On 11/8/2014 4:34 PM, rickman wrote:
On 11/8/2014 3:27 PM, Jerry Stuckle wrote:
On 11/7/2014 8:40 PM, rickman wrote:
On 11/7/2014 7:57 PM, Jerry Stuckle wrote:
On 11/7/2014 7:18 PM, rickman wrote:
On 11/7/2014 5:07 PM, Jerry Stuckle wrote:
On 11/7/2014 4:40 PM, rickman wrote:
On 11/7/2014 4:23 PM, Jerry Stuckle wrote:
On 11/7/2014 3:07 PM, rickman wrote:
On 11/7/2014 1:53 PM, Jerry Stuckle wrote:
On 11/7/2014 1:26 PM, rickman wrote:
On 11/7/2014 1:17 PM, Jerry Stuckle wrote:
On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle 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 agree with Jim. We need many more specifics to
provide a
meaningful
answer. There are a lot of micropower opamps out there
now,
but
the
devil is in the details.

I've only found one detail that is giving me the devil.
That
is the
bandwidth. The signal is 60 kHz. I can't think of any
other
issues I
would have with any amp capable of amplifying this signal
with
a low
power level. What more info do you feel is needed? Can
you ask
questions? Better yet, just point me to any amp that will
meet
my two
stated requirements!


The other posts you made had the info - things like
impedance and
gain
are important, as is frequency of operation (but we already
know
that).

A couple of things to consider, however. The higher the
impedance, the
more susceptible it will be to ambient noise pickup.
You're
starting
with a very small signal and may need to add shielding to
limit
external
noise.

The other problem is you're asking for low impedance
output. Low
impedance limits noise pickup, but increases current
drain. So
how low
of an impedance do you want?

I don't follow on this. How does a low output impedance
drive the
current drain?


There are op amps with very high (in the gigaohm range)
input
impedance
and pretty low quiescent current drain. How much it draws
during use
will be greatly dependent on the output current required,
which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look
through
some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it
also is
affected by the design of the chip. Op amps are not just
single
transistor devices; a lower output impedance also means more
current to
drive the output stage, which affects other components. So
even
if you
have a high impedance load, the lower the output impedance of
the op
amp
(i.e. the more current it can source/sink at a specific
supply
voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not
getting
back to
you quicker - yesterday was pretty busy). Depending on your
needs,
there are hundreds you can choose from. I might recommend
you
check
out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/







. You can pick and choose the parameters you want. Another
one
I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but
you
know the
details of what you want better than I do, so rather than
guess at
what
you might want, I think this would be better. It should give
you a
start.

I have done this before and found nothing. But I did it
again at
both
Mouser and Digikey and found several. One listed by Mouser
looked
especially good only to find rather than 0.75 uA of supply
current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and
another... one
part I'm not sure what to make of it. The selection table
shows
supply
current of 0.034 mA and the data sheet shows 25 A! Yes,
that's
right,
the data sheet shows between 25 and 300 Amps for typical
supply
current!!! I would contact TI about this obvious typo, but
this
part is
not suitable because of the GBW which is also incorrect in the
selection
table.

Same thing at Digikey, everything in the selection table that
meets
these two requirements is a mistake.


A couple of things.

First of all, I've found minor errors in the listings at
Mouser (I
don't
use Digikey much), but never real glaring errors. And this
is th
first
time I've seen a TI datasheet that far off. Looks like someone
dropped
a decimal point . However, I've found Mouser is
interested in
correcting errors; they are input by humans, after all, at some
point in
time, and errors do creep in.

Yes, when you list millions of parts there will be errors. I
have
written digikey many times about listing errors and they always
thank
me. I'm sure Mouser is no different.


Secondly, the current shown is going to be max current, which
will
depend on the output impedance (and the amount that has to be
sourced/sunk). It's not going to pull this all the time; I
would
expect
your actual current draw to be much less since you're 1) going
into a
high impedance load and 2) not going from rail to rail.

I find the opposite. The current listed is under specified
conditions
which usually *do not* include output drive. In fact, it
usually
listed
as a quiescent current.


