Why not use a 2N5089 or 2N7000 in a mini booster?

[will]

Hi,

Just wondering if a high gain bipolar like 2N5089 or Mosfet BS170 or 2N7000 will work even better than a J-Fet in the top transistor position in the mini booster (like the BSIAB)?

The top transistor looks to me like an emitter or source follower. The circuit is designed to provide constant current to maximize the gain of the bottom transistor. The top transistor ideal characteristics would be very high gain and low noise. However, followers having less than unity gain noise is not really much of a problem.  This transistor would not clip in this configuration; it would maximize the bottom transistor’s clipping potential.

One thing I don’t totally understand is the how the low output impedance of the top transistor is simulating an extremely high impedance to maximize the gain of the bottom transistor?

Regards,
Will

[Hal]

socket and try it :-D

it might work...might have too much noise, though.  

depends if you like the sound or not.

[R.G.]

Maybe I ought to a do a "Technology of the Minibooster".

The top transistor looks to me like an emitter or source follower.

It is. It is following the drain voltage of the lower transistor. That is what makes high input impedance critical. The follower must not load the drain of the lower device.

One thing I don’t totally understand is the how the low output impedance of the top transistor is simulating an extremely high impedance to maximize the gain of the bottom transistor?

It's synthesizing a high impedance load by allowing the DC current through for biasing and signal, but moving the DC bias feed around in such a way that the lower transistor does not have to do the moving.

Consider: a seven year old boy trying to lift a heavy weight. The boy's "impedance" is mismatched to the weight he's lifting because his muscle strength is low or less than the weight. Now imagine that behind the boy is standing an Olympic powerlifter. The powerlifter grasps the weight just beside the boy's hands; he watches and feels the boy's efforts on moving the weight and moves the weight in the direction the boy is trying to move it. To the boy, the weight seems to be lighter and easier to move around. The boy's perception is that the weight's "impedance" is higher, and it's easier for him to move it around.

The mu-amp and its ilk are actually push-pull stages. The trick is that the upper stage must have very high impedance so as to not load the lower stage all by itself - that is, the powerlifter has to be fast and perceptive enough to not slow the boy's efforts down all by himself.

[will]

Hi,

Thanks for the explanation R.G.

I think I will give Ed's BSIAB II a try with mosfets in the source follower position, since I've got a bunch of them.

Regards,
Will

[R.G.]

Be sure to try the 1K source/drain isolation resistor I show in "Mod your Mu-amp" at GEO. With that resistor, the source provides a much lower impedance output, not the high impedance output that you get if you take signal directly from the lower drain.