What is this amplifier doing, exactly?

I did a DC test of this circuit from page 2:

I replaced the speaker with a 10R resistor, just in case I damaged it.

With the input grounded the current flow was virtually zero.

With the input at 5V the current flow was 100 mA (which seems to be the limit for the BC546 if the value for "Collector Current (DC)" is the correct one to look at (absolute maximum ratings, heh).

Replacing the 2.2K resistor with 100R gives a higher current (160 mA) but I guess the transistor won't be liking that, huh?

The capacitor doesn't seem to make much difference. With no input voltage, the transistor won't turn on, and current won't flow, whether or not it's there, am I right?

I measured 1.8 mA current drawn from the Arduino (input) when on, so that would seem to be well within the limits of a digital output pin.

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

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?

class C amplifier [‚klas ?s? ?am·pl?‚f?·?r]
(electronics)
An amplifier in which the bias on the control element is appreciably greater than the cutoff valve, so that the output current in each device is zero when no alternating control signal is applied, and flows for appreciably less than half of each cycle when an alternating control signal is applied.
A transistor amplifier in which each transistor is in its active region for significantly less than half the signal cycle.

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.

oric_dan:
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.

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.

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

oric_dan:

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.

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.