Well, yes and no. Op amps typically sink more than they source,
and
the
sink current does not come from the chip. Source current at the
output
is supplied by the chip, of course.

And I've found a wide difference between how op amp specs are
listed;
some show quiescent current, some show average current under
typical
operating conditions. Some even show maximum current which
can be
drawn. So I'll retract that statement above. Wasn't thinking
clearly.


Also, if you use a bipolar supply, then current drain should be
less
because you'll be operating near ground, instead of the
midpoint
of a
single supply voltage (where the output would be at 1/2 Vcc).
Some of
these are quite low voltage, and I would think a couple of the
larger
lithium coin batteries should last quite a while.

Not sure how the ground level would affect the bias currents.
When
the
supply voltage is lowered the GBW lowers as well.


If the output is at ground level, no current will be pulled from
either
rail (at the output). Shifting above or below that will draw a
little
current, reference zero. However, if you're running a single
ended
supply, your output will be at 1/2 Vcc, and will always be
pulling
some
current to maintain that level. The signal will change that
slightly,
increasing or decreasing. But unless you have a square wave with
a 50%
duty cycle, you'll end up needing more current from the single
ended
supply.

What you are saying is only true if your load is ground connected.
The
load for this circuit will be a voltage source through a high
impedance.
The input is differential and to make it as sensitive as
possible a
bias will be applied to one input sufficient to offset the input
bias
voltage. So in reality the load will be biased to approx 1/2 Vcc.


True, but with a bipolar supply, the input is referenced to ground
and
no current flows with no input. The output is also referenced to
ground, so no current flows their, either. And with both input and
output at ground potential, there is less current flowing
internally.

I just explained a scenario where the load will draw current from
the
amp regardless of power supply arrangement. You are making an
assumption that the input and output are ground referenced. That is
independent of the supply arrangement.


But if they aren't ground referenced, then they must be referenced
to an
artificial ground, i.e. 1/2 Vcc. And creating that artificial ground
will require a certain amount of current.

For instance - it's common to bias the input of an op amp running
from a
single ended supply at 1/2 Vcc. This is generally done with a
couple of
resistors, in various configurations. But you will always have a
small
current through those resistors. The lower the impedance of the
input,
the lower the resistors must be.

Output in this case will also be referenced at 1/2 Vcc, which
means the
op amp output is conducting some current all of the time. Even if
the
output is capacitive coupled to the load, internally the op amp must
draw some current to maintain that 1/2 Vcc. Again, the amount of
current is dependent on the output impedance, but it is still there.

With a bipolar supply, the op amp doesn't draw input current with no
input signal, and doesn't have to source or sink any current when you
have 0V output.

I hope this is a bit clearer.

It is not a question of clear. It isn't relevant to the power
consumption of the opamp. No matter what the reference, somebody,
somewhere even if it is in the power supply, is using power sometime
unless there are no voltages on any of the resistors in the
design. But
none of that is relevant to the power consumed by the opamp when in
the
quiescent state.


Yes and no. Op amps are by design bipolar devices; they go plus and
minus from some value. It can be zero volts (ground), or it can be
some
value between Vcc and ground. In the latter case, an artificial ground
must be established; by definition this takes current to establish a
voltage between Vcc and ground.

Actually your characterization of opamps is not accurate. A very few
are designed to use dual supplies but most can work with unipolar
supplies. Basically they don't know where the ground is and they don't
care. The quiescent power the opamp dissipates is not related to where
ground is.


I suggest you build an op amp out of discreet components to understand
how it works. It was one of the projects in an EE class. It was very
educational to see just how they work.

As I said before, "But none of that is relevant to the power consumed by
the opamp when in the quiescent state."


Actually, it does. See above.


I'm trying to pick an opamp. I have no need to evaluate the rest
of the
design when the problem is trying to find an amp with sufficient
GBW and
low quiescent current.


But when you're talking very low power like you are, low quiescent
current is very dependent on the power supply, as well as input and
output impedance and the chip used.

