ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Re:Minibooster Operation

:

9/10/1999 2:10 PM

Re:Minibooster Operation

I don't know if this has any value, but a dual gate d-mosfet

is essentially a cascode-in-a-can (NTE 454, for example)

According to the textbook I've been reading("Introductory DC?AC Electronics" -Nigel Cook) these will emulate a series-connected pair of mosfets --the example given looks identical to the Minibooster. You could build a very small circuit with one of these. The NTE device is a metal can and probably costs more than a handful of jfets, though.

-Eric

9/11/1999 8:42 PM
Jimi	I got a reply from Jack to my email about the minibooster. He sez- "The Mini-Booster design is derived from a circuit first published in February 1970 by National Semiconductor in a publication entitled "AN-32 Fet Circuit Applications". I made my first Mini-Booster in the late 1970s and fell in love with the sound of it." J2K

9/12/1999 1:42 PM

Triode circuit reference for Minibooster

What is essentially the same circuit for triodes is on Page 438, figure 11-21 of Valley and Wallman "Vacuum Tube Amplifiers", part of the MIT Radiation Lab series.

The circuit diagram shows a fixed DC voltage bias for the upper tube, but the operation is the same, as the source-gate cap in the MB holds the Vgs on the upper device constant in the MB, while the illustrated circuit uses a battery source for illustration.

I suspect that for matched JFETs you don't need the two 1M resistors, as they'll balance at mid-supply anyway.

Note that the circuit is a stock JFET common-source amplifier with a constant current source load, not a cascode. A cascode would need a drain load on the upper device, and the output would be taken there as well.

9/16/1999 8:46 PM

Re:Minibooster Operation

It's interesting in the ap note that they use a FET with higher gain/higher cutoff voltage on top and a lower gain/lower cutoff voltage FET on the bottom. The top device has a VGSoff of -2 min whereas the bottom device has a VGSoff of -.6 min. The gain is roughly a 2 to 1 relationship.

It also states that the lower the drain current the higher the gain however you sacrifice input dynamic ranger with increasing gain. Sounds like you could incorporate some sort of 'sag' mechanism using this characteristic.

Says that it's sometimes called the "JFET u amp" because the voltage gain is dependent on the u of the bottom JFET (roughly half). u of a JFET increases as current decreases therefore the gain of the minibooster increases.

FWIW,

--johng

9/16/1999 9:18 PM
Fuzzman	>It also states that the lower the drain current the higher the gain as with all jfet common source preamps >however you sacrifice input dynamic ranger with increasing gain. because of the high gain, not due to anything to do with the u-amp design. it will begin to clip with smaller inputs as the output runs against the supply rails

Page 3 of 3