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Old 17th January 2006, 07:16 AM   #1
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Default Discrete Darlingtons vs. Packaged Darlingtons

This is probably a newbie quetion, but I never understood why:

How come many class AB or Class B solid state amps that use BJTs for their output stages use discrete-built Darlingtons (sometimes upwards to three - two driver pairs and then the output pair) as opposed to fabricated Darlington transistors (the ones that have two transistors in one package)?

I know that fabricated Darlingtons have lower transition frequencies, but I would *think* that making a discrete pair would also have a low ft...
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Old 17th January 2006, 07:33 AM   #2
AndrewT is offline AndrewT  Scotland
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Search for the recent thread that discussed this same Q.

You did search??
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Old 17th January 2006, 07:45 AM   #3
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Discrete components allow Class A drivers and more robust (SOA) parts in general; packaged Darlingtons are usually hard-wired so one is locked into the classic configuration and that limits design choices and
flexibility.

Some designs such as the Leach amplifier use a triple Darlington configuration. I don't know of any packaged devices like that. I've never seen a Sziklai/compound-complementary device either.

Ft varies, but there are plenty of audio power devices that go up to
30 Mhz or so, and drivers that go even higher.

I think there have been some hybrid packages that contain a full
output stage, but that locks one into the choice of devices and usually a considerable limit on output power.

Onsemi may have a good idea in packaging a thermal diode with an output device, even though the device is pricey and requires a 5 pin
package with a non-standard pinout array.
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Old 17th January 2006, 09:09 AM   #4
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Besides what Damon says you can also take into account: What does the "market" need? This is usually the industry. If the industry demanded faster Darlingtons the manufactures would also provide it.
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Old 18th January 2006, 02:23 PM   #5
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What if the pre-packaged darlington doesn't have the specifications wanted?
Using discretes allows for a greater selection of parts parameters.
There are simply more discretes to choose from.
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Old 18th January 2006, 02:53 PM   #6
ilimzn is online now ilimzn  Croatia
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Exactly. In the years past I have tried several darlington packages and just about the only one I had good success with were the MJ11015 and 11016, and even then just for relatively low supply rail voltage (SOA is not too good).
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