It is certainly not dependent on the power supply configuration. I can
design the rest of the circuit. I'm just looking for a low power
device.


Once again, it does. But I see I won't convince you.

No, you won't. If you are talking about the minute differences in
internal biasing, etc, then I can't rule out all effects absolutely, but
if you had a valid point you would be able to explain it other than just
stating the fact repeatedly.


I have tried explaining it several times. However, you haven't accepted
the explanations. I can't help that.


The real point is that this is *LOAD* current, not amplifier
current and
is independent of the amplifier and so considered separately since
selection of the amplifier has no impact on it.


That is true - to an extent. When the op amp is sinking
current, the
current comes from the load, not the op amp. However, internally
there
must still be current flowing to provide that sink.

You aren't grasping the issue. It is not about which direction the
current is flowing, it is about what is responsible for setting the
amount of current. The load determines the current that flows in
or out
of the load and is independent of the opamp characteristics or the
power
supply arrangement. What I can control by picking the opamp is the
current that flows through the opamp that is independent of the
load or
input.


But the direction is important, also. You wanted to know how much
the
op amp itself will draw; when sourcing the load, you will find more
current on Vcc then when the op amp is sinking the load.4

No, I am not asking what the opamp will "draw". I'm asking about low
power amplifiers. I'd be happy knowing how it is done in the chips in
the radio controlled clocks since I'm pretty sure I'm not going to
find
a standard opamp that will do this.


You can; there are a number with that match your requirements. But you
need to design the entire circuit around your requirements, not just
the
op amp.

I only found one that was even close. As I said, the specs listed in
the selection guilds were all in error, usually by three orders of
magnitude.


Interesting - I don't know which ones you checked, but I've never found
that many errors in Mouser's data sheets. Not to say there aren't
errors - I have found several over the years. But not the high
percentage you've found.

It is not a high percentage. There are over 10,000 amps in Mouser's
list. I found around half a dozen or so that were listed incorrectly.
Search for amps that are under 40 uA quiescent current and GBW of 6 or
more.


It's a high percentage of those with the specs you need, which was my point.


But when the op amp is sourcing current, the amplifier has to
provide
the source current plus the drive current. Now you can bias the
output
so that the op amp is always sinking, but then you have a steady
drain
from the standby current.

When you're using a bipolar supply and input is at ground, the
output
will also be at ground, and very little current will be flowing.

You can create an artificial ground at 1/2 Vcc, but even creating
that
artificial ground draws current.

The difference in current drain is not important in the vast
majority of
cases. But they become important when you're talking the low
drain you
wish.

None of this is relevant to the issue of picking an amplifier.


It is if you're trying to minimize power usage to the maximum
extent. I
admit my op amp theory is around 40 years old, but I don't think the
laws of physics have changed in that time

No, and the laws of relevancy haven't changed either. The power
consumed by the load doesn't impact my selection of amplifier.


In many cases, it really doesn't matter much. But it does, when you're
down in the microamp range. That's what you don't understand.

Ok, thanks for your comments.







--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 11/8/2014 4:44 PM, Jerry Stuckle wrote:
On 11/8/2014 4:34 PM, rickman wrote:
On 11/8/2014 3:27 PM, Jerry Stuckle wrote:
On 11/7/2014 8:40 PM, rickman wrote:
On 11/7/2014 7:57 PM, Jerry Stuckle wrote:
On 11/7/2014 7:18 PM, rickman wrote:
On 11/7/2014 5:07 PM, Jerry Stuckle wrote:
On 11/7/2014 4:40 PM, rickman wrote:
On 11/7/2014 4:23 PM, Jerry Stuckle wrote:
On 11/7/2014 3:07 PM, rickman wrote:
On 11/7/2014 1:53 PM, Jerry Stuckle wrote:
On 11/7/2014 1:26 PM, rickman wrote:
On 11/7/2014 1:17 PM, Jerry Stuckle wrote:
On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle 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 agree with Jim. We need many more specifics to
provide a
meaningful
answer. There are a lot of micropower opamps out there
now,
but
the
devil is in the details.

