Search OpAmp chip as a voltage follower

[lvwener@gmail.com]

Hello, I am now looking for an Operation Amplifer chip able to be a
voltage follower, which I need to use as a buffer between my sensor
circuit and the digitizer. Can anybody help me?

Since the output impedance of my sensor circuit is about 0.2 Gohm, the
follower needs to have a input impedance much higher than that (Maybe
a JFET one could work).

The input signal is always sinusoidal, with an amplitude = 1~2V, and a
variable frequency = 10KHz~1MHz, and the noise at the output is
required to be <1uV.

I tried AD712, but its bandwidth seems not enough for the 1MHz signal.
And the AD746 showed a terrible self-excitation as being used as a
follower.

The price would not be a problem if it is lower than 50 US$.

A million thanks!

 

[Vladimir Vassilevsky]

Hello, I am now looking for an Operation Amplifer chip able to be a
voltage follower, which I need to use as a buffer between my sensor
circuit and the digitizer. Can anybody help me?

Since the output impedance of my sensor circuit is about 0.2 Gohm, the
follower needs to have a input impedance much higher than that (Maybe
a JFET one could work).

The input signal is always sinusoidal, with an amplitude = 1~2V, and a
variable frequency = 10KHz~1MHz, and the noise at the output is
required to be <1uV.

Your requirements do not make any sense. Change it to something more
realistic.

0.2GOhm @ 1MHz = ~ 8e-4 pF. Not feasible.

1uV @ 1MHz = ~ 1nV/sqrt(Hz). Not feasible.

1uV @ 1Mhz @ 0.2GOhm =~ 5e-3fA/sqrt(Hz). Not feasible.

0.2GOhm @ 300K @ 1MHz = ~1.8mV of thermal noise already.

I tried AD712, but its bandwidth seems not enough for the 1MHz signal.
And the AD746 showed a terrible self-excitation as being used as a
follower.

I suggest OPA2350. But your requirements just can't be met by any
technology.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

 

[Winfield]

Hello, I am now looking for an Operation Amplifer chip able to be a
voltage follower, which I need to use as a buffer between my sensor
circuit and the digitizer. Can anybody help me?

Since the output impedance of my sensor circuit is about 0.2 Gohm, the
follower needs to have a input impedance much higher than that (Maybe
a JFET one could work).

The input signal is always sinusoidal, with an amplitude = 1~2V, and a
variable frequency = 10KHz~1MHz, and the noise at the output is
required to be <1uV.

I tried AD712, but its bandwidth seems not enough for the 1MHz signal.
And the AD746 showed a terrible self-excitation as being used as a
follower.

The price would not be a problem if it is lower than 50 US$.

We'll ignore the difficulty of reconciling a 200M source
with a 1MHz bandwidth (what about capacitance?). Let me
suggest you evaluate the TI Burr-Brown OPA657 family.
These are JFET opamps with 1.6GHz bandwidth and 700V/us
slew rate -- that should be fast enough!

 

[Jan Panteltje]

Your requirements do not make any sense. Change it to something more
realistic.

0.2GOhm @ 1MHz = ~ 8e-4 pF. Not feasible.

1uV @ 1MHz = ~ 1nV/sqrt(Hz). Not feasible.

1uV @ 1Mhz @ 0.2GOhm =~ 5e-3fA/sqrt(Hz). Not feasible.

0.2GOhm @ 300K @ 1MHz = ~1.8mV of thermal noise already.


I suggest OPA2350. But your requirements just can't be met by any
technology.

Perhaps, if the source allows it, he can drive the sensor directly into
a low impedance source, like a BJT.
Then it behaves as current source, and capacitance becomes not important.

 

[Ken S. Tucker]

Hello, I am now looking for an Operation Amplifer chip able to be a
voltage follower, which I need to use as a buffer between my sensor
circuit and the digitizer. Can anybody help me?

Since the output impedance of my sensor circuit is about 0.2 Gohm, the
follower needs to have a input impedance much higher than that (Maybe
a JFET one could work).

The input signal is always sinusoidal, with an amplitude = 1~2V, and a
variable frequency = 10KHz~1MHz, and the noise at the output is
required to be <1uV.

I tried AD712, but its bandwidth seems not enough for the 1MHz signal.
And the AD746 showed a terrible self-excitation as being used as a
follower.

