On 11/16/2014 1:54 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.

Thanks for the suggestion. Noise shouldn't be a problem in this app.
The noise is typically dominated by terrestrial sources of interference.
The antenna has a Q of 90 but the signal is still very low level.

The thing I don't get is that the BF862 data sheet says the gate source
capacitance is in the 10 pF ballpark. But in the simulation it seems to
be more like 300 pF. The frequency response curves don't look anything
like capacitive loading either. Is this some strange non-linear thing
because I am using the part with a very low drain current ~5 uA?

Someone here pointed out to me once that at low collector currents the
gain falls off. That didn't make a lot of sense until just now I was
looking at the ID vs VG1 of the BBF998 and I realized how it is like a
leaky faucet. You can easily change the flow rate from 1 gal/min to 1.1
gal/min. But trying to change it from 1 drop per minute to 1.1 drop per
minute is not so easy. The curve is asymptotic to the X axis making it
very hard to get much change in current as it approaches 0.

--

Rick