LM4702 triple darlington output

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Why hasn't everyone decided to go triple-darlington with the LM4702? You could put as many output transistors as you wanted to maximize the available voltage. Sure, you'd probably drop a bundle on transformers and caps but why not go for the gold with this chip? Just curious. I have a schematic in mind. There must be some con's to this. Let's hear them.
 
The allure of the chipamp is typically the lower cost, higher integration, and short feedback path.

I'm not entirely confident most are looking to just increase output wattage as much as possible either, "more" is not always better, particularly if you have to lose hearing to realize the benefit of the design.

That doesn't make it a bad idea though, but it also drifts away from the chipamp forum idea, because this isn't then going to be a chipamp, it's just a chip driver.
 
As I see it this isn't a typical chip amp because it lacks a power output stage. I'm simply adding a much MUCH larger current gain stage. So I may be guilty of being a wattage hog but I'm sure a lot of DIY'ers building large passive speakers are interested in at least a part chip amp. A little work is done for you. Good work National Semi once again.
 
tf1216 said:
As I see it this isn't a typical chip amp because it lacks a power output stage. I'm simply adding a much MUCH larger current gain stage. So I may be guilty of being a wattage hog but I'm sure a lot of DIY'ers building large passive speakers are interested in at least a part chip amp. A little work is done for you. Good work National Semi once again.


It isn't a chipamp, it's a chip voltage gain stage, essentially a high voltage tolerance opamp. "Everyone" in the chipamp forum isn't doing it because it's not a chipamp and you would not see a non-chipamp in the chipamp forum.

Is it a "bit" like a chipamp? Yes, but lots of other things are a bit like one too, even all the way to using old school discrete parts for everything.

So what it has in common: amp
What it doesn't: the word chip in front of "amp".


THEN there is the real chipamp, what we use in this forum. You asked why not everyone and the aswer was given. That doesn't make a driver topology evil or bad, but if you ask why CHIPAMP builders aren't building this instead, it's because that's not what they want or at least not on-topic in this forum.

I suppose the more relevant question with a larger current amp is this: Given the larger expense, is the LM4702 appropriate for that class of amp? It might end up sounding better, or worse, or about the same but louder, and yet in the end only you have to be happy with it.

Finding amps with separate current gain output stages is the rule, not the exception... except of course, in the chipamp forum. Do you see what I mean?

What you propose isn't unique at all except quite specifically in a chipamp forum. To a certain extent it is the opposite of a chipamp.
 
Perhaps this isn't the correct forum to talk about a driver CHIP, but there are plenty of threads here about the LM4702 (all with output transistors or FET's.) So what are a few more components? If you want to be technical, it's not a chip amp because it isn't the whole amp in a single IC. I can share my ideas somewhere else then.
 
Well, this thread has been a huge waste of time so far. Thanks for the ramblings, "!".:rolleyes:

Back on topic:

A quick back of the envelope calculation regarding current needed to drive the three darlingtons leaves me a little short. I may be doing some math wrong (or assuming the wrong Hfe for your darlingtons) but I don't think the ~5mA the LM4702 puts out will be enough to drive three output darlingtons.

I do like the idea, so to make sure we are on the same page, what rails are you using, what output power do you want, and what is the Hfe of your darlingtons?

I think a pretty simple two stage voltage follower with multiple output pairs would do the same thing without taxing the chip as much, but using the darlingtons would indeed be a little more simple. I think it is worth exploring.

David



PS As to the appropriate forum for these questions, since the device is a chip/ss amp hybrid, I would think either forum would be fair to post in. Why is this even an issue???:confused:
 
Assumption mode enabled:

I would imagine that the average person attracted to the CHIP AMP forum comes here because they really don't have a good grasp of discrete amplifiers. What they do understand is how to read a manufacturers chip amp Datasheet.

It would make sense to me that you would get much less feedback as a result of this and might get more input from a different forum more dedicated to a discussion of Analog and Transistor based amps.

Just a thought.
 
Originally posted by dfdye PS As to the appropriate forum for these questions, since the device is a chip/ss amp hybrid, I would think either forum would be fair to post in. Why is this even an issue???:confused:

Well it's not like I was trying to imply it's "bad" to post per se, only that if one is asking "why don't I see many in this (chipamp) forum", then a good answer would be "because it's not a chipamp".
 
I think a problem with this chip is that people with enough savvy to design the Vbe, driver, output stage would rather design the front end also.
And people without the design knowledge may find building an amp with this chip intimidating.
As such it falls between the cracks as far as defining the forum to use so let's not belabor the point.

I agree that driving three seperate darlingtons is probably not the way to go. Nor am I impressed with the SAP/N/15/16 specs. Not enough drive from the 4702.

My thoughts are to use either a pair of 2SA1930/C5171 or 2SK216/J79 as emitter/source followers driving as many 2SA1216/C2922 as one wants. Specs for these can be found at AmpsLab. Both can provide plenty of drive from a few mils.

http://www.ampslab.com/components.htm

I would be interested in using a Sziklai configuration but they have the reputation of oscillating and I am reluctant to attempt it, so will probably stick with the voltage follower config.

Comments?
 
! said:
Well it's not like I was trying to imply it's "bad" to post per se, only that if one is asking "why don't I see many in this (chipamp) forum", then a good answer would be "because it's not a chipamp".
Fair enough. :D

I agree it isn't properly a chipamp, but it is a nice bridge into the world of descrete amps, then I think we are on the same page as it being fair game in either forum.

Now, back to the topic:

I had a nice little brain fart and thought when Alexcd invoked "tripple darlingtons" he was referring to three parallel darlington output transistor pairs. Of course a tripple emitter follower/tripple darlington would be fine, and you could of course parallel the final power stage in order to get more power into harder loads. The drawback is that three stage outputs can add in a little more noise than single darlingtons or two stage outputs. (BTW, I usually only use "Darlington" as a reference to a single device to avoid this type of confusion)
 
pwillard said:
Assumption mode enabled:

I would imagine that the average person attracted to the CHIP AMP forum comes here because they really don't have a good grasp of discrete amplifiers. What they do understand is how to read a manufacturers chip amp Datasheet.

It would make sense to me that you would get much less feedback as a result of this and might get more input from a different forum more dedicated to a discussion of Analog and Transistor based amps.

Just a thought.

Be careful with your words. I didn't post my schematic because I wasn't sure if people were interested. I think you'll find it a simple but worthwhile circuit. There wasn't a good "hybrid" forum for the LM4702 so this is the final destination for it.

Triple darlington is done to raise the effective Hfe, so current is not an issue. If you plan on doing any high-power high-current applications with this chip you better look into this. I can use as many output pairs as I need. 2ohm, 1ohm, maybe less is possible.
 
Triple Darlington IC?

Hi,

Only IC triple Darlington I could find was a BIP 350 for

automotive ignition at

www.semiconductors.bosch.de/pdf/BIP350_product_Info.pdf

and there are easy to find Darlington pairs in ICs so there may be

integration problems, because it is certainly a circuit of choise for

car audio amplifiers. At least BIP 350 gives schematic for what a

Dalington triple is and is not.

Mark
 
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