op-amp suggestions - 20MHz, 30mA, hi-z output

Malcolm Reeves · Jan 19, 2005

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better. I
also need to be able to turn the output high impedance.

Is there such an op-amp? And is it cheap?

TIA

--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
(mreeves@fullcircuit.com, mreeves@fullcircuit.co.uk or mreeves@iee.org).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - www.CharteredConsultant.co.uk - The Consultant A-List

John Larkin · Jan 19, 2005

On Wed, 19 Jan 2005 09:36:21 +0000, Malcolm Reeves
<mreeves@fullcircuit.com> wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.

TI has lots of +-15 rail opamps that are *very* fast. THS3061 and
such. Fairly cheap.

also need to be able to turn the output high impedance.

*That* I don't understand.

John

Jim Thompson · Jan 19, 2005

On Wed, 19 Jan 2005 08:13:28 -0800, John Larkin
<jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

On Wed, 19 Jan 2005 09:36:21 +0000, Malcolm Reeves
mreeves@fullcircuit.com> wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.

TI has lots of +-15 rail opamps that are *very* fast. THS3061 and
such. Fairly cheap.

also need to be able to turn the output high impedance.

*That* I don't understand.

John

480)460-2350 | |
| E-mail Address at Website Fax

John Larkin · Jan 19, 2005

On Wed, 19 Jan 2005 10:05:58 -0700, Jim Thompson
<thegreatone@example.com> wrote:

On Wed, 19 Jan 2005 08:13:28 -0800, John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

On Wed, 19 Jan 2005 09:36:21 +0000, Malcolm Reeves
mreeves@fullcircuit.com> wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.

TI has lots of +-15 rail opamps that are *very* fast. THS3061 and
such. Fairly cheap.

also need to be able to turn the output high impedance.

*That* I don't understand.

John

He needs to "tri-state" his OpAmp.

...Jim Thompson

Think that's what he meant? Downstream analog switch, then.

John

Jim Thompson · Jan 19, 2005

On Wed, 19 Jan 2005 10:44:24 -0800, John Larkin
<jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

On Wed, 19 Jan 2005 10:05:58 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Wed, 19 Jan 2005 08:13:28 -0800, John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

On Wed, 19 Jan 2005 09:36:21 +0000, Malcolm Reeves
mreeves@fullcircuit.com> wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.

TI has lots of +-15 rail opamps that are *very* fast. THS3061 and
such. Fairly cheap.

also need to be able to turn the output high impedance.

*That* I don't understand.

John

He needs to "tri-state" his OpAmp.

...Jim Thompson

Think that's what he meant? Downstream analog switch, then.

John

480)460-2350 | |
| E-mail Address at Website Fax

Jim Thompson · Jan 19, 2005

On Wed, 19 Jan 2005 15:57:06 -0500, John Popelish <jpopelish@rica.net>
wrote:

Jim Thompson wrote:

On Wed, 19 Jan 2005 10:44:24 -0800, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

On Wed, 19 Jan 2005 10:05:58 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Wed, 19 Jan 2005 08:13:28 -0800, John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:

On Wed, 19 Jan 2005 09:36:21 +0000, Malcolm Reeves
mreeves@fullcircuit.com> wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.

TI has lots of +-15 rail opamps that are *very* fast. THS3061 and
such. Fairly cheap.

also need to be able to turn the output high impedance.

*That* I don't understand.

John

He needs to "tri-state" his OpAmp.

Think that's what he meant? Downstream analog switch, then.

Malcolm said "30mA". That's a bit rough with an analog switch.

More like a fet based relay.

480)460-2350 | |
| E-mail Address at Website Fax

Jim Thompson · Jan 19, 2005

On Wed, 19 Jan 2005 16:42:05 -0500, John Popelish <jpopelish@rica.net>
wrote:

Jim Thompson wrote:

On Wed, 19 Jan 2005 15:57:06 -0500, John Popelish <jpopelish@rica.net
wrote:

More like a fet based relay.

Hey! Hey! Hey! Hey! A technical discussion. Will miracles never
cease!

As I said before, I'd use booster discretes at the output, making it a
piece-a-cake to tri-state.
Sounds good. The devil would be in the details of opening the
feedback loop and the recovery on closing it, again. Especially if
this had to happen quickly or often.

480)460-2350 | |
| E-mail Address at Website Fax

Ken Smith · Jan 20, 2005

In article <n4asu0hifv9ncojgpf6quvhslbm9k4g2ip@4ax.com>,
Malcolm Reeves <mreeves@fullcircuit.com> wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better. I
also need to be able to turn the output high impedance.

Is there such an op-amp? And is it cheap?

30mA into what impedance? This matters a lot because at smaller swings,
the problem gets easier.

How big is the signal when it is tri-stated? Simply putting a power MOSFET
in the signal path may be enough.

--
--
kensmith@rahul.net forging knowledge

Ken Smith · Jan 20, 2005

In article <cqitu096s9icfgruj3tb5cfauervs1tqi4@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[...]

As I said before, I'd use booster discretes at the output, making it a
piece-a-cake to tri-state.

The requirement wasn't clear but I wonder if the LT1206 wouldn't do the
job. It isn't exactly low cost, but it may run cheaper than a
multicomponent circuit.

I believe that the LT1206's output is high impedance when you disable it.

--
--
kensmith@rahul.net forging knowledge

Ken Smith · Jan 20, 2005

In article <41EED42D.6C6F1B@rica.net>,
John Popelish <jpopelish@rica.net> wrote:
[...]

