Oscillating inductor/buffer

On Jan 18, 12:09 pm, George Herold <gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

       |\  opa2134
+--+---|+\
|  |   |  >-+out
L  R  +|-/  |
L  R  ||/   |
L  R  +-----+
|  |
+--+
GND

R = 1 Meg ohm

With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.

The inductors are 2200R series from
Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf

But I’ve got some wimpier 1mH’s that show similar behavior.

Here’s a ‘scope shot. frequency about 1.1
MHz.http://imageshack.us/photo/my-images/718/tek0039.png/

The upper trace is with the supply voltage lowered to +/-6 volts.  The
other is +/-15V.

Here’s a 100uH
inductor.http://imageshack.us/photo/my-images/585/tek0040.png/ Freq.
about 3.4 MHz.

and a 10uH.  (scale
change)http://imageshack.us/photo/my-images/849/tek0041.png/ ~10MHz,
which is beyond the GBP of the opamp (OPA2134)

If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.

Can anyone give me a hint as to what the heck is going on?

The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)

Thanks

George H.



Hello Gentlemen (and any Ladies), Just want a ‘reality’ check.


+---cf---+
| |
+ |\ | opa2134
+---+--+---|+\ |
| | | | >-+out
C L R +|-/ |
C L R ||/ |
C L R +-----+
| | |
+---+--+
GND

So here’s my current model of the oscillations and feedback. (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground. cf
is some feed back capacitance from output to input.

RRR is a parallel resistance that I add to kill the oscillations. In a
hand-wavy way the added resistance should about equal the cf impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of the
dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input. Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)

Test RRR
shield 45.7k ohm
no shield 38.9k
2.2pF 27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit better
opamp.

George Herold a écrit :





I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

       |\  opa2134
+--+---|+\
|  |   |  >-+out
L  R  +|-/  |
L  R  ||/   |
L  R  +-----+
|  |
+--+
GND

R = 1 Meg ohm

With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.

The inductors are 2200R series from Murata,
http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf

But I’ve got some wimpier 1mH’s that show similar behavior.

Here’s a ‘scope shot. frequency about 1.1 MHz.
http://imageshack.us/photo/my-images/718/tek0039.png/

The upper trace is with the supply voltage lowered to +/-6 volts.  The
other is +/-15V.

Here’s a 100uH inductor.
http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.

and a 10uH.  (scale change)
http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)

If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.

Can anyone give me a hint as to what the heck is going on?

The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)

Thanks

George H.

OK, George I see you're still at it.

I've have that very pb when I designed my 200pV/rtHz preamplifier
(several paralleled huge input jets which means lots of parasitic
capacitance) and I think I reported it here in its time.

That pb comes from when you use a closed loop amplifier with a
gain-bandwidth product GBW, which means that, at a high enough frequency
above the low frequency pole, the amplifier differential input voltage
has 90degree phase shift WRT to the input (or output) voltage.
This phase shifted voltage then injects some current into the opamp
positive input node, thanks to the opamp differential input capacitance.
You can do the math for a perfect single pole opamp and a cd capacitor
shunting the inputs.

You then obtain a synthesized input impedance which is:

Zin= - (2 pi GBW + j w)/(cd w^2)

When you decompose this in parallel real and imaginary impedances, that
gives you:

Rp = -(4 pi^2 GBW^2 + w^2)/(2 pi cd GWB w)

Cp = cd w^2 /(4 pi^2 GBW^2 + w^2)

As you can see your stage input impedance has an always negative
parallel admittance which depends only on your opamp GBW and capacitance.

Now for the oscillating frequency, apart from the obvious additional
parasitic capacitance you also have to add the opamp CM input capacitance..

And now again, when you take the limit for w -> infinity, the parallel
resistance limit is:

RpLimit = -1/(2 pi cd GBW)

which is roughly -10K for your example OPA2134 and have no high
frequency limit!
That is, no frequency limit for a perfect one pole opamp model plus
differential input capacitance.
The additional poles above GBW for real world opamps change that
somewhere above the GBW frequency, but it's no surprising at all that
you see some oscillation above the 8MHz GBW limit of your 2134.

