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Old 17th March 2002, 05:17 PM   #1
JoeBob is offline JoeBob  Canada
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Default BJT Buffer Circuits?

I was looking at the JFET article on www.borbelyaudio.com, and noticed the source-followers shown and was thinking of using such a circuit in a "buffered passive pre-amp" so to speak. When I fell across HarryHaller's thread on his passive preamp and he said he was thinking of doing something along those lines as well.

I was looking at the 16b diagram. But I was thinking, could such a thing be done with BJTs? Or would the input impedence be too low? I'd rather use BJTs for I have some on hand and I'm cheap and all.

Or if annyone knows of simmilar circuits which use BJTs and run off close too +/- 24v, do let me know.

Or if BJTs are not suited for this at all, are MOSFETs more so?
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Old 17th March 2002, 07:39 PM   #2
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Default BJT buffer

This circuit is from the The Audio Amateur 2/88 and I adapted from 15 volt to 24V rails. You might want to put an output cap since I don't know what the offset voltage is. I would use C2240s and A970 if possible.

H.H.
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Old 17th March 2002, 07:46 PM   #3
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Once again....
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Old 17th March 2002, 07:49 PM   #4
JoeBob is offline JoeBob  Canada
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Ah, thanks. I'll give it a try.

But you wouldn't by any chance know what the input and output impedences are supposed to be with this circuit?
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Old 17th March 2002, 08:02 PM   #5
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The Borbeley ones are one of the best I know
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Old 17th March 2002, 09:15 PM   #6
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Default BJT buffer

The input impedance will depend on the Hfe of the transistors and the load that the buffer is driving. Wiith the SC2240 and A970s I would estimate it to be several megaohms. I would use transistors with Hfe greater than 200 at 2 mA. Output impedance should be less than 25 ohms.

H.H.
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Old 18th March 2002, 03:08 AM   #7
JoeBob is offline JoeBob  Canada
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The diagram you posted had originally 2N3904's and 2N3906's, but could one use higher power devices (even though it'd be a waste of money, I already have them) like MJL21193/MJL21194 or MJL3281A/MJL1302A. They only have their hfe listed for 8A and 16A, so I dunno. I could always go buy some little small signal transistors, but was wondering if these could do the job, why bother...
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Old 18th March 2002, 03:10 AM   #8
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Here is an article by Walt Jung describing a discrete BJT buffer:

http://www.planetee.com/planetee/ser...ArticleID=2800
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Old 18th March 2002, 07:41 AM   #9
sonnya is offline sonnya  Denmark
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It is a bit overkill with MJLxxxx for a basicbuffer.
You will get some good results, when using ztx453/553 as output types biased around 20 - 30 mA... i think 1$/each.. As a start use them and 2sa970/2240 as input @ 2mA like Harry writes... Then you should be flying... Can be better but it is a really good start.

Sonny
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Old 18th March 2002, 12:24 PM   #10
JoeBob is offline JoeBob  Canada
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I mean, the thing is I can only buy 2N3906s and 2N3904s and simmilar locally. And since I have some MJLXXXXXs, I know it's overkill, but would it work? I just don't want to have to wait for a mail order shipment from the states (but, if I have to, I have to.
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