Sounds good. The devil would be in the details of opening the
feedback loop and the recovery on closing it, again. Especially if
this had to happen quickly or often.

You could use two pairs of MOSFETs as switches. One MOSFET would isolate
the output from the load and the other would connect a dummy load. This
would keep the loop in bounds except for the switching charges. The
switching would partially cancel.

--
--
kensmith@rahul.net forging knowledge

Winfield Hill · Jan 20, 2005

John Larkin wrote...

It's surprising how many people post a question and then
vanish from the thread.

They are taking their answer off the air.

--
Thanks,
- Win

Jim Thompson · Jan 20, 2005

On Thu, 20 Jan 2005 01:20:54 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

In article <n4asu0hifv9ncojgpf6quvhslbm9k4g2ip@4ax.com>,
Malcolm Reeves <mreeves@fullcircuit.com> wrote:
Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better. I
also need to be able to turn the output high impedance.

Is there such an op-amp? And is it cheap?

30mA into what impedance? This matters a lot because at smaller swings,
the problem gets easier.

How big is the signal when it is tri-stated? Simply putting a power MOSFET
in the signal path may be enough.

--
Me? I fret over charge injection.

480)460-2350 | |
| E-mail Address at Website Fax

John Larkin · Jan 20, 2005

On 19 Jan 2005 19:44:30 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

John Larkin wrote...

It's surprising how many people post a question and then
vanish from the thread.

They are taking their answer off the air.

But I wish they'd check in now and then to clarify just what the hell
they really need.

John

Jeroen Belleman · Jan 20, 2005

John Larkin wrote:

John Larkin wrote...

It's surprising how many people post a question and then
vanish from the thread.
[...]

But I wish they'd check in now and then to clarify just what the hell
they really need.

John

The world is round remember? This fellow is in the UK, for god's
sake! While you are bitching about his lack of response, he's
probably fast asleep.

Jeroen Belleman

Winfield Hill · Jan 20, 2005

Malcolm Reeves wrote...

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.
I also need to be able to turn the output high impedance.

Hmm, various folks are wagging their tongues, failing to address
your question. Several opamps meet your needs, such as these:

part f_T Vsupply Iout slew tristate
number manuf MHz max max V/us output? comments
------ ----- --- ----- ---- ---- ------- ------------
CLC431 NSC 120 33V 60mA 2000 yes G = 2 min
CLC411 NSC 200 36V 70mA 2300 yes +/-6V Vout max

Check the rest of the Comlinear (now NSC) product line. There are
lots of muscular opamps, like the Analog Devices AD815. If you can
run the opamp at lower voltages (e.g. add three-terminal regulators)
there are lots of good choices.

--
Thanks,
- Win

Fred Bloggs · Jan 20, 2005

Malcolm Reeves wrote:

Hi,

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better. I
also need to be able to turn the output high impedance.

Is there such an op-amp? And is it cheap?

You should be asking yourself, and answering, this question before you
go off into a drug induced dreamland over the composite circuit
configuration, bit head.

Ken Smith · Jan 20, 2005

In article <fhauu0ttfr3srj2bi9kvlo61joe3ffhmkh@4ax.com>,
Jim Thompson <thegreatone@example.com> wrote:
[...]

Me? I fret over charge injection.

No, not you. The questions were aimed at the OP. I was partly pointing
out that you were having to assume stuff that may not be true.

As for charge injection: it depends on how long the circuit has to
recover.

--
--
kensmith@rahul.net forging knowledge

Stephan Goldstein · Jan 21, 2005

In article <csocbg01rl3@drn.newsguy.com>, Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Malcolm Reeves wrote...

I need an op-amp to drive 30mA, on 24V supply, signal is quite high
frequency, 120kHz so I reckon I want at least 40dB to give low
distortion, i.e. at least GBW 20MHz but 200MHz would be better.
I also need to be able to turn the output high impedance.

Hmm, various folks are wagging their tongues, failing to address
your question. Several opamps meet your needs, such as these:

part f_T Vsupply Iout slew tristate
number manuf MHz max max V/us output? comments
------ ----- --- ----- ---- ---- ------- ------------
CLC431 NSC 120 33V 60mA 2000 yes G = 2 min
CLC411 NSC 200 36V 70mA 2300 yes +/-6V Vout max

Check the rest of the Comlinear (now NSC) product line. There are
lots of muscular opamps, like the Analog Devices AD815. If you can
run the opamp at lower voltages (e.g. add three-terminal regulators)
there are lots of good choices.

Sorry Win, not AD815. I used that one a few years ago in a calibrator
and it doesn't have tri-state unless my age-enfeebled mind is worse than
I thought!

steve

Winfield Hill · Jan 21, 2005

Stephan Goldstein wrote...

Sorry Win, not AD815. I used that one a few years ago in a calibrator
and it doesn't have tri-state unless my age-enfeebled mind is worse
than I thought!

That's correct, it's merely a muscular opamp, like I said.

--
Thanks,
- Win

Pig Bladder · Jan 21, 2005

On Wed, 19 Jan 2005 14:05:51 -0700, Jim Thompson wrote:

.... Will miracles never

cease!

Not as long as _I_ have anything to say about it! ;-D ;-D ;-D
--
The Pig Bladder From Uranus, Still Waiting for
Some Hot Babe to Ask What My Favorite Planet Is.

op-amp suggestions - 20MHz, 30mA, hi-z output

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