What is this amplifier doing, exactly?

oric_dan:

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.

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

No. No it will not. :frowning:

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.

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.

BillO:

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!

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.

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

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.

D'you think the moose would care too much?

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

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

oric_dan:
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.

oric_dan:
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.

Moose.jpg

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.

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.

BillO:
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.

I intended no offense, just offering up my thoughts and trying to be sure that I understand the circuit. :slight_smile: When it comes to these microcontrollers, anything can happen. The output pin being left high wouldn't be an unusual situation during development, so how else should the circuit be analyzed? I'm thinking that just adding a blocking cap on the base along with a pulldown and it should be well protected from DC. At 50% duty cycle, everything should be fine, but you might want to use a smaller resistor on the base since hfe at large currents is as low as 20, unless I got it all wrong.

I do understand what you are saying, at least partially. I'm not sure where you got the hfe of 20. My spec sheet (Farichild 09/99) states a worst case hfe of 40 @ 500mA and shows a typical of around 80 @ 500ma. The transistor I am using actually measures at 121, but that is besides the point. Looking at DC characteristics in a circuit like this is pointless. The average current through the transistor is much less than 500ma. In fact, at 400hz the circuit only draws about 200ma.

I've come up with a 'better' circuit that will be (almost) idiot proof. I am just going to build it and test it to make sure it meets the design specs. However, I am cannot seem to come up with fool-proof design based on the simple switch we've been discussing here, so the new circuit is an AC-coupled class 'A'. I'll post the schematic when I'm done the tests.

Oops, you're right the minimum hfe is around 40. But average current isn't the point, this is about idiot-proof non-self-destructive design. :wink: If the average is 200mA then the max is 400mA which will melt things if it gets stuck on.

I don't see any need for a class A amp, common emitter class B should be plenty good enough and use less power. Sound quality certainly isn't the issue. An emitter follower would be good enough I think.

Sounds like the discussion has come back around to this.
AC coupled, transistor turns off when Input gets held High or Low, no DC going thru the speaker either.

I have 5 of these running, I have two Tones to make a nice output warble like a loud cell phone ring. Have been running since Jan 2011, so 2+ years. Ignore the header/switch at the top left, just use a series resistor from V+ to the transistor.

This article goes into depth of the single gate amplifier. Scroll down to page 8.

We don't need it biased to operate in the linear region since the tone output is either on or off - hence the amplifiers output can be on or off as well.

Yeah, like Crossroads showed. Class B and no wasted energy with DC idle currents.

afremont:
Oops, you're right the minimum hfe is around 40. But average current isn't the point, this is about idiot-proof non-self-destructive design. :wink: If the average is 200mA then the max is 400mA which will melt things if it gets stuck on.

Well, I'm not sure that would be self-destructive. In any case, as I said before, just rate the collector resistor at 3w. Problem solved. But read on...

afremont:
I don't see any need for a class A amp, common emitter class B should be plenty good enough and use less power. Sound quality certainly isn't the issue. An emitter follower would be good enough I think.

Class 'A' because I'm sure someone will likely find fault with any other solution. Since they have not seen it before, they will assume it just doesn't work. Not that they won't find fault with a class 'A'. Also, class 'A' will only require a couple of extra parts. Since the very simple was apparently too simple, why not go the extra step and make it something everyone can look up and wrap their heads around. BTW, I can't seem to get my head around an emitter follower solution. Not that it won't work.

So, below is the class 'A' I came up with. Draws 210mA @ 5V and delivers 125mw into 8ohms from 20Hz to 6KHz. Has typical single stage class 'A' distortion on sine a triangle waveforms and is biased to start clipping more or less symmetrically at Vin = about 4.5V. Square waves produce an expected over-shoot spike which is controlled by C3. If you don't care about the over-shoot, you can leave C3 out. The over-shoot on square waves causes and appreciable increase in gain at frequencies above 6KHz. Increasing C3 can help with this.

It works as described and does not get hot, explode, glow, kick the dog or beat the wife despite what you may believe. You can leave the drive pin high or low as you wish. If you want to prove it is does anything other than what I've stated, build one and show me actual results. So, all you 'Bash Street Kids', have at it.

CrossRoads:
This article goes into depth of the single gate amplifier. Scroll down to page 8.
http://www.ece.ucsb.edu/Faculty/rodwell/Classes/ece2c/labs/Lab_2_2C_2007.pdf
We don't need it biased to operate in the linear region since the tone output is either on or off - hence the amplifiers output can be on or off as well.

I agree.

... I will update the design and test it to make sure it is idiot proof.

From a practical viewpoint, this should be Rule #1, since everybody who's ever done
anything at all knows how large the idiot population really is.