On 11/16/2014 11:14 AM, 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.

Can you explain this? The info I have on the BF862 shows 10 pF range
gate to source. If I replace the FET gate with a 10 pF load on the
first stage the output looks normal. With the FET in place the
frequency response of the first stage output looks very bad with a
complex curve rather than just a simple capacitor loading.


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.

Dual gate FETs are another option. An example, although this one still
has 2pF at gate 1:

http://www.nxp.com/documents/data_sheet/BF998.pdf

Have you tried BJTs? Only sad thing is, many of the very low power
Japanese ones have been discontinued.

I know nothing about dual gate FETs. The BJTs I have looked at won't
get the high input impedance or the low biasing currents I want. The
JFET is operating with about 5 uA of drain current.

--

Rick