SIT measurements, Mu Follower, and amplifier build
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lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
DEFiSIT is a Mu Follower Circuit

I performed a similar analysis of the FirstWatt DEFiSIT circuit shown in first image below. The second image shows the simplified circuit which was analyzed. The DEFiSIT is a mu follower with equations very similar those for the common source mu follower.

"Degenerated Transconductance of M1:
gm[M1e] = 1/(Rlo+1/gm[M1])

Mu Follower Current Gain:
β = ΔI[Rload]/ΔI[SIT] = gm[M1e]*(1/gm[SIT]+Rmu) + 1

RL = ΔX/ΔI[SIT] = Rload*β + Rmu

Push-Pull Ratio:
ΔI[M1]/ΔI[SIT] = β-1

As a result, the "sweet-spot" condition for the DEFiSIT can be derived from the equation derived in post #10:

Id = A*Vd + B/RL + C*Vd/RL + D

which is fairly well approximated by the equation from post #49:

Id = C*Vd/RL
Attached Images
 DEFiSIT-Pass.jpg (86.5 KB, 140 views) DEFiSIT-abstract-1.asc.jpg (144.7 KB, 143 views)

lhquam
diyAudio Member

Join Date: Mar 2010
Location: Olalla, Oregon: Land of the 100 Valleys
DEFiSIT sweet spot calculations

DEFiSIT is not a mu follower as I asserted in the previous post. However the sweet spot can be calculated in a manner similar to the mu follower.

The equations in the previous post appear to be correct and can be used to calculate RL in order to achieve the sweet spot. Furthermore, using a triode model for the SIT gives additional useful equations.

Being able to determine gm[M1E] allows one to choose the value of the degeneration resistor for PFET M1:

Rlo = 1/(1/gme[M1]-1/gm[M1])
Attached Images
 DEFiSIT-eqns-1.jpg (126.5 KB, 68 views)

Last edited by lhquam; 23rd February 2020 at 07:50 PM.

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