiers operate as class C. A Low duty cycle packs all that power into a very narrow bandwidth, like a CW transmitter as well. 

afremont

#50
Apr 03, 2013, 01:24 am Last Edit: Apr 03, 2013, 01:31 am by afremont Reason: 1

Quote
flows for appreciably less than half of each cycle

So what is the class if current flows for half of each cycle, as here?

As I recall, Class C RF amps rely on the low-duty "ping" to stimulate the LC tank,
and set it to oscillating.



Class AB amps as in HF ham radio linear amplifiers rely on the tank circuit to generate the entire other half of the waveform.  Class C amplifiers are non-linear, but that doesn't matter for FM or CW amplification.
Experience, it's what you get when you were expecting something else.

Krupski


Very nice - but will it give Tone outputs that nice warm Tube sound? 8)


No. No it will not.  :(
Gentlemen may prefer Blondes, but Real Men prefer Redheads!


Oof, 1/2 Amp through the transistor when input held high. Nice 2.5W glow worm.


Well, maybe you can show me the math on that?  Vce measured at .5V in my test rig with that rascally little ole input held high.  That's .25W where I live.  And if you're real paranoid about it, decouple the DC with a cap on the input.

Just wondering though, why would you leave the input high?  Do you not have control over that in your world?  Interesting.  Here in Canada microcontrollers do what you tell them to.  Unusual, I know, but must be a Great White North thing.
Facts just don't care if you ignore them.

oric_dan



Oof, 1/2 Amp through the transistor when input held high. Nice 2.5W glow worm.

Well, maybe you can show me the math on that?  Vce measured at .5V in my test rig with that rascally little ole input held high.  That's .25W where I live.  And if you're real paranoid about it, decouple the DC with a cap on the input.

5V/10ohms = 0.5 Amp.
5V*5V/10ohms = 2.5 Watts.

Plus, it's much much worse if you're not providing enough base drive to saturate transistor.
Worst case, 1.25 watts --> transistor=poof!

Quote
Just wondering though, why would you leave the input high?  Do you not have control over that in your world?  Interesting.  Here in Canada microcontrollers do what you tell them to.  Unusual, I know, but must be a Great White North thing.

Read post #43.

retrolefty

Quote
Just wondering though, why would you leave the input high?  Do you not have control over that in your world?  Interesting.  Here in Canada microcontrollers do what you tell them to.  Unusual, I know, but must be a Great White North thing.


Lots of things work well in Canada, but too bad their beer is so bad!

Lefty

oric_dan

You've obviously never had a Moosehead, lefty, but apparently their GreatNorthern
Transistors don't go poof, even if your code forgets to set the I/O pin low.

Docedison

D'you think the moose would care too much?
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard


5V/10ohms = 0.5 Amp.
5V*5V/10ohms = 2.5 Watts.


Brilliant.  Thanks for that.  Now I see where your signature line comes from.

Plus, it's much much worse if you're not providing enough base drive to saturate transistor.
Worst case, 1.25 watts --> transistor=poof!


There is enough drive.  You keep talking about situations that, unless your a total zonk, should never occur.  What if you attach the power to your Atmega328 backwards?  What if you try to monitor a 12V signal directly?  There are so many things you could do wrong, so I am not sure where you are coming from on this.  That circuit is up a n running in my lab.  It does not get hot, explode or glow if used the way it was intended.  Just like so many things.

Read post #43.


Just because you said something from the OneD TenT file earlier does not make it any more realistic if you reference it again.  You simply execute a digitalWrite(LOW) to the pin in question once you are done making pretty sounds.  This is so obvious I'm embarrassed to have to spell it out.

Again, it you are a total zonk, then I could suggest some small alterations to the circuit that should allow it to deal with your shortcomings.

1) Put a DC blocking capacitor on the input.  This will require the addition of a cap and a resistor
2) Use a 3W resistor on the collector

... and ... change the base resistor to 500 ohms.

However, none of this is necessary as it works just fine as it is.  BTW, any Canadian beer is better than Old Milwaukee!!

Even this guy know what I'm talking about.
Facts just don't care if you ignore them.

afremont

#58
Apr 03, 2013, 02:08 pm Last Edit: Apr 03, 2013, 02:11 pm by afremont Reason: 1
On BILLO's circuit.  According to the Fairchild datasheet for a 2n2222a, Vce(sat) is 1V for "large" currents.  That means that a maximum of 1V will be dropped by the transistor and a minimum of 4V will be dropped by the resistor.  That puts the max collector current at 4V / 10R = 400mA.  That means the resistor will be dissipating 4V * 400mA = 1.6W which will exceed the resistors max dissipation.  The 2n2222a will be dissipating 1V * 400mA = 400mW which will exceed the derated maximum dissipation for the transistor as it heats up.  Tell me where I got it wrong please.
Experience, it's what you get when you were expecting something else.

I think I've entered the twilight zone.

Okay, one more time.... I did not design the circuit to be turned on and left that way.

So, since every single person who looks at this is assuming that they will be forced against there better judgement to use it incorrectly, I will update the design and test it to make sure it is idiot proof.
Facts just don't care if you ignore them.

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