Svart

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« on: September 14, 2004, 07:46:52 PM »
Can a few people give me some advice on using FET buffers or BJT buffers(or opamp buffers)?  I've read so much opinion all over the internet about either one of these, I'm not sure which one I would rather use!  I would like to pose a debate:
 
Why or why not use FET or BJT buffers in preamps.

or at least point me to some neutral reading material!

here are the rules.

1. No hair pulling
2. No making fun of the opponent's Star-wars and/or Star Trek dolls/collectables, unless of course they are over 10 years of age, which deems such acts acceptable.
3. No one with less than  :guinness:  :guinness:  :guinness:  in their bloodstream is allowed to enter said match.
4. TWO MEN ENTER, ONE MAN... oh wait, wrong fight..
5. loser must actually USE a LM741 in their next peice of gear in the audio chain as punishment.

anyone caught cheating or not following the rules set forth will be held in contempt and subject to the following punishment:

Your Name will be turned over to B*gTree and will be charged with making fun of the wig he was wearing in his avatar....


 :green:

now get to it!
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


NewYorkDave

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #1 on: September 14, 2004, 07:50:29 PM »
Use a BJT buffer with a FET input stage.

There, that was easy.

[email protected]

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #2 on: September 14, 2004, 07:53:41 PM »
Um, what kind of buffer again? Input, output? What does it buffer from what? What are the expected input and output impedances? Can you provide something more specific?

Tamas

Svart

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #3 on: September 14, 2004, 07:59:02 PM »
thanks dave, but i'd actually like to know why someone would choose to use that combo also.  I plan on using a THS6012 (maybe ssm2019?) as the gain element, but I'm not keen on settling for using an opamp(ths6012) for input like i've seen others design, I'd like to see something more discrete like a FET/bjt stage.  I'm open to suggestions but i'd really like to learn why someone would choose one over the other wherever they use it.  I don't have much time other than a few minutes here and there to check replies or post questions or i would have done much more than i have been able to proto.  That's what makes this forum great though, is that we can get answers like this from people who know right away.

thanks!
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

Svart

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #4 on: September 14, 2004, 08:01:42 PM »
sorry i got caught up making a funny post..

Input buffer for a preamp, before the gain element.

i've seen:

BJT
FET
OpAmp

I'd just like to know how/why people pick what they use in their designs.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

Bjorn Zetterlund

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #5 on: September 14, 2004, 08:16:51 PM »
err...preamps normally don't have an input buffer as such.  When you say buffer I'm thinking high input impedance and low output impedance and no gain, which I believe is the conventional definition.  In general,  discretes ahead of an op-amp are not really buffers but are there inside the feedback loop of the op-amp.  Have only had 3 cans of stella and am therefore disqualified, though!

Bjorn

Svart

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #6 on: September 14, 2004, 08:22:11 PM »
ok, maybe my definition doesn't fit i guess.  Yes, the goal would be to create high input impedence, low impedence to the input of the gain stage..  I thought that would be a buffer?? I dunno, that's why i asked!

thanks!

Alcohol of any nature is welcome here brother!  er... no wine coolers though, sorry..
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

NewYorkDave

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #7 on: September 14, 2004, 08:45:53 PM »
Preamp input stages should be designed not to load the anticipated source, and therefore should not require a separate buffer circuit. Now, output buffers... those are useful!  :wink:

PRR

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #8 on: September 14, 2004, 10:27:52 PM »
Good designs are possible either way.

The difference is in the details, not the devices.

But I'm disqualified on rule #3.

Svart

Theory requested: FET buffer VS. BJT buffer. Round 1. FIGHT!
« Reply #9 on: September 15, 2004, 07:12:28 AM »
Thanks PRR.

Well no one is making it any easier on me here!  :wink:
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.


 

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