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Chip Amps Amplifiers based on integrated circuits

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Old 25th March 2014, 07:10 PM   #1
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Default For a laugh

With all the talk over counterfeit/fallout chips I got a giggle out of this. Saw the $.75 TDA2002 chip and thought I would take a look at it. Ad says "house numbered" and if you look at the numbers on the illustration/picture...

I'm sure it was just a mistake on the web site i.e. they wanted a 5 pin audio amp chip so he just grabbed what was available.

The TDA2002 is of practically no interest in this day and age. Only thing interesting about it is in one of the designs they use a pattern on the circuit board to make an inductor to block RF.
TDA2002
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Old 27th March 2014, 08:25 AM   #2
marce is offline marce  United Kingdom
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Got a link please?
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Old 27th March 2014, 05:34 PM   #3
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The data sheet was on jameco.com

I copied the relevant part of the pdf. Only mildly interesting in that everyone wants to reduce board area to a minimum so doing a RF inductor in copper clad pc board does not seem to be on anybody's hit list. I didn't bother to do any calculations of course. From experience with tuned circuits I would guess that size of inductor would be suitable for something in the 100-2000 MHz range. Depends on what cap you stick with it of course.
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Old 27th March 2014, 05:47 PM   #4
macboy is offline macboy  Canada
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That is not a PCB inductor. It will have some inductance, but not much. Since it doubles-back on itself, the mutual inducatance of the parallel pairs of traces (with equal and opposing currents) will cause partial cancellation of the inductance. The inductance of this will be less than that of the equivalent length of straight wire. If it was a spiral shape, then it would definitely be an inductor. This is more reminiscent of a delay line used in high speed digital, such as memory circuits, where they are trying to match signal propagation times in a wide bus, making some traces longer by snaking them back and forth. Take a look at the traces leading to the memory chips on a PC video card or motherboard dimm sockets. In those cases, inductance is definitely not desired.

This appears to be a low value resistor. It is not clear what the purpose is, but it may be to set the DC gain. The AC gain seems to be set by capacitor ratios instead of resistor ratios. That's clever.
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Old 27th March 2014, 06:15 PM   #5
mjf is offline mjf  Austria
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the tda2002 was used in cheap car radio applications.

schematic.......looks like current feedback. the speaker is the load (probably 4 ohms) and the spiral shape on pcb is a low ohm res (rfb) .
cfb (c feedback) is paralell to the speaker and limits the bandwith i think........
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Old 27th March 2014, 07:10 PM   #6
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"That is not a PCB inductor."

But then, you say it is an inductor, just a low value one so I think we all agree on that point. I've also made kits that operated in the 2 GHz range that used that same technique and called it an inductor.

"cfb (c feedback) is paralell to the speaker and limits the bandwith i think"

I think you are mostly right. I see that section of the circuit as operating in two ranges, otherwise: Why the inductor?

In the amplifier range we are interested in, cfb would be chosen to cut off bandwidth in a reasonable range. Adding an arguably extremely low value inductor, it would function just as if the inductor were ground. In the radio frequencies around 100 Mhz, it becomes a LC tuned circuit.

Since as you noted, it was a cheap car amplifier IC, IMO this was added to eliminate or reduce rfi from the ignition system.

You don't have to see what I am trying to say since it is academic at this point. I don't think anyone is going to build a TDA2002 based car stereo or hifi amp given the better solutions. I am a fan of killing two birds with one stone and economy of design. It looks to me like they just used circuit board traces to add an inductor and made the bandwidth capacitor function as both bandwith adjustment and RFI filter. Pretty clever if it actually worked! Of little practical value unless you live next to a radio station or maybe want to put an amp inside your computer.

Last edited by ricortes; 27th March 2014 at 07:21 PM.
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Old 27th March 2014, 07:36 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

It is not an inductor and your just talking nonsense.

http://www.jameco.com/Jameco/Products/ProdDS/32897.pdf

It is a low value resistor that clearly sets the gain : Av-1 = RL/Rfb.

rgds, sreten.
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Last edited by sreten; 27th March 2014 at 07:43 PM.
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Old 28th March 2014, 03:20 AM   #8
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~2.3 GHz microwave reciever<VCO mixer/down converter> I built ~30 years ago and took a picture of about an hour ago. PC board inductor in addition to the others.

The cut on the PC board and glob of solder was just tuning to get it in the desired range. Very common technique for anyone that has even a basic knowledge of electronics.
Planar spiral coil inductor calculator
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Old 28th March 2014, 07:25 AM   #9
mjf is offline mjf  Austria
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This is an audio amp..............(and not a microwave -......)
rfb means feedback resistor.
r = resistor, fb = feedback.

Last edited by mjf; 28th March 2014 at 07:38 AM.
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Old 28th March 2014, 12:18 PM   #10
macboy is offline macboy  Canada
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Let's be clear, every conductor is an inductor, because current through any conductor produces a magnetic field and voila: inductance. That does not imply that every trace on a PCB is designed as an inductor, though some are. But it also means that you can't ignore the inductance of your PCB traces. In this case, the long snaking trace is designed as a resistor, but it will have some parasitic inductance. That inductance can likely be ignored at frequencies that this amplifier can operate within.
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