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Old 20th December 2010, 06:42 PM   #1
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Default D-amp for DC coupled applications

Hello,

I need to amplify a DC coupled input signal to drive a EM mechanical shaker (for structure testing). I typically prototype electronics in stripboard (0.1" spacing). So, TPA3125D2 draws my attention since I can buy from Digikey and it has DIP package. Also, in BTL configuration, it can be used as a mono amplifier, which gives >20 W power.

However, I have no experience working with D-amp. My question is, if I take out the input capacitors and output inductor/capacitors shown in the spec sheet, can it work?

Bear with me if I am being naive.

In addition, what do you recommend it that chip is not good for the purpose.

Thanks
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Old 20th December 2010, 07:01 PM   #2
81bas is offline 81bas  Germany
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for DC coupled class D application you need to use BTL configuration. Otherwise, bus pumping effect can occur...
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Old 20th December 2010, 07:10 PM   #3
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Ok, thanks. That is fine I can definitely use BTL.

How about no input capacitor and output stage?
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Old 20th December 2010, 07:47 PM   #4
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I don't think there is a way to DC couple the inputs on these single supply TI chips. The inputs of the TPA3125D2 have a bias voltage of AVcc/8.

EDIT: Actually, there might be a way, but it would require an isolated power supply. It might work if you use a single ended signal between +IN and -IN. I have a TPA3122D2 board I can test this on. Give me few minutes.

Last edited by theAnonymous1; 20th December 2010 at 07:59 PM.
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Old 20th December 2010, 08:10 PM   #5
81bas is offline 81bas  Germany
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Quote:
Originally Posted by theAnonymous1 View Post
I don't think there is a way to DC couple the inputs on these single supply TI chips. The inputs of the TPA3125D2 have a bias voltage of AVcc/8.

EDIT: Actually, there might be a way, but it would require an isolated power supply. It might work if you use a single ended signal between +IN and -IN. I have a TPA3122D2 board I can test this on. Give me few minutes.
fully differential opamp OPA1632 has an special Vocm (output common-mode voltage) pin for these purposes. Feeding AVcc/8 to this pin (i.e. connecting it to the BYPASS pin of TPA IC), differential outputs will be referenced to this voltage then. In this case it is possible to connect this opamp without decoupling capacitors directly to TPA IC...
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Old 20th December 2010, 08:24 PM   #6
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Quote:
Originally Posted by 81bas View Post
fully differential opamp OPA1632 has an special Vocm (output common-mode voltage) pin for these purposes. Feeding AVcc/8 to this pin (i.e. connecting it to the BYPASS pin of TPA IC), differential outputs will be referenced to this voltage then. In this case it is possible to connect this opamp without decoupling capacitors directly to TPA IC...
You are right, and the board I am testing with actually has one of these on the front end. When I built the board I didn't consider doing this for whatever reason and just use caps between it's outputs on the the inputs of the amp. I had the VOCM pin grounded.

The board is pretty messed up from various tests. I will have to see if I can put humpty back together again.

Anyway, my above suggestion seems to work (kind of). I ran the board from a battery and hooked the the + and - signal inputs to my my signal generator. The DC offset trim on the generator moves my test speaker in and out, so DC coupled it is. The only problem is RF noise when I touch the speaker or any of the wiring, I assume since the inputs are "floating" and not referenced to ground.
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Old 20th December 2010, 08:53 PM   #7
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Thank very much both of you!

If split power supply is not an issue, any more straightforward solutions? Even a type AB...
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Old 20th December 2010, 09:18 PM   #8
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I tried connecting VOCM to BYPASS but just end up with PVcc/2 on the output of the OPA1632 which causes the TPA3122D2 to shutdown.

I don't really have time to mess around with it more right now.
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Old 20th December 2010, 09:23 PM   #9
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Thanks for your great effort, theAnonymous1.
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