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Author Topic: What is this amplifier doing, exactly?  (Read 4720 times)
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I found a 220 uF cap lying around and that gives this:



I presume I put the + side on the Arduino side? That's the more positive side of it, right?



As for the 100 ohm between emitter and ground, that made it much softer:



Did you mean to replace the 1K with 100 ohm between base and ground?
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The 220 uF looks much better than 0.33 uF. The quick decay is gone, and you'll
not have any dc-currents through the speaker.

I thought the 100R in the emitter would help improve the low-freq response, but
it also kills the gain too much. So you might go back to tying the emitter to gnd,
and use a 10K in series with the 220 uF cap on the base. I might also use a larger R
on the base to gnd, eg go back to the 100K.

I'm not sure why you're getting the overshoot on the leading edge. Do you have the
scope probe ground lead tied close to the same point as the speaker gnd, or right
at the emitter?
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The Arduino Gnd pin, so there was a bit of a cable run. I moved it to next to the emitter, but no real change on the display.

By a coincidence I got these in the mail today from eBay:



2 Channels 3W PAM8403 Class D Audio Amplifier Board 5V

$US 2.84 each.

Wired one up to my iPhone and those old speakers. Quite nice sound out of it.
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So you might go back to tying the emitter to gnd, and use a 10K in series with the 220 uF cap on the base.

Nah, that just killed the sound altogether (the 10K resistor).
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Your question on the 10 ohm resistor was good. It is exactly for current limiting. Remember the 6-8 ohm speaker is an impedance (Xl = 2*pi*f*l) usually measured at 1000 HZ. So with no ac signal, the dc value of that speaker is basically a short circuit.
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Well, actually, not "quite" a Class A design. Class A is intended to bias the transistor
into its linear region of operation, but you'll never get that with the ckt as shown.

It looks like the person who designed it had the intent of Class A, but you would need
a resistor in the emitter lead, so the 100Ks on the base have something to bias in
a stable fashion. As it is, they simply turn on the transistor, and whether or not the
collector sits at Vcc/2 is strictly a factor of the hFE [beta] dc current-gain of the
transistor.

Advice - throw this ckt away, use something better.


Sure it's Class A. Class A is defined as bias current flowing all the time. Class A doesn't HAVE to also be "linear".

And, given the choice of Class A, Class AB, Class B, Class C or Class D, what would YOU call it?  smiley

The emitter resistor isn't really needed because the bias-stabilizing negative feedback comes from the fact that the top base resistor is connected to the COLLECTOR and not to VCC.

I agree though, that circuit is better off in the trash can!  smiley
« Last Edit: April 02, 2013, 09:04:08 am by Krupski » Logged

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This is the speaker. I am quite happy with them.
http://www.mpja.com/4-Ohm-Mini-Speaker/productinfo/14618%20SP/


For a tiny speaker amp, why not an LM-386?
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By a coincidence I got these in the mail today from eBay:



2 Channels 3W PAM8403 Class D Audio Amplifier Board 5V

$US 2.84 each.

Wired one up to my iPhone and those old speakers. Quite nice sound out of it.

Nice!

Here's my 300 watts per channel Class-D power amplifier (300W into 4 ohms, 150W into 8 ohms and 50V output swing +50/-50). Bwahahaaa!!! smiley


* amp-side.jpg (219.63 KB, 800x600 - viewed 34 times.)
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Very nice - but will it give Tone outputs that nice warm Tube sound? smiley-cool
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So you might go back to tying the emitter to gnd, and use a 10K in series with the 220 uF cap on the base.

Nah, that just killed the sound altogether (the 10K resistor).
I don't know what works best from 5000 miles away, so you have to play with values, :-).

But I would definitely keep the cap in line to the base and the R to gnd on the base [but
larger than 1K], so the amp is turned off with no signal input, and so no DC flows through
the speaker. With these in place, you can remove the 10 ohm R from the speaker line.

Quote
And, given the choice of Class A, Class AB, Class B, Class C or Class D, what would YOU call it?

The emitter resistor isn't really needed because the bias-stabilizing negative feedback comes from the fact that the top base resistor is connected to the COLLECTOR and not to VCC.
My feeling was, the original ckt was an "attempt" at a Class A linear design, but not
especially well done, ;-).

And you're right, I overlooked that the upper base R is really connected for "self-biasing"
and not normal Class A linear operation, which brought the comment about an emitter R.
So, I get a demerit for that [bad dog]. OTOH, I've tried self-biasing ckts like that, and
never found them to be very stable with beta and temperature variations. The emitter R
is the way to get real gain stability.

Also, of course, if you are going to do either bias scheme, then you need to isolate
the speaker using a really big cap, as previously mentioned.

So, would Nick's ckt as is be a Class C?
« Last Edit: April 02, 2013, 12:50:01 pm by oric_dan » Logged

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If the device (tube or transistor) operates into 'cutoff', it will meet the class C fingerprint.
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It's really just acting like a switch.  I'd hazard to say you could call it a class 'D' amplifier, but even that would not be right.  Class 'C' would, I guess, be closest.

Edit:  On second thought, no, it's not a class 'C'.  It's just a switch.  It gets driven between saturation and cut-off and is not linear in any way.  For lack of a proper name, we could call it a class 'S' amplifier.
« Last Edit: April 02, 2013, 03:28:30 pm by BillO » Logged

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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?
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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?
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class C amplifier [‚klas ′sē ′am·plə‚fī·ər]
(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.
 
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