Have fun with that :-)

--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -

On 1/19/2012 10:35 AM, George Herold wrote:



On Jan 18, 12:09 pm, George Herold<gher...@teachspin.com>  wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

        |\  opa2134
+--+---|+\
|  |   |>-+out
L  R  +|-/  |
L  R  ||/   |
L  R  +-----+
|  |
+--+
GND

R = 1 Meg ohm

With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.

The inductors are 2200R series from Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf

But I’ve got some wimpier 1mH’s that show similar behavior.

Here’s a ‘scope shot. frequency about 1.1 MHz.http://imageshack.us/photo/my-images/718/tek0039.png/

The upper trace is with the supply voltage lowered to +/-6 volts.  The
other is +/-15V.

Here’s a 100uH inductor.http://imageshack.us/photo/my-images/585/tek0040.png/
Freq. about 3.4 MHz.

and a 10uH.  (scale change)http://imageshack.us/photo/my-images/849/tek0041.png/
~10MHz, which is beyond the GBP of the opamp (OPA2134)

If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.

Can anyone give me a hint as to what the heck is going on?

The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)

Thanks

George H.

Hello Gentlemen (and any Ladies),  Just want a ‘reality’ check.

         +---cf---+
         |        |
         +   |\   |    opa2134
  +---+--+---|+\  |
  |   |  |   |>-+out
  C   L  R  +|-/  |
  C   L  R  ||/   |
  C   L  R  +-----+
  |   |  |
  +---+--+
      GND

So here’s my current model of the oscillations and feedback.  (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground.
cf is some feed back capacitance from output to input.

RRR is a parallel resistance that I add to kill the oscillations.  In
a hand-wavy way the added resistance should about equal the cf
impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of
the dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit
better opamp.

Thanx again for the help,
George H

  Hi George,
Looks like my hypothesis missed the mark, (I didn't note it wasn't
supposed to be an osc.)
   However, does the frequency get lower with additional capacitance
across the inductor.

                     Mikek- Hide quoted text -

- Show quoted text -

On Jan 19, 12:45 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Thu, 19 Jan 2012 08:35:53 -0800, George Herold wrote:
On Jan 18, 12:09 pm, George Herold <gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

       |\  opa2134
+--+---|+\
|  |   |  >-+out
L  R  +|-/  |
L  R  ||/   |
L  R  +-----+
|  |
+--+
GND

R = 1 Meg ohm

With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.

The inductors are 2200R series from
Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf

But I’ve got some wimpier 1mH’s that show similar behavior.

Here’s a ‘scope shot. frequency about 1.1
MHz.http://imageshack.us/photo/my-images/718/tek0039.png/

The upper trace is with the supply voltage lowered to +/-6 volts.  The
other is +/-15V.

Here’s a 100uH
inductor.http://imageshack.us/photo/my-images/585/tek0040.png/Freq.
about 3.4 MHz.

and a 10uH.  (scale
change)http://imageshack.us/photo/my-images/849/tek0041.png/~10MHz,
which is beyond the GBP of the opamp (OPA2134)

If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.

Can anyone give me a hint as to what the heck is going on?

The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)

Thanks

George H.

Hello Gentlemen (and any Ladies),  Just want a ‘reality’ check.

        +---cf---+
        |        |
        +   |\   |    opa2134
 +---+--+---|+\  |
 |   |  |   |  >-+out
 C   L  R  +|-/  |
 C   L  R  ||/   |
 C   L  R  +-----+
 |   |  |
 +---+--+
     GND

So here’s my current model of the oscillations and feedback.  (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground. cf
is some feed back capacitance from output to input.

RRR is a parallel resistance that I add to kill the oscillations.  In a
hand-wavy way the added resistance should about equal the cf impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of the
dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit better
opamp.

If you were to mention what you were actually _doing_ with that coil it
might help us help you.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

Hi Tim, Well let me first say that the opamp is the input to a wide
band amplifier stage. (DC? to 2-3 MHz.... exact HF spec once the pcb
is stuffed.) It's sort of a general purpose amp.
One of the applications will be the front end of an AM radio. (You
guys already helped me with that!) So the antenna is just a piece of
wire... but if I hang an inductor on the input (where the wire enters
the amp) I can get about 10dB more signal.

