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Small unity gain 5V output stage sought....
Small unity gain 5V output stage sought....
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Old 23rd September 2009, 03:48 PM   #1
HankMcSpank is offline HankMcSpank  United Kingdom
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Default Small unity gain 5V output stage sought....

Having tested a few audio output ICs (TDA7052A etc), I'm looking for a 'better' solution. (better here being 'better for my application'!)

What I seek is an output stage that will not apply any voltage gain at all (even though th TDA7052 can be set to zero voltage gain, I have the impression that it can't deliver enough grunt quick enough).

I'm using a 5V rail (I'm pretty much tied to this as I'm using another device to host the supply for this circuit - & it only has 5V on it) ...& rail to rail opamps will feed this output stage. The output is a 4 ohm inductive load. The AC signal varies between 1V to about 4.8V ....the frequency range is about 80Hz to 4kHz. I need it to have very little distortion (the output won't be heard, but nevertheless, it needs to be free of sistortion becuase it's causing a string to resonate & for that to happen well, it has to be an accurate representation of the input.

Would discreet components (vs an integrated output IC) be a better solution?

Can someone give me some pointers please...I'm running out of ideas (not that I had many in the first place!!)
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Old 23rd September 2009, 04:09 PM   #2
AndrewT is offline AndrewT  Scotland
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1Vac requires +-1.4Vp=2.8Vpp at the output.
4.8Vac will require ~14Vpp.
To give this output you will need at least 15Vdc either as a single supply or as a +-7.5Vdc dual polarity supply.
5V has no hope of giving more than 1.5Vac as an output and that into 4ohms or even 4r0 is a mighty ask.
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Old 23rd September 2009, 04:11 PM   #3
wg_ski is offline wg_ski  United States
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Any audio amp IC capable of 4 ohms running off a 5V rail will give almost no output swing. Even a diamond buffer would only give 3.5V p-p, which may be enough. If you want to get really close to the rail, you need complemetary FETs in common source - and that's not something you sit down and 'design' in 5 minutes (an afternoon, maybe).

If it's being used to excite a string, distortion doesn't matter much. It will respond mostly to the fundamental.

What is the app, some sort of guitar sustain gizmo?
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Old 23rd September 2009, 05:57 PM   #4
HankMcSpank is offline HankMcSpank  United Kingdom
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Thanks guys..perhaps I should have outlined, those were peak to peak figures not RMS!

Yes, it's to exctie a string, but the distortion does matter (if you listen to the output from the guitar, whilst exciting the string with a sine vs a square, there's a massive difference in tone ...a sine sounds pure with no coloration of the string's sound, whereas driving the string with a square isn't a nice sound at all). Same when using the guitar string as the source....if the circuit clips, you hear it in the guitar's output.

Perhaps we're at cross purposes, so let me put my dilemna another way.....

(these are all sine wave, peak to peak figures!)

Having used a sig gen to 'excite the top E string' sufficiently (for non-guitarists - this being the thinnest string with the least amoiunt of ferrous material for the driver to 'get a grip on' - therefore the trickiest to resonate with the driver coil), it needs 5V peak to peak (1.7V RMS) across the 4 ohm driver coil that I've wound (as measured on a scope).

Now to give the preceding AGC circuit enough dynamic range to work with, I'd have liked to have simply had a unity gain output circuit ...ie 1V into the output stage, 1V seen across the coil (again peak to peak figures) 4.8V into the output stage ....4.8V seen across the coil.

I can work with small signals (eg 2.5V into the output stage with a voltage gain of two....but what I'm finding is that the convenient audio amp IC, are a little sluggish at delivering the necessary drive into the string.
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Old 23rd September 2009, 07:09 PM   #5
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by HankMcSpank View Post
I can work with small signals (eg 2.5V into the output stage with a voltage gain of two....but what I'm finding is that the convenient audio amp IC, are a little sluggish at delivering the necessary drive into the string.
Audio amp chips lose 4 to 6 volts of signal swing. Operated from only a 5 volt supply you get 1V p-p if you're lucky. If you're unlucky, it won't bias up at all.

I would be looking at discretes, and not be afraid to use a transformer.
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Old 23rd September 2009, 07:57 PM   #6
theAnonymous1 is offline theAnonymous1  United States
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So you need 4.8Vp-p into a 4R load from a 5V supply?

That's only ~0.72W. There are a few IC's that can do this, but most of them have built in gain.

How about the TPA0211? With Ri = 125k in BTL mode the gain should be -1V/V.

http://focus.ti.com/lit/ds/symlink/tpa0211.pdf
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Old 23rd September 2009, 11:51 PM   #7
jcx is offline jcx  United States
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AD8397 might work - limited input cm range so use a gain of +2 (or -1 with an extra pair of resistors)
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Old 26th September 2009, 03:46 PM   #8
HankMcSpank is offline HankMcSpank  United Kingdom
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theAnonymous1 - that's a great 'lead' on the TPA0211 ...it might just be what I'm after (I've spent quite a bit of time on Digikey's site using their filtering mechanism ...but that one slipped me by!)

jcx...unless I'm missing something, it's not obvious to me how the AD8937 work for my needs? I'm looking for something that can deliver circa 1W into 4 Ohms @330Hz.

I've managed to coax the TDA7053A into a form of what I need, but I'd like more grunt in reserve.

here's a quick youtube video I made of the end result (just to give you an idea of what this is all about)....

http://www.youtube.com/watch?v=DXitv...e=channel_page

5 strings sustain fine...the top E string is a little bit more stroppy! (that said, the circuit was only running at 4.5V vs the 5V I was using on the bench)
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Old 27th September 2009, 05:51 PM   #9
Elvee is online now Elvee  Belgium
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You can also go discreet: here is an example of moderate complexity and performance. It gives 4.4Vpp output at 0.5% distortion and is unity gain.
The quiescent current is 20 to 30mA and is thermally compensated by attaching D1 or D2 to the output transistors. The other one compensates for Q3.
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Old 27th September 2009, 07:30 PM   #10
jcx is offline jcx  United States
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sorry, missed the current requirement - although paralleling 2 packages worth of the ad8397 duals would get the I out it's not real convenient – and the internal compensation is a little light for low gains – needs good bypassing, pwr/gnd planes

another TI chip that looks OK is the OPA569 - the soic-w 20 package is considerably more hackable than the TPA0211 msop
http://focus.ti.com/lit/ds/symlink/opa569.pdf
you can "dead-bug" the soic-20 by bending its legs and then gluing a heat sink to the belly up power pad – I’ve used this upside down mounting with TPA6120

you do need a little more than your supply V if you need 1 Wrms into 4 Ohms so bridged output chips would be good - also reduces rail-to-rail requirement

if you don't have a power limit then relatively simple discrete options become more viable - Class A bias is usually much easier to get low distortion from

Last edited by jcx; 27th September 2009 at 07:57 PM.
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