The price would not be a problem if it is lower than 50 US$.

A million thanks!

Input impedance using Tera Ohm IC's or the
equivalent transistor circuits are fairly easy.
(It's in the book)
Much lower than that and you'll be counting
electrons .
Ken

 

[Wener]

Thanks so much for the kindly replies!

Well, I am sorry about the typo in my post: the OUT IMPEDANCE of my
sensor circuit is typically "20 MOhm" (but NOT 20G).

The OPA2350 and OPA657 seem exacly what I am looking for. However, I
am still not sure whether they could be used as a voltage follower.
You know, sometimes a low closed-loop gain will cause a terrible self-
excitation, and that always happens for the wide-band op-amps. And
that is exactly the reason why I abandoned op37 or ad746. Well, I
guess I really need to read more in electronics... : )

 

[Phil Allison]

"Wener"

Thanks so much for the kindly replies!

Well, I am sorry about the typo in my post: the OUT IMPEDANCE of my
sensor circuit is typically "20 MOhm" (but NOT 20G).

** Even with 20 Mohms, noise a 1 MHz band is 570 uV.

( En = sqrt 4kTBR )

You have not got the tiniest clue what are crapping on about.



......... Phil

 

[John Larkin]

Hello, I am now looking for an Operation Amplifer chip able to be a
voltage follower, which I need to use as a buffer between my sensor
circuit and the digitizer. Can anybody help me?

Since the output impedance of my sensor circuit is about 0.2 Gohm, the
follower needs to have a input impedance much higher than that (Maybe
a JFET one could work).

The input signal is always sinusoidal, with an amplitude = 1~2V, and a
variable frequency = 10KHz~1MHz, and the noise at the output is
required to be <1uV.

I tried AD712, but its bandwidth seems not enough for the 1MHz signal.
And the AD746 showed a terrible self-excitation as being used as a
follower.

The price would not be a problem if it is lower than 50 US$.

A million thanks!

A 1 MHz, 0.2 Gohm signal basically can't exist in any form that can be
connected to an opamp. And at that resistance, the noise density is
almost 2 uV per root Hertz.

John

 

[Ken S. Tucker]

A 1 MHz, 0.2 Gohm signal basically can't exist in any form that can be
connected to an opamp. And at that resistance, the noise density is
almost 2 uV per root Hertz.

John

I thought perhaps something like the LH0063 is what the fella
might want...
http://www.datasheetcatalog.com/datasheets_pdf/L/H/0/0/LH0063.shtml
Ken

 

[clicliclic@freenet.de]

Thanks so much for the kindly replies!

Well, I am sorry about the typo in my post: the OUT IMPEDANCE of my
sensor circuit is typically "20 MOhm" (but NOT 20G).

The OPA2350 and OPA657 seem exacly what I am looking for. However, I
am still not sure whether they could be used as a voltage follower.
You know, sometimes a low closed-loop gain will cause a terrible self-
excitation, and that always happens for the wide-band op-amps. And
that is exactly the reason why I abandoned op37 or ad746. Well, I
guess I really need to read more in electronics... : )

Yes! - as others have remarked, you do not seem to know what you are
talking about. By now, you have mentioned three different values for
your sensor source impedance: 0.2 Gohm (that's 200 Mega-Ohm), 20 MOhm
(I take this to be Mega-Ohm), and 20 G (Giga-Ohm?). Before somebody
succeeds in selling you a $50 opamp, I suggest you try this:

Cut off most of the signal pin of your sensor, leaving the smallest
amount of metal that you can solder to. Make sure no other metal parts
are close to the pin. Obtain a FET (field-effect transistor) with a
small reverse capacitance - the 2SK161 comes to mind. That should cost
you less than $1. Cut off as much of the gate pin of the FET as you
dare and solder it directly to the sensor pin. Pick off the signal at
the source pin of the FET, using a resistor from source to ground to
feed the FET a current of a few milliamps (chose the resistor such
that the current stays below Idrain-source at zero Vgate-source, so
measure your FET first). Connect the drain pin of the FET to a
positive voltage supply (not exceeding 18V with the 2SK161).