I've only found one detail that is giving me the devil.
That
is the
bandwidth. The signal is 60 kHz. I can't think of any
other
issues I
would have with any amp capable of amplifying this signal
with
a low
power level. What more info do you feel is needed? Can
you ask
questions? Better yet, just point me to any amp that will
meet
my two
stated requirements!


The other posts you made had the info - things like
impedance and
gain
are important, as is frequency of operation (but we already
know
that).

A couple of things to consider, however. The higher the
impedance, the
more susceptible it will be to ambient noise pickup.
You're
starting
with a very small signal and may need to add shielding to
limit
external
noise.

The other problem is you're asking for low impedance
output. Low
impedance limits noise pickup, but increases current
drain. So
how low
of an impedance do you want?

I don't follow on this. How does a low output impedance
drive the
current drain?


There are op amps with very high (in the gigaohm range)
input
impedance
and pretty low quiescent current drain. How much it draws
during use
will be greatly dependent on the output current required,
which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look
through
some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it
also is
affected by the design of the chip. Op amps are not just
single
transistor devices; a lower output impedance also means more
current to
drive the output stage, which affects other components. So
even
if you
have a high impedance load, the lower the output impedance of
the op
amp
(i.e. the more current it can source/sink at a specific
supply
voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not
getting
back to
you quicker - yesterday was pretty busy). Depending on your
needs,
there are hundreds you can choose from. I might recommend
you
check
out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/







. You can pick and choose the parameters you want. Another
one
I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but
you
know the
details of what you want better than I do, so rather than
guess at
what
you might want, I think this would be better. It should give
you a
start.

I have done this before and found nothing. But I did it
again at
both
Mouser and Digikey and found several. One listed by Mouser
looked
especially good only to find rather than 0.75 uA of supply
current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and
another... one
part I'm not sure what to make of it. The selection table
shows
supply
current of 0.034 mA and the data sheet shows 25 A! Yes,
that's
right,
the data sheet shows between 25 and 300 Amps for typical
supply
current!!! I would contact TI about this obvious typo, but
this
part is
not suitable because of the GBW which is also incorrect in the
selection
table.

Same thing at Digikey, everything in the selection table that
meets
these two requirements is a mistake.


A couple of things.

First of all, I've found minor errors in the listings at
Mouser (I
don't
use Digikey much), but never real glaring errors. And this
is th
first
time I've seen a TI datasheet that far off. Looks like someone
dropped
a decimal point . However, I've found Mouser is
interested in
correcting errors; they are input by humans, after all, at some
point in
time, and errors do creep in.

Yes, when you list millions of parts there will be errors. I
have
written digikey many times about listing errors and they always
thank
me. I'm sure Mouser is no different.


Secondly, the current shown is going to be max current, which
will
depend on the output impedance (and the amount that has to be
sourced/sunk). It's not going to pull this all the time; I
would
expect
your actual current draw to be much less since you're 1) going
into a
high impedance load and 2) not going from rail to rail.

I find the opposite. The current listed is under specified
conditions
which usually *do not* include output drive. In fact, it
usually
listed
as a quiescent current.


Well, yes and no. Op amps typically sink more than they source,
and
the
sink current does not come from the chip. Source current at the
output
is supplied by the chip, of course.

And I've found a wide difference between how op amp specs are
listed;
some show quiescent current, some show average current under
typical
operating conditions. Some even show maximum current which
can be
drawn. So I'll retract that statement above. Wasn't thinking
clearly.


Also, if you use a bipolar supply, then current drain should be
less
because you'll be operating near ground, instead of the
midpoint
of a
single supply voltage (where the output would be at 1/2 Vcc).
Some of
these are quite low voltage, and I would think a couple of the
larger
lithium coin batteries should last quite a while.

Not sure how the ground level would affect the bias currents.
When
the
supply voltage is lowered the GBW lowers as well.