But having the inductor turn the amp into an oscillator is not so
good. :^)


I’ve been trolling digikey for other opamps without much luck. I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

George H.

On Thu, 19 Jan 2012 08:35:53 -0800, George Herold wrote:
On Jan 18, 12:09 pm, George Herold <gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

       |\  opa2134
+--+---|+\
|  |   |  >-+out
L  R  +|-/  |
L  R  ||/   |
L  R  +-----+
|  |
+--+
GND

R = 1 Meg ohm

With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.

The inductors are 2200R series from
Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf

But I’ve got some wimpier 1mH’s that show similar behavior.

Here’s a ‘scope shot. frequency about 1.1
MHz.http://imageshack.us/photo/my-images/718/tek0039.png/

The upper trace is with the supply voltage lowered to +/-6 volts.  The
other is +/-15V.

Here’s a 100uH
inductor.http://imageshack.us/photo/my-images/585/tek0040.png/Freq.
about 3.4 MHz.

and a 10uH.  (scale
change)http://imageshack.us/photo/my-images/849/tek0041.png/~10MHz,
which is beyond the GBP of the opamp (OPA2134)

If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.

Can anyone give me a hint as to what the heck is going on?

The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)

Thanks

George H.

Hello Gentlemen (and any Ladies),  Just want a ‘reality’ check.

        +---cf---+
        |        |
        +   |\   |    opa2134
 +---+--+---|+\  |
 |   |  |   |  >-+out
 C   L  R  +|-/  |
 C   L  R  ||/   |
 C   L  R  +-----+
 |   |  |
 +---+--+
     GND

So here’s my current model of the oscillations and feedback.  (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground. cf
is some feed back capacitance from output to input.

RRR is a parallel resistance that I add to kill the oscillations.  In a
hand-wavy way the added resistance should about equal the cf impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of the
dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit better
opamp.

If you were to mention what you were actually _doing_ with that coil it
might help us help you.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

RRR is a parallel resistance that I add to kill the oscillations.  In
a hand-wavy way the added resistance should about equal the cf
impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of
the dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a
tad lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit
better opamp.

If you were to mention what you were actually _doing_ with that coil it
might help us help you.

--
My liberal friends think I'm a conservative kook. My conservative
friends think I'm a liberal kook. Why am I not happy that they have
found common ground?

Tim Wescott, Communications, Control, Circuits &
Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

Hi Tim, Well let me first say that the opamp is the input to a wide
band amplifier stage. (DC? to 2-3 MHz.... exact HF spec once the pcb is
stuffed.) It's sort of a general purpose amp. One of the applications
will be the front end of an AM radio. (You guys already helped me with
that!) So the antenna is just a piece of wire... but if I hang an
inductor on the input (where the wire enters the amp) I can get about
10dB more signal.

But having the inductor turn the amp into an oscillator is not so good.
:^)


I’ve been trolling digikey for other opamps without much luck. I think
I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

On Thu, 19 Jan 2012 12:54:26 -0800, George Herold wrote:

RRR is a parallel resistance that I add to kill the oscillations.  In
a hand-wavy way the added resistance should about equal the cf
impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of
the dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a
tad lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit
better opamp.

If you were to mention what you were actually _doing_ with that coil it
might help us help you.

--
My liberal friends think I'm a conservative kook. My conservative
friends think I'm a liberal kook. Why am I not happy that they have
found common ground?

Tim Wescott, Communications, Control, Circuits &
Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

Hi Tim, Well let me first say that the opamp is the input to a wide
band amplifier stage. (DC? to 2-3 MHz.... exact HF spec once the pcb is
stuffed.) It's sort of a general purpose amp. One of the applications
will be the front end of an AM radio. (You guys already helped me with
that!) So the antenna is just a piece of wire... but if I hang an
inductor on the input (where the wire enters the amp) I can get about
10dB more signal.