Note that even with a FET capacitance of 0.1 pF, 100 MHz signals will
be damped very significantly: the R-C time constant of the setup would
be 20 Mohm * 0.1 pF = 2 µs (micro-seconds). And, as others have
remarked, your noise requirements were nonsense anyway: if kept at
room temperature, the 20 Gohm sensor itself will be the dominant noise
source (look up the Nyquist formula) - you can forget the FET noise.
Once it works, you should measure the frequency response of your
sensor-plus-preamp setup and correct all later measurements for it.

That's it.

Martin.

 

[Jan Panteltje]

Cut off most of the signal pin of your sensor, leaving the smallest
amount of metal that you can solder to. Make sure no other metal parts
are close to the pin. Obtain a FET (field-effect transistor) with a
small reverse capacitance - the 2SK161 comes to mind. That should cost
you less than $1. Cut off as much of the gate pin of the FET as you
dare and solder it directly to the sensor pin. Pick off the signal at
the source pin of the FET, using a resistor from source to ground to
feed the FET a current of a few milliamps (chose the resistor such
that the current stays below Idrain-source at zero Vgate-source, so
measure your FET first). Connect the drain pin of the FET to a
positive voltage supply (not exceeding 18V with the 2SK161).

Note that even with a FET capacitance of 0.1 pF, 100 MHz signals will
be damped very significantly: the R-C time constant of the setup would
be 20 Mohm * 0.1 pF = 2 µs (micro-seconds). And, as others have
remarked, your noise requirements were nonsense anyway: if kept at
room temperature, the 20 Gohm sensor itself will be the dominant noise
source (look up the Nyquist formula) - you can forget the FET noise.
Once it works, you should measure the frequency response of your
sensor-plus-preamp setup and correct all later measurements for it.

That's it.

Martin.

Very nice solution, but let's look again:

+12
|
2k2 R1
|------------------------------- out
c
------------------------ b T1 NPN low noise
| Z1 e beta ~500
20MOhm |
| 2Vpp |
~ |
| |
/// ///


So what happens?
Delta IbT1 is 2 / 20.000.000 = .1uA
Delta IcT1 is 500x.1 uA = 50uA
Delta Ur1 is 50uA x 2200 = 110mV. in 2k2!
Any cable and component capacitance at the input sees the base emitter capacitance of T1
in parallel, changing the time constant to much shorter (will be a few kOhm).

Although the output voltage swing is lower, you have a much wider freq response in
a nice low impedance.
I have only showed the AC path, one should bias the emitter of T1 for the right Ic
(or the generator).
Simpler even if AC coupling is allowed.
The non-linearity cause by the be junction of T1 is insignificant when you see that IbT1
is mainly set by Ugenerator / Zgenerator.

In some cases this circuit may be a very good solution,.
I have used to as preamp in a vidicon based camera, where there is also high impedances
like this, and lot of noise from defection coils etc in the immediate vincinity.
Worked very well.

Transistors are good current amplifiers, why not use it?

 

[Winfield]

Thanks so much for the kindly replies!

Well, I am sorry about the typo in my post: the OUT IMPEDANCE of
my sensor circuit is typically "20 MOhm" (but NOT 20G).

Perhaps you can tell us exactly what you're trying to do,
and we can help from there.

 

[clicliclic@freenet.de]

Very nice solution, but let's look again:

+12
|
2k2 R1
|------------------------------- out
c
------------------------ b T1 NPN low noise
| Z1 e beta ~500
20MOhm |
| 2Vpp |
~ |
| |
/// ///


So what happens?
Delta IbT1 is 2 / 20.000.000 = .1uA
Delta IcT1 is 500x.1 uA = 50uA
Delta Ur1 is 50uA x 2200 = 110mV. in 2k2!
Any cable and component capacitance at the input sees the base emitter capacitance of T1
in parallel, changing the time constant to much shorter (will be a few kOhm).

Although the output voltage swing is lower, you have a much wider freq response in
a nice low impedance.
I have only showed the AC path, one should bias the emitter of T1 for theright Ic
(or the generator).
Simpler even if AC coupling is allowed.
The non-linearity cause by the be junction of T1 is insignificant when you see that IbT1
is mainly set by Ugenerator / Zgenerator.

In some cases this circuit may be a very good solution,.
I have used to as preamp in a vidicon based camera, where there is also high impedances
like this, and lot of noise from defection coils etc in the immediate vincinity.
Worked very well.

Transistors are good current amplifiers, why not use it?