If the output is at ground level, no current will be pulled from
either
rail (at the output). Shifting above or below that will draw a
little
current, reference zero. However, if you're running a single
ended
supply, your output will be at 1/2 Vcc, and will always be
pulling
some
current to maintain that level. The signal will change that
slightly,
increasing or decreasing. But unless you have a square wave with
a 50%
duty cycle, you'll end up needing more current from the single
ended
supply.

What you are saying is only true if your load is ground connected.
The
load for this circuit will be a voltage source through a high
impedance.
The input is differential and to make it as sensitive as
possible a
bias will be applied to one input sufficient to offset the input
bias
voltage. So in reality the load will be biased to approx 1/2 Vcc.


True, but with a bipolar supply, the input is referenced to ground
and
no current flows with no input. The output is also referenced to
ground, so no current flows their, either. And with both input and
output at ground potential, there is less current flowing
internally.

I just explained a scenario where the load will draw current from
the
amp regardless of power supply arrangement. You are making an
assumption that the input and output are ground referenced. That is
independent of the supply arrangement.


But if they aren't ground referenced, then they must be referenced
to an
artificial ground, i.e. 1/2 Vcc. And creating that artificial ground
will require a certain amount of current.

For instance - it's common to bias the input of an op amp running
from a
single ended supply at 1/2 Vcc. This is generally done with a
couple of
resistors, in various configurations. But you will always have a
small
current through those resistors. The lower the impedance of the
input,
the lower the resistors must be.

Output in this case will also be referenced at 1/2 Vcc, which
means the
op amp output is conducting some current all of the time. Even if
the
output is capacitive coupled to the load, internally the op amp must
draw some current to maintain that 1/2 Vcc. Again, the amount of
current is dependent on the output impedance, but it is still there.

With a bipolar supply, the op amp doesn't draw input current with no
input signal, and doesn't have to source or sink any current when you
have 0V output.

I hope this is a bit clearer.

It is not a question of clear. It isn't relevant to the power
consumption of the opamp. No matter what the reference, somebody,
somewhere even if it is in the power supply, is using power sometime
unless there are no voltages on any of the resistors in the
design. But
none of that is relevant to the power consumed by the opamp when in
the
quiescent state.


Yes and no. Op amps are by design bipolar devices; they go plus and
minus from some value. It can be zero volts (ground), or it can be
some
value between Vcc and ground. In the latter case, an artificial ground
must be established; by definition this takes current to establish a
voltage between Vcc and ground.

Actually your characterization of opamps is not accurate. A very few
are designed to use dual supplies but most can work with unipolar
supplies. Basically they don't know where the ground is and they don't
care. The quiescent power the opamp dissipates is not related to where
ground is.


I suggest you build an op amp out of discreet components to understand
how it works. It was one of the projects in an EE class. It was very
educational to see just how they work.

As I said before, "But none of that is relevant to the power consumed by
the opamp when in the quiescent state."


Actually, it does. See above.


I'm trying to pick an opamp. I have no need to evaluate the rest
of the
design when the problem is trying to find an amp with sufficient
GBW and
low quiescent current.


But when you're talking very low power like you are, low quiescent
current is very dependent on the power supply, as well as input and
output impedance and the chip used.

It is certainly not dependent on the power supply configuration. I can
design the rest of the circuit. I'm just looking for a low power
device.


Once again, it does. But I see I won't convince you.

No, you won't. If you are talking about the minute differences in
internal biasing, etc, then I can't rule out all effects absolutely, but
if you had a valid point you would be able to explain it other than just
stating the fact repeatedly.


I have tried explaining it several times. However, you haven't accepted
the explanations. I can't help that.

Yes, and in each case I have explained why your description of what is
happening is not valid.

The issue I am discussing is the power supply current that is determined
by the selection of the amplifier. You seem to want to replace that
with all power that flows into the power supply pins of the device even
when that power is determined by other parts of the circuit or even
power consumed in totally separate parts of the circuit. In other
words, you want to address other problems than the one I am addressing.


The real point is that this is *LOAD* current, not amplifier
current and
is independent of the amplifier and so considered separately since
selection of the amplifier has no impact on it.