But having the inductor turn the amp into an oscillator is not so good.
:^)


I’ve been trolling digikey for other opamps without much luck. I think
I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

They can be made, but I don't think that's the real problem here.

Why don't you make a real AM radio front end, with a real tank, and
follow that with a real RF amplifier that works in real applications?

Just because an op-amp is a versatile thing, doesn't mean that it is
_every_ thing.

But having the inductor turn the amp into an oscillator is not so
good. :^)


IÂ’ve been trolling digikey for other opamps without much luck. I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

They can be made, but I don't think that's the real problem here.

Why don't you make a real AM radio front end, with a real tank, and
follow that with a real RF amplifier that works in real applications?

with real transistors

On Thu, 19 Jan 2012 12:54:26 -0800 (PST), George Herold





gher...@teachspin.com> wrote:
On Jan 19, 12:45 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Thu, 19 Jan 2012 08:35:53 -0800, George Herold wrote:
On Jan 18, 12:09 pm, George Herold <gher...@teachspin.com> wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

       |\  opa2134
+--+---|+\
|  |   |  >-+out
L  R  +|-/  |
L  R  ||/   |
L  R  +-----+
|  |
+--+
GND

R = 1 Meg ohm

With a 1 mH inductor the thing oscillates at about 1/2 the self
resonant frequency.

The inductors are 2200R series from
Murata,http://www.murata-ps.com/data/magnetics/kmp_2200r.pdf

But I’ve got some wimpier 1mH’s that show similar behavior.

Here’s a ‘scope shot. frequency about 1.1
MHz.http://imageshack.us/photo/my-images/718/tek0039.png/

The upper trace is with the supply voltage lowered to +/-6 volts.  The
other is +/-15V.

Here’s a 100uH
inductor.http://imageshack.us/photo/my-images/585/tek0040.png/Freq.
about 3.4 MHz.

and a 10uH.  (scale
change)http://imageshack.us/photo/my-images/849/tek0041.png/~10MHz,
which is beyond the GBP of the opamp (OPA2134)

If I reduce R to 100kOhm the oscillations stop, but it’s right on the
edge.

Can anyone give me a hint as to what the heck is going on?

The oscillation frequencies are about 1/2 the SRF’s (I’m not sure if
that’s a clue or a red herring)

Thanks

George H.

Hello Gentlemen (and any Ladies),  Just want a ‘reality’ check..

        +---cf---+
        |        |
        +   |\   |    opa2134
 +---+--+---|+\  |
 |   |  |   |  >-+out
 C   L  R  +|-/  |
 C   L  R  ||/   |
 C   L  R  +-----+
 |   |  |
 +---+--+
     GND

So here’s my current model of the oscillations and feedback.  (thanks
for the help in letting me see it.)
CCC is the capacitance in the coil, opamp and any strays to ground. cf
is some feed back capacitance from output to input.

RRR is a parallel resistance that I add to kill the oscillations.  In a
hand-wavy way the added resistance should about equal the cf impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of the
dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a tad
lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit better
opamp.

If you were to mention what you were actually _doing_ with that coil it
might help us help you.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com-Hide quoted text -

- Show quoted text -

Hi Tim,  Well let me first say that the opamp is the input to a wide
band amplifier stage.  (DC? to 2-3 MHz.... exact HF spec once the pcb
is stuffed.)  It's sort of a general purpose amp.
One of the applications will be the front end of an AM radio.  (You
guys already helped me with that!)  So the antenna is just a piece of
wire... but if I hang an inductor on the input (where the wire enters
the amp) I can get about 10dB more signal.

But having the inductor turn the amp into an oscillator is not so
good.  :^)

I’ve been trolling digikey for other opamps without much luck.  I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

George H.

As others have pointed out, you have pin-to-pin feedback.  You might
try some shielding along with guarding the +Input.

                                        ...Jim Thompson
--
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I love to cook with wine.     Sometimes I even put it in the food.- Hide quoted text -

- Show quoted text -

On Thu, 19 Jan 2012 12:54:26 -0800, George Herold wrote:
RRR is a parallel resistance that I add to kill the oscillations.  In
a hand-wavy way the added resistance should about equal the cf
impedance.

