Classic bootstrap buffer

Those little 3-terminal ingaas mmics are cool, stable gain blocks. A jfet follower ahead of one of them would be fast and simple.

Why R11?

History I think, can't quite recall. I could use it to invert the signal, or cut and join the C7 track to stabilize the power. See them adjacent on the PCB layout. If you see the photo, R11 is installed as a wire.

Clifford Heath.

You can make the circuit first-order d.c.-accurate by matching rather than your intentionally mis-matching of the JFETs.

This set of tweaks does that (the "lower" JFET's current produces a Vgs-cancelling offset across the "upper" JFET's source resistor), has a flatter, more accurate response, and uses about half as much power.

Cheers, James Arthur

Version 4 SHEET 1 880 1004 WIRE 64 -160 -128 -160 WIRE 256 -160 64 -160 WIRE 432 -160 256 -160 WIRE 64 -128 64 -160 WIRE 432 -128 432 -160 WIRE -128 -64 -128 -160 WIRE 432 -32 432 -48 WIRE 432 -32 336 -32 WIRE 64 -16 64 -48 WIRE 64 -16 -64 -16 WIRE 432 -16 432 -32 WIRE 336 0 336 -32 WIRE 64 16 64 -16 WIRE -128 80 -128 32 WIRE 432 112 432 64 WIRE -304 144 -512 144 WIRE -176 144 -304 144 WIRE -512 176 -512 144 WIRE -304 176 -304 144 WIRE -128 224 -128 176 WIRE 256 288 256 -160 WIRE -512 304 -512 256 WIRE -304 304 -304 256 WIRE -128 336 -128 304 WIRE 192 336 -128 336 WIRE 416 336 192 336 WIRE -128 384 -128 336 WIRE 64 416 64 80 WIRE 256 416 256 384 WIRE 256 416 64 416 WIRE -176 448 -224 448 WIRE -128 512 -128 480 WIRE 256 512 256 416 WIRE -224 624 -224 448 WIRE -128 624 -128 592 WIRE 256 624 256 592 FLAG -304 304 0 FLAG 432 112 Vss FLAG 336 0 0 FLAG -224 624 Vss FLAG -128 624 Vss FLAG -512 304 0 FLAG 416 336 Out IOPIN 416 336 Out FLAG 256 624 Vss SYMBOL njf -176 80 R0 SYMATTR InstName Q1 SYMATTR Value 2N5484 SYMBOL njf -176 384 R0 SYMATTR InstName Q2 SYMATTR Value 2N5484 SYMBOL res -144 496 R0 SYMATTR InstName R1 SYMATTR Value 250 SYMBOL res -320 160 R0 SYMATTR InstName R2 SYMATTR Value 1Meg SYMBOL voltage 432 -144 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL voltage 432 -32 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL voltage -512 160 R0 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value2 AC 1 SYMATTR InstName V3 SYMATTR Value SINE(0 1 100k) SYMBOL npn -64 -64 M0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL res 48 -144 R0 SYMATTR InstName R4 SYMATTR Value 33k SYMBOL res -144 208 R0 SYMATTR InstName R5 SYMATTR Value 250 SYMBOL npn 192 288 R0 SYMATTR InstName Q4 SYMATTR Value 2N3904 SYMBOL res 240 496 R0 SYMATTR InstName R6 SYMATTR Value 10k SYMBOL zener 80 80 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value BZX84C8V2L SYMATTR Description Diode SYMATTR Type diode TEXT -536 376 Left 2 !.ac oct 20 100 100Meg TEXT -536 432 Left 2 !;tran 0 10u 0

Thanks, that looks real good in the sim and I like the DC coupling. Problem is the drive current capability is quite limited (granted I didn't specify that so no biggie.)

I came up with this as "Revision 3", it can drive a 75 ohm load pretty well using an output PNP loaded by the CCS for more current gain. The FET is biased in the "triode region", I don't know if that's the best place for it but into a 1k load at least the circuit seems to have a large amount of "loop gain" and the overall gain is extremely close to 0 dB over most of the frequency response:

Version 4 SHEET 1 1388 1052 WIRE -128 -736 -128 -864 WIRE 272 -736 -128 -736 WIRE 608 -736 272 -736 WIRE -128 -560 -128 -736 WIRE 272 -560 272 -736 WIRE -384 -432 -384 -464 WIRE 608 -352 608 -736 WIRE -544 -320 -608 -320 WIRE -384 -320 -384 -352 WIRE -384 -320 -464 -320 WIRE -384 -304 -384 -320 WIRE -128 -304 -128 -480 WIRE 544 -304 -128 -304 WIRE -608 -288 -608 -320 WIRE -384 -176 -384 -224 WIRE -128 -144 -128 -304 WIRE 272 -96 272 -480 WIRE 272 -96 -64 -96 WIRE -128 80 -128 -48 WIRE 272 96 272 -96 WIRE -368 144 -608 144 WIRE -176 144 -368 144 WIRE -608 176 -608 144 WIRE -368 176 -368 144 WIRE -128 240 -128 176 WIRE 608 240 608 -256 WIRE 608 240 -128 240 WIRE 752 240 608 240 WIRE 960 240 816 240 WIRE 1152 240 960 240 WIRE 1280 240 1152 240 WIRE -608 304 -608 256 WIRE -368 304 -368 256 WIRE -128 336 -128 240 WIRE 1152 352 1152 240 WIRE -128 496 -128 416 WIRE -64 544 -80 544 WIRE 272 544 272 160 WIRE 272 544 -64 544 WIRE 560 544 272 544 WIRE 960 544 960 240 WIRE 960 544 624 544 WIRE 1152 544 1152 432 WIRE 272 640 272 544 WIRE -128 816 -128 592 WIRE 272 816 272 720 FLAG -128 -864 Vdd FLAG -384 -464 Vdd FLAG -384 -176 Vss FLAG -608 -288 0 FLAG -128 816 Vss FLAG -608 304 0 FLAG 1280 240 Out IOPIN 1280 240 Out FLAG -368 304 0 FLAG 272 816 Vss FLAG 1152 544 0 SYMBOL njf -176 80 R0 SYMATTR InstName J1 SYMATTR Value 2N5484 SYMBOL res -384 160 R0 SYMATTR InstName R2 SYMATTR Value 1Meg SYMBOL pnp 544 -256 M180 SYMATTR InstName Q2 SYMATTR Value 2N3906 SYMBOL res -144 -576 R0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL voltage -384 -448 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL voltage -384 -320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL voltage -608 160 R0 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR Value2 AC 1 SYMATTR InstName V3 SYMATTR Value SINE(0 1 10Meg) SYMBOL npn -64 -144 M0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL res 256 624 R0 WINDOW 3 36 68 Left 2 SYMATTR Value 47k SYMATTR InstName R9 SYMBOL cap 816 224 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 100n SYMBOL res 1136 336 R0 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL cap 624 528 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 100n SYMBOL References\\LT1389-5 272 128 R0 SYMATTR InstName U1 SYMBOL res 256 -576 R0 SYMATTR InstName R4 SYMATTR Value 47k SYMBOL res -448 -336 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 1m SYMBOL pnp -64 592 R180 SYMATTR InstName Q1 SYMATTR Value 2N3906 SYMBOL res -144 320 R0 SYMATTR InstName R7 SYMATTR Value 100 TEXT -744 376 Left 2 !.tran 0.1 TEXT -856 -8 Left 2 !;tran 0.1

I think it makes sense to attach the cathode of the reference to the emitter of Q1. Then if I use the appropriate reference voltage and pick the resistances right I should be able to eliminate the output coupling capacitor

You could take the output from the emitter follower, too.

One caution: if noise matters, bypass the zener and maybe R5 too.

Interesting: at 100 MHz, that circuit presents about an 800 ohm load to the input source, and has a roughly 550 ohm output impedance. So at that frequency it's not a lot better than a piece of wire. That's one gotcha about these buffer circuits: they may not have much actual power gain at high frequencies, but do have a of noise.

This one is fun. The fet runs constant-current. It's simple and pretty fast. Adding a drain bootstrap would increase gain a bit more and reduce input capacitance.

Version 4 SHEET 1 880 788 WIRE 384 -32 224 -32 WIRE 544 -32 384 -32 WIRE 544 16 544 -32 WIRE 224 112 224 -32 WIRE 544 128 544 96 WIRE -16 176 -64 176 WIRE 112 176 64 176 WIRE 176 176 112 176 WIRE -64 224 -64 176 WIRE 384 224 384 -32 WIRE 224 272 224 208 WIRE 272 272 224 272 WIRE 320 272 272 272 WIRE 224 320 224 272 WIRE -64 336 -64 304 WIRE 224 448 224 400 WIRE 384 448 384 320 WIRE 384 448 224 448 WIRE 480 448 384 448 WIRE 592 448 480 448 WIRE 384 496 384 448 WIRE 384 640 384 576 WIRE 384 768 384 720 FLAG 544 128 0 FLAG -64 336 0 FLAG 384 768 0 FLAG 112 176 IN FLAG 272 272 B FLAG 480 448 OUT SYMBOL njf 176 112 R0 WINDOW 0 101 23 Left 2 WINDOW 3 74 57 Left 2 SYMATTR InstName J1 SYMATTR Value 2N5484 SYMBOL npn 320 224 R0 WINDOW 0 97 32 Left 2 WINDOW 3 91 65 Left 2 SYMATTR InstName Q1 SYMATTR Value 2N5769 SYMBOL voltage -64 208 R0 WINDOW 0 87 86 Left 2 WINDOW 3 37 121 Left 2 WINDOW 123 77 162 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value SINE(0 1 100e6) SYMATTR Value2 AC 1 SYMBOL voltage 544 0 R0 WINDOW 0 70 42 Left 2 WINDOW 3 76 78 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 5 SYMBOL voltage 384 624 R0 WINDOW 0 61 39 Left 2 WINDOW 3 60 73 Left 2 SYMATTR InstName V3 SYMATTR Value -10 SYMBOL res 368 480 R0 WINDOW 0 69 38 Left 2 WINDOW 3 63 77 Left 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL res -32 192 R270 WINDOW 0 83 60 VTop 2 WINDOW 3 73 62 VBottom 2 SYMATTR InstName R2 SYMATTR Value 1m SYMBOL res 208 304 R0 WINDOW 0 57 43 Left 2 WINDOW 3 51 77 Left 2 SYMATTR InstName R3 SYMATTR Value 350 TEXT -32 472 Left 2 !;tran 1u TEXT -72 520 Left 2 !.ac dec 50 2Meg 200Meg TEXT -8 688 Left 2 ;JL May 2017 TEXT 24 632 Left 2 ;Buffer