That is true - to an extent. When the op amp is sinking
current, the
current comes from the load, not the op amp. However, internally
there
must still be current flowing to provide that sink.

You aren't grasping the issue. It is not about which direction the
current is flowing, it is about what is responsible for setting the
amount of current. The load determines the current that flows in
or out
of the load and is independent of the opamp characteristics or the
power
supply arrangement. What I can control by picking the opamp is the
current that flows through the opamp that is independent of the
load or
input.


But the direction is important, also. You wanted to know how much
the
op amp itself will draw; when sourcing the load, you will find more
current on Vcc then when the op amp is sinking the load.4

No, I am not asking what the opamp will "draw". I'm asking about low
power amplifiers. I'd be happy knowing how it is done in the chips in
the radio controlled clocks since I'm pretty sure I'm not going to
find
a standard opamp that will do this.


You can; there are a number with that match your requirements. But you
need to design the entire circuit around your requirements, not just
the
op amp.

I only found one that was even close. As I said, the specs listed in
the selection guilds were all in error, usually by three orders of
magnitude.


Interesting - I don't know which ones you checked, but I've never found
that many errors in Mouser's data sheets. Not to say there aren't
errors - I have found several over the years. But not the high
percentage you've found.

It is not a high percentage. There are over 10,000 amps in Mouser's
list. I found around half a dozen or so that were listed incorrectly.
Search for amps that are under 40 uA quiescent current and GBW of 6 or
more.


It's a high percentage of those with the specs you need, which was my point.

I'm not sure what you mean. There are *no* parts available with the
specs I need. So I guess you could look at it as 100% which is indeed a
high percentage.


But when the op amp is sourcing current, the amplifier has to
provide
the source current plus the drive current. Now you can bias the
output
so that the op amp is always sinking, but then you have a steady
drain
from the standby current.

When you're using a bipolar supply and input is at ground, the
output
will also be at ground, and very little current will be flowing.

You can create an artificial ground at 1/2 Vcc, but even creating
that
artificial ground draws current.

The difference in current drain is not important in the vast
majority of
cases. But they become important when you're talking the low
drain you
wish.

None of this is relevant to the issue of picking an amplifier.


It is if you're trying to minimize power usage to the maximum
extent. I
admit my op amp theory is around 40 years old, but I don't think the
laws of physics have changed in that time

No, and the laws of relevancy haven't changed either. The power
consumed by the load doesn't impact my selection of amplifier.


In many cases, it really doesn't matter much. But it does, when you're
down in the microamp range. That's what you don't understand.

Ok, thanks for your comments.









--

Rick

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

On Sat, 15 Nov 2014 22:17:38 -0500, 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 haven't seen the original post, but are you building some type of
clock receiver ? Those work for a year with a single battery.

What kind of antenna are you using ? Do you really need a preamp ?

Do you have room for a tank circuit (L/C) on the collector/drain ?

On 11/16/2014 3:18 AM, wrote:
On Sat, 15 Nov 2014 22:17:38 -0500, 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 haven't seen the original post, but are you building some type of
clock receiver ? Those work for a year with a single battery.

Yes, it is a radio controlled clock.


What kind of antenna are you using ? Do you really need a preamp ?

I was planning on a loop antenna made from RG6 cable, but if I have to
add an amplifier I may use a ferrite loop.


Do you have room for a tank circuit (L/C) on the collector/drain ?

Room should not be a problem. But what is the point of a tank?

--

Rick

On Sun, 16 Nov 2014 03:47:44 -0500, rickman wrote:

On 11/16/2014 3:18 AM, wrote:
On Sat, 15 Nov 2014 22:17:38 -0500, 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 haven't seen the original post, but are you building some type of
clock receiver ? Those work for a year with a single battery.

Yes, it is a radio controlled clock.


What kind of antenna are you using ? Do you really need a preamp ?

I was planning on a loop antenna made from RG6 cable, but if I have to
add an amplifier I may use a ferrite loop.

