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JoeBob 17th March 2002 06:17 PM

BJT Buffer Circuits?
I was looking at the JFET article on, 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?

HarryHaller 17th March 2002 08:39 PM

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.


HarryHaller 17th March 2002 08:46 PM

Retry Attach
2 Attachment(s)
Once again....

JoeBob 17th March 2002 08:49 PM

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?

promitheus 17th March 2002 09:02 PM

The Borbeley ones are one of the best I know

HarryHaller 17th March 2002 10:15 PM

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.


JoeBob 18th March 2002 04:08 AM

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...

rpell 18th March 2002 04:10 AM

Here is an article by Walt Jung describing a discrete BJT buffer:

sonnya 18th March 2002 08:41 AM

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.


JoeBob 18th March 2002 01:24 PM

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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