On 11/16/2014 01:54 PM, John Larkin wrote:
On Sun, 16 Nov 2014 08:14:11 -0800, Joerg
wrote:
rickman wrote:
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.
You need a FET with an input capacitance an order of magnitude lower.
Got to run now and can't find it so quickly but ask John Larkin. He
suggested a FET a while ago that is IIRC under 1pF.
NE3509 maybe... a bit under 1 pF. Phemts have high 1/f noise corners,
so I don't know how well they might work at 60 KHz and low current.
Phil probably has lf noise data on a Skyworks part.
The key to low-noise, low-power gain in narrowband amps is proper
input network tuning. A tuned circuit makes voltage gain for zero
power consumption. Ditto interstage coupling. This problem may not
actually need a super-low-capacitance part.
My data are pretty limited--a few examples of the SKY65050 had flatband
1-Hz noise of about 0.4 nV and a 1/f corner of about 50 MHz. (!)
The Avago ATF38143s I tested were quieter, 0.35 nV and 10 MHz, which is
still pretty high--JFETs usually come in at about 1 kHz and good
bipolars, 10 Hz.
Besides the 1/f noise, the main down side of pHEMTs is that they have
pathetically low drain impedances, down in the hundreds of ohms. That
makes them useless for followers, for instance, and seriously limits the
voltage gain you can get out of them. Cascoding them helps a lot.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net