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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Ye olde bootstrappe buffere
Date: Mon, 9 Sep 2024 19:44:18 +0100
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On 09/09/2024 6:28 pm, bitrex wrote:
> Looks promising, with a midband gain of about 0.97.
> 
> Use a PNP/JFET cascode as the slower input (as I guess sub-GHz PNPs is 
> all we have, now) and fast NPNs on the output.
> 
> It's definitely not expensive, anyway:
> 
> <https://imgur.com/a/MTh2kl7>
> 
> Version 4
> SHEET 1 5120 680
> WIRE 592 -1024 592 -1104
> WIRE 592 -1024 -240 -1024
> WIRE -240 -848 -240 -1024
> WIRE -240 -656 -240 -768
> WIRE 176 -656 -240 -656
> WIRE 800 -656 240 -656
> WIRE -240 -528 -240 -656
> WIRE 592 -416 592 -1024
> WIRE -240 -368 -240 -448
> WIRE 528 -368 -240 -368
> WIRE -240 -256 -240 -368
> WIRE 592 -192 592 -320
> WIRE 592 -192 -192 -192
> WIRE 800 -192 800 -656
> WIRE 800 -192 592 -192
> WIRE -240 0 -240 -160
> WIRE -576 48 -672 48
> WIRE -416 48 -512 48
> WIRE -304 48 -416 48
> WIRE 592 128 592 -192
> WIRE -240 176 -240 96
> WIRE 528 176 -240 176
> WIRE -832 240 -832 176
> WIRE -672 240 -672 48
> WIRE -416 288 -416 48
> WIRE -240 288 -240 176
> WIRE 592 304 592 224
> WIRE -832 464 -832 320
> WIRE -672 464 -672 320
> WIRE -416 464 -416 368
> WIRE -240 464 -240 368
> WIRE 592 464 592 384
> FLAG -240 464 0
> FLAG 592 464 0
> FLAG -416 464 0
> FLAG -672 464 0
> FLAG -832 464 0
> FLAG -832 176 Vcc
> FLAG 592 -1104 Vcc
> SYMBOL njf -192 -256 M0
> SYMATTR InstName J1
> SYMATTR Value 2N3819
> SYMBOL res -256 272 R0
> SYMATTR InstName R1
> SYMATTR Value 1k
> SYMBOL npn 528 -416 R0
> SYMATTR InstName Q1
> SYMATTR Value BFR91A
> SYMBOL npn 528 128 R0
> SYMATTR InstName Q2
> SYMATTR Value BFR91A
> SYMBOL res 576 288 R0
> SYMATTR InstName R2
> SYMATTR Value 470
> SYMBOL res -432 272 R0
> SYMATTR InstName R3
> SYMATTR Value 10k
> SYMBOL cap -512 32 R90
> WINDOW 0 0 32 VBottom 2
> WINDOW 3 32 32 VTop 2
> SYMATTR InstName C1
> SYMATTR Value 100n
> SYMBOL voltage -672 224 R0
> WINDOW 123 24 124 Left 2
> WINDOW 39 0 0 Left 0
> SYMATTR Value2 AC 1
> SYMATTR InstName V1
> SYMATTR Value SINE(0 0.2 1Meg)
> SYMBOL voltage -832 224 R0
> WINDOW 123 0 0 Left 0
> WINDOW 39 0 0 Left 0
> SYMATTR InstName V2
> SYMATTR Value 9
> SYMBOL pnp -304 96 M180
> SYMATTR InstName Q3
> SYMATTR Value MPSH81
> SYMBOL res -256 -544 R0
> SYMATTR InstName R4
> SYMATTR Value 1k
> SYMBOL res -256 -864 R0
> SYMATTR InstName R5
> SYMATTR Value 1k
> SYMBOL cap 240 -672 R90
> WINDOW 0 0 32 VBottom 2
> WINDOW 3 32 32 VTop 2
> SYMATTR InstName C2
> SYMATTR Value 100n
> TEXT -704 488 Left 2 !;tran 0.1
> TEXT -640 368 Left 2 !.ac oct 10 10k 1G
> TEXT 1208 -472 Left 2 !.MODEL  BFR91A   NPN\n+              IS = 
> 1.32873E-015\n+              BF = 1.02000E+002\n+              NF = 
> 1.00025E+000\n+             VAF = 5.19033E+001\n+             IKF = 
> 8.15511E+000\n+             ISE = 1.39029E-014\n+              NE = 
> 1.51292E+000\n+              BR = 1.76953E+001\n+              NR = 
> 9.94038E-001\n+             VAR = 3.28032E+000\n+             IKR = 
> 1.00000E+001\n+             ISC = 1.04297E-015\n+              NC = 
> 1.18993E+000\n+              RB = 1.00000E+001\n+             IRB = 
> 1.00000E-006\n+             RBM = 1.00000E+001\n+              RE = 
> 7.63636E-001\n+              RC = 9.00000E+000\n+              EG = 
> 1.11000E+000\n+             XTI = 3.00000E+000\n+             CJE = 
> 2.03216E-012\n+             VJE = 6.00000E-001\n+             MJE = 
> 2.90076E-001\n+              TF = 6.55790E-012\n+             XTF = 
> 3.89752E+001\n+             VTF = 1.09308E+001\n+             ITF = 
> 5.21078E-001\n+             CJC = 1.00353E-012\n+             VJC = 
> 3.40808E-001\n+             MJC = 1.94223E-001\n.ENDS
> TEXT 1216 472 Left 2 !.model MPSH81 PNP(Is=10f Xti=3 Eg=1.11 Vaf=100 
> Bf=133.8 Ise=1.678p Ne=2.159 Ikf=.1658 Nk=.901 Xtb=1.5 Var=100 Br=1 
> Isc=9.519n Nc=3.88 Ikr=5.813 Rc=7.838 Cjc=2.81p Mjc=.1615 Vjc=.8282 
> Fc=.5 Cje=2.695p Mje=.3214 Vje=.7026 Tr=11.32n Tf=97.83p Itf=69.29 
> Xtf=599u Vtf=10 Vceo=20 Icrating=50m mfg=Fairchild)

What does the JFET get you that an NPN BJT in that position wouldn't do?

piglet