Three remarks:

- Loading the 20 Mohm sensor with (say) 2kohm will divide the signal
by 10^4 and the noise by 10^2. So the signal-to-noise ratio will
decrease to 1/100 - which is bad.

- I fear that at frequencies around 100 MHz the Miller capacitance of
real beta=500 transistors will bring down your output signal to around
1 mV - as bad as with the FET circuit.

- My phrase "using a resistor from source to ground" should have been
"using a resistor from source a negative voltage supply" - after all,
we have to accomodate a large voltage swing. The total voltage across
the 2SK161 must not exceed 18 V.

Two $1 solutions - it's up to the original poster.

Martin.

 

[Jan Panteltje]

Three remarks:

- Loading the 20 Mohm sensor with (say) 2kohm will divide the signal
by 10^4 and the noise by 10^2. So the signal-to-noise ratio will
decrease to 1/100 - which is bad.

I think you see this the wrong way.
You are using the current in the circuit, and the current is in no way
attenuated.

- I fear that at frequencies around 100 MHz the Miller capacitance of
real beta=500 transistors will bring down your output signal to around
1 mV - as bad as with the FET circuit.

The OP mentioned *1MHz* IIRC.

Two $1 solutions - it's up to the original poster.

Yep.

The circuit I mentioned worked for me though.
I had this camera preamp, with nice FET input stage, and no way
could I get it 'noise' free, and here it was not so much thermal noise,
but the fact that these extremely high impedances pick up _any_ noise
from nearby (and probably far away too) sources.
So I tried about 3 or 4 different high impedance FET circuits.
None if these worked like I hoped, and then accidently I touched the
target wire (from the vidicon tube) on the input of the NEXT stage
(a normal NPN common emitter amplifier).
Great picture, no noise, flat frequency response!!
It was one of those moments where you sit for some minutes, stunned,
wondering 'how can that be?'.
I figured it out, removed the FET stage, changed a few things to optimise it,
and it worked OK ever after.
In those days (sixties) you had no spice...You had to build things to try it.
As to the Miller cap, you can drive the collector into the emitter of a next
NPN, that has the base at say +5, so Uce is constant (cascode).

 

[clicliclic@freenet.de]

I think you see this the wrong way.
You are using the current in the circuit, and the current is in no way
attenuated.

I must disagree: the noise injected by a base-emitter junction that
constitutes an input resistance of 2 kohm roughly corresponds to the
voltage (and current) noise of a 2 kohm resistor. But we should let
others comment on this point.

The OP mentioned *1MHz* IIRC.

You're right! That's so much easier for both circuits. (And we may
also hope for the sensor impedance to be corrected to 2 Mohm!)

(Of course my correction should have read "using a resistor from
source _to_ a negative voltage supply".)

Martin.

 

[Jan Panteltje]

I must disagree: the noise injected by a base-emitter junction that
constitutes an input resistance of 2 kohm roughly corresponds to the
voltage (and current) noise of a 2 kohm resistor. But we should let
others comment on this point.

OK, but for that to be amplified, there must be Ib flowing, that requires
an external path, and in this case that path is via the 20 MOhm generator Ri?
That would result in a really really low contribution to Ib...
But indeed, others may be better at theoretical noise calculations then me.
I only know for sure it worked for my application
And then mainly to reduce noise pickup from external sources.

 

[default]

Hello, I am now looking for an Operation Amplifer chip able to be a
voltage follower, which I need to use as a buffer between my sensor
circuit and the digitizer. Can anybody help me?

Since the output impedance of my sensor circuit is about 0.2 Gohm, the
follower needs to have a input impedance much higher than that (Maybe
a JFET one could work).

The input signal is always sinusoidal, with an amplitude = 1~2V, and a
variable frequency = 10KHz~1MHz, and the noise at the output is
required to be <1uV.

I tried AD712, but its bandwidth seems not enough for the 1MHz signal.
And the AD746 showed a terrible self-excitation as being used as a
follower.

The price would not be a problem if it is lower than 50 US$.

A million thanks!

You aren't looking for an op amp. What you want is an electrometer
amplifier.

 

[Jamie]

A 1 MHz, 0.2 Gohm signal basically can't exist in any form that can be
connected to an opamp. And at that resistance, the noise density is
almost 2 uV per root Hertz.

John

So, does that mean we're coming to the end of anything new and exciting?