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what happens if transistors are not matched? especially output stage..
what happens if transistors are not matched? especially output stage..
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Old 15th November 2012, 04:38 PM   #11
dry joint is offline dry joint  United Kingdom
diyAudio Member
Join Date: Oct 2006
People matching small signal transistors might buy 100 for a pair or a quad, its just not sensible to do that with power transistors, a commercial amp that did that would have to sell for tens of thousands. It would be a revelation if matching was the best use for an extra X hundred Pounds/Dollars/Euros in parts, normally people go for PSU and parts upgrades, Hifi brands also go for flashier cases and assorted methods of persuasion.

Maybe amp manufacturers that use outputs with multiple chip amps, like Jeff Rowland, are economically matching their output transistors by passing the cost of doing so back down to the IC supplier.
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Old 15th November 2012, 05:28 PM   #12
Struth is offline Struth  Canada
Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

You can look at Doug Self's example:

None of the designs in his books or articles use matched devices and the results are pretty good. When he doubles up or expands the output stage with more pairs, then he adds emitter resistors for current sharing. The fact is that for BJTs, the turn-on characteristic is very uniform from one sample to another, and even from one device to another.

When you order BJTs from a supplier, you will get ones from the same rail and thus the same batch, and this is usually matched as close as you need for 99% of circuits.

Small-signal transistors should be matched for lowest distortion, particularly in symmetric circuits. If they are not matched, then hopefully there is emitter degeneration in use to reduce the sensitivity to device parameter spreads. Leach says he never matched any devices in his circuits and had pretty good performance.

Note that for output BJTs, higher Re will enforce better current sharing but will increase output impedance. Having 14 pairs versus 2 pairs allows the multi-pairs to smooth out variations so the push-pull sides are likely to be more balanced.

With proper cooling and protection circuit design, nearly the full rating of the output device can be utilised on a peak basis.

Have fun
Kevin O'Connor
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Old 15th November 2012, 06:48 PM   #13
Ian Finch is offline Ian Finch  Australia
diyAudio Member
Join Date: Apr 2010
Location: Coffs Harbour
Many of the questions you ask here have been answered so many times that people have written books and webpages about them. What's more, the answers likely are better thought out than just good guesses or suggestions guys try to offer when they are uncertain what you are really after.

Why not Search or Google your questions first before starting dozens of threads with the same questions others ask here regularly? The right answers and discussions are just seconds away. If you still need more information, then post in an old thread that is close to what you want so the info. remains together for others to search conveniently. Here's most of what you need to know about matching; Matching Power and Driver Transistors

In fact, if you read that site systematically, you'd probably learn enough to tackle anything you might want to DIY.
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