Ye olde bootstrappe buffere

Sep 09, 2024 Last reply: 1 year ago 3 Replies

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:



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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

The p-to-n emitter-to-emitter connection, a variant on the cascode, is intersting, as is the hybrid nfet/pnp source-to-emitter connection.

Your input could have been into the jfet, for hi-z.

An NPN there seems to cause some appreciable peaking around the HF cutoff, though I haven't dug into this topology far enough to understand exactly why..

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