Are you going to use a big (several meters) loop with the RG-6 center
conductor as a loop and cutting the shield at the top and using the
rest of the cable shield as a grounded static shield and using a small
coupling loop into the receiver ? With the main loop resonated by a
capacitor to 60 kHz, you should get quite decent signal without
preamplifier.

For anything smaller, a 5 cm ferrite bar is quite adequate due to the
high band noise, even if the ferrite antenna gain might be -40 dBi or
even -60 dBi.


Do you have room for a tank circuit (L/C) on the collector/drain ?

Room should not be a problem. But what is the point of a tank?

1. if you do not have a frequency selective antenna, this tank circuit
will provide the selectivity. Since this stage has a low gain at
unwanted frequencies, this reduces the risk of IP3 distortion, which
becomes critical at low collector/drain currents.

2. you get at least twice the voltage swing compared to the battery
voltage. Tapping the inductor or capacitor chain will provide nice
impedance matching avoiding the need for a cascaded stage.

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.

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.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

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.



--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On 11/16/2014 5:08 AM, wrote:
On Sun, 16 Nov 2014 03:47:44 -0500, rickman wrote:

On 11/16/2014 3:18 AM, wrote:
On Sat, 15 Nov 2014 22:17:38 -0500, 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 haven't seen the original post, but are you building some type of
clock receiver ? Those work for a year with a single battery.

Yes, it is a radio controlled clock.


What kind of antenna are you using ? Do you really need a preamp ?

I was planning on a loop antenna made from RG6 cable, but if I have to
add an amplifier I may use a ferrite loop.

Are you going to use a big (several meters) loop with the RG-6 center
conductor as a loop and cutting the shield at the top and using the
rest of the cable shield as a grounded static shield and using a small
coupling loop into the receiver ? With the main loop resonated by a
capacitor to 60 kHz, you should get quite decent signal without
preamplifier.

That's the general idea but in an 8 turn 2 foot loop. I may add another
50 foot of RG6 (helps with the split) to boost the signal further.
"Decent" must be defined. This signal is not so strong, 100 uV/m and
this loop will only give 26 uV counting a Q of 90 which might not fully
materialize by the time it is plugged into the receiver.

As I think about this (I do more thinking than designing sometimes) I am
becoming less and less convinced I can do this without a preamp.


For anything smaller, a 5 cm ferrite bar is quite adequate due to the
high band noise, even if the ferrite antenna gain might be -40 dBi or
even -60 dBi.

Not sure why you would compare the ferrite antenna to an isotropic
antenna, but when compared to the 2 foot loop the equations show the 2
foot loop provides a stronger signal. Now that I am considering a
preamp I may return to the idea of the ferrite loop antenna. Lol, if I
do that I can explore the joys of Litz wire.


Do you have room for a tank circuit (L/C) on the collector/drain ?

Room should not be a problem. But what is the point of a tank?

1. if you do not have a frequency selective antenna, this tank circuit
will provide the selectivity. Since this stage has a low gain at
unwanted frequencies, this reduces the risk of IP3 distortion, which
becomes critical at low collector/drain currents.

The antenna is already highly tuned to the frequency of interest.
Unlike voice broadcasts the bandwidth of this signal is just double
digit Hz so a Q as high as feasible is useful. That is why RG6 was
picked, with a center conductor that pushes the skin effect the Q will
be as good as practical (without using Litz wire, lol).


2. you get at least twice the voltage swing compared to the battery
voltage. Tapping the inductor or capacitor chain will provide nice
impedance matching avoiding the need for a cascaded stage.

Impedance matching to what? My "receiver" is an FPGA with a rather high
impedance input, measured in Megaohms in parallel with single digit pF.

I don't need to worry about Vcc (or Vdd) limiting voltage swing even
with the amp the voltage is low. If the tuned circuit will boost the
voltage otherwise I would consider it. Would a tank circuit be put in
series with a resistance? Otherwise how is the DC point established? I
am using a source resistor with bypass cap to bias the gate-source
voltage (reminds me of tube circuits) but a resistor is also needed in
the drain connection, no?

--

Rick

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