On Wed, 02 Mar 2011 19:26:29 -0500, Bitrex
wrote:
On 3/2/2011 6:30 PM, John Larkin wrote:
On Wed, 02 Mar 2011 17:21:23 -0600, Vladimir Vassilevsky
wrote:
John Larkin wrote:
I've always sort of liked the classic "GE" tape head/mic preamp
circuit:
ftp://jjlarkin.lmi.net/GEcircuit.jpg
Here is a couple of very low noise audio circuits for the same purpose:
http://www.abvolt.com/misc/inputs_audio.jpg
I like BJT and JFET in cascode.
The one on the left, with the base and gate connected, is sure cute.
John
I'm having a little trouble with the operation of that one. The way
it's set up, assuming the output sits at 0 volts won't the input BJT be
cut off?
Jfets usually run at negative gate voltage. Assume the transistor base
is at +0.7. The jfet source will be at some more positive voltage,
+2.5 maybe. That's enough to run the transistor.
Actually, you can cascode a transistor into the source of a fet that
has a grounded gate. In that case, the source/collector voltage might
be a volt or two. You would have to look at the fet transfer curve,
and know the design operating current, to see exactly what that
voltage might be.
The problem with jfets is the huge part-to-part variation in Idss and
transfer curves. A 10:1 datasheet spread in Idss isn't unusual.
+10v
|
|
|
d
gnd---------g
s
|
|
+------ Vs
|
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10K
|
|
|
gnd
For a typical vanilla jfet in this circuit, Vs might be +1 to +4 volts
maybe.
John