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rickman · November 5th 14, 10:34 PM posted to rec.radio.amateur.equipment

On 11/5/2014 2:56 PM, wrote:
rickman wrote:
On 11/5/2014 1:57 PM, wrote:
rickman wrote:
On 11/4/2014 8:29 PM, wrote:
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?

Specifying the output, impedance, and degree of linearlity required
might get a meaningful response.

The Maxim MAX480 opamp will run from a single supply down to 1.6V with
a supply current of less tha 20 uA.

Most of the micropower opamps I've seen run in the range of 30 to 50 uW
quiescent which doesn't leave a lot for output.

Thanks for the reply. I'm not sure what you are asking regarding the
output and impedance. The input would be high impedance and the output
low driving another high impedance input. I can design the circuit
around the amp, I just can't find anything very low power. The main
problem seems to be getting enough gain-bandwidth product to actually
amplify the signal.

The output and impedance determine E^2/R, which subtracts from the 100 uW
leaving you with approximetly the quiescent power budget.

The degree of linearity has a big effect on efficiency which has a
big effect on the power budget.

Also the desired gain would be a nice thing to know as in would a
single transistor be enough.

20 dB would be a good start.

Off the top of my head that seems to be a bit high for a single transistor.

If the current is low enough two could be used each with a gain of 10 or
more.

The part you suggest has a number of problems. One is that it is not
recommended for new designs. The other is that it only has a gain
bandwidth product of 20 kHz.

It was just a micropower opamp off the top of my head.

I assume you have looked into designs using various FET's?

Such as?

Well, you could start with an amplifier design that uses a theoretical
transistor which meets your goals then search maker spec sheets for a
real transistor that has the needed qualities.

I thought you said you had Spice?

I don't think spice is needed to design a single transistor amp. I was
actually looking for an IC that would do the job.

If using a transistor someone with experience in transistors would have
an idea of the limits. I have been told that bipolar devices have very
low gain at low collector currents which becomes a limitation as the
bias current is reduced. I don't know about FETs. It is not at all
clear to me that transistor models are accurate enough to give a good
representation of this in spice.

If I were doing it, I would start with some common transistor then
tweak the transistor specs until it did what I wanted or became
obvious it can't be done with a single transistor.

If there were no real transistor with the required specs or it
is not possible to do with a single transistor, I would go to
two transistors and repeat.

If I can't find someone with more experience I may do that. The actual
goal is to do this without a preamp, but I'm starting to have some
doubts about being able to pull it off. I need to characterize my
digital inputs. Then I will have a better idea of the requirements, but
that is more easily said than done.

--

Rick