On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing. Any
suggestions?

I had found one op amp that might get me in the ballpark of power
consumption and I did some spice simulation on it. The current ends up
being in the 50 uA range which is more than I would like and the gain is
only around 100 before the bandwidth limits are felt which is less than
I would like. At 50 uA there is not the power to add a second stage.

Instead I was looking at some JFETs and found one I like, BF862 made by
NXP. I can construct a stage that gives a gain of 40 dB at only a
handful of uA. But when I try to cascade a second stage I have trouble.

The input capacitance is stated in the data sheet to be in the range of
10 pF. If I add a 10 pF cap to the output of the first stage I get
close to 40 dB of gain at the frequency of interest, 60 kHz. But when a
second stage is added with capacitive coupling the gain of the first
stage drops to 19 dB at 60 kHz while maintaining 40 dB at 1 kHz.

As a simple test, I put a capacitor in series with the gate and drove it
from a voltage source. I found the gate was at about half the voltage
of the voltage source when the capacitor was 300 pF. That says to me
the JFET model has 300 pF of capacitance. That just doesn't sound right.

I have seen other oddities from trying to drive the input of this part.
I have it biased correctly so the gate is not conducting. Any
suggestions? I am including the LTspice files below. I found one
thread on an audio web site where someone "improved" the model file.

Model file - spice_BF862.prm - put in "Simulations" directory below
schematic location
*******************
* BF862 SPICE MODEL MARCH 2007 NXP SEMICONDUCTORS
* ENVELOPE SOT23
* JBF862: 1, Drain, 2,Gate, 3,Source
Ld 1 4 L= 1.1nH
Ls 3 6 L= 1.25nH
Lg 2 5 L= 0.78nH
Rg 5 7 R= 0.535 Ohm
Cds 1 3 C= 0.0001pF
Cgs 2 3 C= 1.05pF
Cgd 1 2 C= 0.201pF
Co 4 6 C= 0.35092pF
JBF862 model parameters:
..model JBF862 NJF(Beta=47.800E-3 Betatce=-.5 Rd=.8 Rs=7.5000
Lambda=37.300E-3 Vto=-.57093
+ Vtotc=-2.0000E-3 Is=424.60E-12 Isr=2.995p N=1 Nr=2 Xti=3 Alpha=-1.0000E-3
+ Vk=59.97 Cgd=7.4002E-12 M=.6015 Pb=.5 Fc=.5 Cgs=8.2890E-12 Kf=87.5E-18
+ Af=1)
ENDS BF862


Schematic file - LowPowerPreAmp_JFET.asc
*******************
Version 4
SHEET 1 1340 680
WIRE 32 -128 -16 -128
WIRE 128 -128 32 -128
WIRE 368 -128 368 -160
WIRE 1008 -128 1008 -160
WIRE 128 -112 128 -128
WIRE -16 -96 -16 -128
WIRE -16 0 -16 -16
WIRE 128 0 128 -48
WIRE 368 0 368 -48
WIRE 416 0 368 0
WIRE 448 0 416 0
WIRE 608 0 512 0
WIRE 768 0 608 0
WIRE 1008 0 1008 -48
WIRE 1152 0 1008 0
WIRE 1264 0 1152 0
WIRE 368 32 368 0
WIRE 1008 32 1008 0
WIRE 1264 48 1264 0
WIRE 240 96 -16 96
WIRE 320 96 240 96
WIRE 768 96 768 0
WIRE 832 96 768 96
WIRE 960 96 832 96
WIRE 240 144 240 96
WIRE 368 144 368 128
WIRE 448 144 368 144
WIRE 496 144 448 144
WIRE 1008 144 1008 128
WIRE 1088 144 1008 144
WIRE 1136 144 1088 144
WIRE -16 160 -16 96
WIRE 768 160 768 96
WIRE 368 176 368 144
WIRE 1008 176 1008 144
WIRE 496 192 496 144
WIRE 1136 192 1136 144
WIRE 1264 224 1264 112
WIRE 240 256 240 224
WIRE -16 288 -16 240
WIRE 368 288 368 256
WIRE 496 288 496 256
WIRE 496 288 368 288
WIRE 1008 288 1008 256
WIRE 1136 288 1136 256
WIRE 1136 288 1008 288
WIRE 368 336 368 288
WIRE 768 336 768 240
WIRE 1008 336 1008 288
FLAG 368 336 0
FLAG -16 0 0
FLAG 32 -128 V2.2
FLAG -16 96 Vin
FLAG 240 256 0
FLAG -16 288 0
FLAG 128 0 0
FLAG 368 -160 V2.2
FLAG 448 144 Vs
FLAG 1008 336 0
FLAG 1152 0 Vout
FLAG 1008 -160 V2.2
FLAG 1088 144 Vs2
FLAG 768 336 0
FLAG 416 0 G1
FLAG 608 0 Vin2
FLAG 832 96 Vin3
FLAG 1264 224 0
SYMBOL voltage -16 -112 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR InstName V1
SYMATTR Value 2.2v
SYMATTR SpiceLine Rser=1
SYMBOL voltage -16 144 R0
WINDOW 123 24 152 Left 2
WINDOW 39 24 124 Left 2
SYMATTR InstName V2
SYMATTR Value SINE(0 50uV 60K)
SYMATTR Value2 AC 1
SYMATTR SpiceLine Rser=10
SYMBOL res 224 128 R0
SYMATTR InstName R1
SYMATTR Value 10Meg
SYMBOL cap 112 -112 R0
SYMATTR InstName C5
SYMATTR Value 100µF
SYMBOL res 352 -144 R0
SYMATTR InstName R3
SYMATTR Value 100k
SYMBOL njf 320 32 R0
SYMATTR InstName T1
SYMATTR Value JBF862
SYMBOL res 352 160 R0
SYMATTR InstName R2
SYMATTR Value 100k
SYMBOL cap 480 192 R0
SYMATTR InstName C1
SYMATTR Value 10µF
SYMBOL res 992 -144 R0
SYMATTR InstName R6
SYMATTR Value 100k
SYMBOL njf 960 32 R0
SYMATTR InstName T2
SYMATTR Value JBF862
SYMBOL res 992 160 R0
SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL cap 1120 192 R0
SYMATTR InstName C3
SYMATTR Value 1000nf
SYMBOL cap 448 16 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C2
SYMATTR Value 10µF
SYMBOL res 752 144 R0
SYMATTR InstName R4
SYMATTR Value 10Meg
SYMBOL cap 1248 48 R0
SYMATTR InstName C4
SYMATTR Value 10pF
TEXT 502 -200 Left 2 !.ac dec 10 0.1 10Meg
TEXT -24 400 Left 2 !.lib Simulations\\spice_BF862.prm

--

Rick