As a test I tightened up the layout and then added a copper shield
between the output and input. (here’s a pic, you can see a corner of
the dip sticking out from the copper tape.)

http://imageshack.us/photo/my-images/408/dscf0018hb.jpg/

So I measure the resistance needed to kill the oscillations with the
shield, without the shield and then with an added 2.2 pF between the
output and input.  Oscillation frequency were all near 1.5 MHz. (a
tad lower with 2.2 pF C)

Test          RRR
shield       45.7k ohm
no shield    38.9k
2.2pF        27k

The numbers don’t quite add up.. but the trend is pretty clear.

So I’ll search for a 1mH inductor with a high SRF, and also a bit
better opamp.

If you were to mention what you were actually _doing_ with that coil it
might help us help you.

--
My liberal friends think I'm a conservative kook. My conservative
friends think I'm a liberal kook. Why am I not happy that they have
found common ground?

Tim Wescott, Communications, Control, Circuits &
Softwarehttp://www.wescottdesign.com-Hide quoted text -

- Show quoted text -

Hi Tim,  Well let me first say that the opamp is the input to a wide
band amplifier stage.  (DC? to 2-3 MHz.... exact HF spec once the pcb is
stuffed.)  It's sort of a general purpose amp. One of the applications
will be the front end of an AM radio.  (You guys already helped me with
that!)  So the antenna is just a piece of wire... but if I hang an
inductor on the input (where the wire enters the amp) I can get about
10dB more signal.

But having the inductor turn the amp into an oscillator is not so good.
:^)

I’ve been trolling digikey for other opamps without much luck.  I think
I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

They can be made, but I don't think that's the real problem here.

Why don't you make a real AM radio front end, with a real tank, and
follow that with a real RF amplifier that works in real applications?

Just because an op-amp is a versatile thing, doesn't mean that it is
_every_ thing.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

On Thu, 19 Jan 2012 15:09:48 -0700, Jim Thompson wrote:
But having the inductor turn the amp into an oscillator is not so
good. :^)

I’ve been trolling digikey for other opamps without much luck.  I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

They can be made, but I don't think that's the real problem here.

Why don't you make a real AM radio front end, with a real tank, and
follow that with a real RF amplifier that works in real applications?

with real transistors ;-)

I suggested a JFET off the top of the coil when I didn't know what he was
doing.

Now I'll suggest that he either tap down on the coil, or intentionally
resonate it with both a fixed capacitor and a variable.  Further, he can
make the fixed capacitor into a nice voltage divider, and drive the base
of a transistor with that.  Yes, the voltage will be lower -- but with
the right circuit, the overall noise will be lower than a circuit
involving the misuse of an op-amp.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.

With a 1 mH inductor the thing oscillates

I’ve been trolling digikey for other opamps without much luck. I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

On Thursday, January 19, 2012 12:54:26 PM UTC-8, George Herold wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
With a 1 mH inductor the thing oscillates
I’ve been trolling digikey for other opamps without much luck.  I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

That's unlikely to get you much happiness... try instead just

          |\  opa2134
 +--+-R2--|+\
 |  |     |  >-+out
 L  R    +|-/  |
 L  R    ||/   |
 L  R    +--R2-+
 |  |    +--C--+
 +--+
 GND

I'd try R2 = 1k ohm, C= 33 pF.

On Thursday, January 19, 2012 12:54:26 PM UTC-8, George Herold wrote:
I’ve got an inductor across a unity gain buffer, with 1 meg ohm in
parallel.
With a 1 mH inductor the thing oscillates
I’ve been trolling digikey for other opamps without much luck.  I
think I’ll have to stick with ‘good enough’.

Any clues to finding inductors with high SRF?

That's unlikely to get you much happiness... try instead just

          |\  opa2134
 +--+-R2--|+\
 |  |     |  >-+out
 L  R    +|-/  |
 L  R    ||/   |
 L  R    +--R2-+
 |  |    +--C--+
 +--+
 GND

I'd try R2 = 1k ohm, C= 33 pF.