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Author Topic: Small Push-Pull Amplifier  (Read 13899 times)

Kaz Kylheku

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Re: Small Push-Pull Amplifier
« Reply #15 on: December 10, 2012, 05:46:23 PM »
Is it really OK to put the voltage feedback through 220 uF couplers in that TDA2005 circuit? (Is that from the datasheet? Too lazy to look).

Even if the datasheet example circuit has that feature, it seems dodgy. You have no DC feedback for basic DC stability, and large gain at low frequencies.

It seems that a better way is to AC-couple the voltage divider itself to ground, and DC-couple the feedback branch.

Then, from a DC analysis point of view, you just have a DC connection from output to (-) through some resistance: the stage is basically a voltage follower for DC, and has a low gain for low frequencies. For AC above the RC network's corner frequency, the feedback is determined by the voltage divider, same as now.

I don't think the cap to ground needs to be anywhere near 220 uF, either.
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J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #16 on: December 11, 2012, 09:03:52 AM »
The datasheet application is correct.
What's confusing you is that it's *drawn* similar to a standard Op Amp ... which it's not. :(
A TDA2005 is 2 x TDA2003 into a single larger chip.
A TDA2002 (the original version) was a *very* clever idea when it was invented by a Genius, the Italian Engineer Bruno Murari, head designer at ST Semiconductors.
Up to that point, the Japanese had been making car radio amplifiers, but they needed from 8 to 12 pins for auxiliary components; his claim to glory was making a successful one inside s TO220 case, with only 5 pins *and* self centering when using single supply (without external resiostors), plus being robust enough to survive up to 40V peaks, self protect thermally, resist inverted battery connection, the works.
A real genius.
His idea was so good, that we *still* have 5 pin TDA20xx chips *everywhere*.
Read the "LM383 story", where its designer explains that they blatantly copied TDA2002 and how it works:
http://www.idea2ic.com/LM383/index.html
The original application suggestion:

and its internal circuit is:

The bright idea was to balance current from 2 resistors: a 40K one from the +V rail, and a 20K one from the output.
To get that, voltage across the 20K one must be 1/2 voltage across the 40K one. Brilliant!!!
So now you *need* that external capacitor to avoid it messing with the DC balancing circuit and also need it to be large (and the feedback resistors low value) because impedance at the mixing point is low.
Very intelligent guy this Bruno Murari.
http://www.idea2ic.com/LM383/_Milan__a_Subtle_Artisan_.pdf

Kaz Kylheku

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Re: Small Push-Pull Amplifier
« Reply #17 on: December 21, 2012, 04:58:30 PM »
Interesting that this thing has a "quasi-complementary" output stage: both transistors are NPN.

The MC33078's op-amps an all NPN output stage also. They might make a good "wine pairing" with this power amp. :)

Thanks for that; I will read the paper.  I can see in the diagram how the current mirror sucks the same current through the 40K and 20K, so the top of the 20K is approximately V/2.
« Last Edit: December 21, 2012, 06:36:44 PM by Kaz Kylheku »
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J M Fahey

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Re: Small Push-Pull Amplifier
« Reply #18 on: December 22, 2012, 09:44:52 AM »
Yes, a real genius.
In the paper he explains why he used all NPNs: with the 70's technology the power PNP's which would fit there were "slow" and messed with compensation and stability big time.
And I've been making SS amps since the early 70's, I remember almost *everybody* used NPN's , 2N3055 or some of its variants.

Roly

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Re: Small Push-Pull Amplifier
« Reply #19 on: January 29, 2013, 08:17:20 PM »
After fifty-odd years dealing with mains power in industrial and domestic situations I could tell you some hair raising stories, but JM, phatt, and I are at one on this; you treat the power mains lightly at your extreme peril.

This is not being over the top or namby-pamby - I have witnessed what the mains can do to flesh in a few moments, and it ain't pretty; 6000 degrees will leave you with burns that will scar you for a lifetime, if you survive.  No bloody joke.

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The Electric Guitar is one of the rare situations where the metal parts on the guitar are Directly connected to the Chassis of the equipment.
If you say theory and practice don't agree you haven't applied enough theory.

CamKrist

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Re: Small Push-Pull Amplifier
« Reply #20 on: February 05, 2013, 07:38:14 AM »
Similar subject was being discussed at yahoo answers last week. I can post the link if needed.
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