bias current

Walter Harley · Sep 25, 2004

Bipolar opamps often have substantial input bias current - for instance, the
LM833 has typical 0.5uA, which is presently causing me a design headache.

As I was thinking about how to solve my problem (short of using a JFET amp,
which introduces other problems), it occurred to me that one solution could
be to put a current sink on each input of the opamp; it wouldn't be exact,
but I could probably get it pretty close.

Which in turn made me wonder: why isn't that current sink built right into
the opamp? Are there any opamps that do it that way, or is there some
reason it doesn't work?

Genome · Sep 25, 2004

"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:R5WdnQB-NKyilcvcRVn-gw@speakeasy.net...
| Bipolar opamps often have substantial input bias current - for instance,
the
| LM833 has typical 0.5uA, which is presently causing me a design headache.
|
| As I was thinking about how to solve my problem (short of using a JFET
amp,
| which introduces other problems), it occurred to me that one solution
could
| be to put a current sink on each input of the opamp; it wouldn't be exact,
| but I could probably get it pretty close.
|
| Which in turn made me wonder: why isn't that current sink built right into
| the opamp? Are there any opamps that do it that way, or is there some
| reason it doesn't work?
|
|

It's done.

AoE? talks somewhere about input bias current cancellation whereby current
mirrors sampling base current of a cascode transistor above the input stage
inject cancellation current into the actual input stage to counteract its
input bias current.

It must work because they do it.... on the other hand it sounds like a
wonderful opportunity for positive feedback.

Can't give any type numbers tho.

DNA

My name is not Guy Macon

Walter Harley · Sep 26, 2004

"Genome" <genome@nothere.net> wrote in message
news:KTn5d.212$bL6.37@newsfe3-gui.ntli.net...

AoE? talks somewhere about input bias current cancellation whereby current
mirrors sampling base current of a cascode transistor above the input
stage
inject cancellation current into the actual input stage to counteract its
input bias current.

It must work because they do it.... on the other hand it sounds like a
wonderful opportunity for positive feedback.

Found an allusion to "bias-cancellation schemes" but I don't see the part
about how it works. I need GoogAoE.

Pooh Bear · Sep 26, 2004

Walter Harley wrote:

Bipolar opamps often have substantial input bias current - for instance, the
LM833 has typical 0.5uA, which is presently causing me a design headache.

As I was thinking about how to solve my problem (short of using a JFET amp,
which introduces other problems), it occurred to me that one solution could
be to put a current sink on each input of the opamp; it wouldn't be exact,
but I could probably get it pretty close.

Which in turn made me wonder: why isn't that current sink built right into
the opamp? Are there any opamps that do it that way, or is there some
reason it doesn't work?

I'm sure there are some examples. It sounds kinda familair. LM101 ? I expect
they date back several decades at least.
Then there were the 'super-beta' input op-amps !

Graham.

John Larkin · Sep 26, 2004

On Sun, 26 Sep 2004 00:16:42 GMT, "Genome" <genome@nothere.net> wrote:

AoE? talks somewhere about input bias current cancellation whereby current
mirrors sampling base current of a cascode transistor above the input stage
inject cancellation current into the actual input stage to counteract its
input bias current.

It must work because they do it.... on the other hand it sounds like a
wonderful opportunity for positive feedback.

Can't give any type numbers tho.

OP-27, OP-37, LT1028 that I know of.

DNA

My name is not Guy Macon

And my name is not Fred.

John

Tim Wescott · Sep 26, 2004

Walter Harley wrote:

Bipolar opamps often have substantial input bias current - for instance, the
LM833 has typical 0.5uA, which is presently causing me a design headache.

As I was thinking about how to solve my problem (short of using a JFET amp,
which introduces other problems), it occurred to me that one solution could
be to put a current sink on each input of the opamp; it wouldn't be exact,
but I could probably get it pretty close.

Which in turn made me wonder: why isn't that current sink built right into
the opamp? Are there any opamps that do it that way, or is there some
reason it doesn't work?

I assume you know of the technique to set the parallel equivalent of all

the resistances on the V+ input to the same on the V- input, so equal
offsets are generated?

Like this:

||
.-----||---.
| || |
R1 | |
Vin ___ | |\ |
------|___|----o---|-\ | Vout
| >---o------------
.----|+/
| |/
|
.-.
| |
| | R2 Make sure to set
'-' R2 = R1
|
|
===
GND

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Spehro Pefhany · Sep 26, 2004

On Sat, 25 Sep 2004 21:33:49 -0700, the renowned "Walter Harley"
<walterh@cafewalterNOSPAM.com> wrote:

Thanks for the leads. I had not realized that there were bipolar opamps
that had the other parameters I need, but also had low Ib. The AD8672 looks
like a remarkable component, although a bit pricey and...

Just out of curiosity, why the insistence on bipolar? What are the
"other problems" that using a JFET or CMOS input op-amp would cause?
Noise?

...available only in
SOIC. No schematic on the datasheet, unfortunately.

Both part of a trend, which is unfortunate (especially the latter).

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Jim Thompson · Sep 26, 2004

On Sun, 26 Sep 2004 03:28:31 +0100, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

Walter Harley wrote:

Bipolar opamps often have substantial input bias current - for instance, the
LM833 has typical 0.5uA, which is presently causing me a design headache.

As I was thinking about how to solve my problem (short of using a JFET amp,
which introduces other problems), it occurred to me that one solution could
be to put a current sink on each input of the opamp; it wouldn't be exact,
but I could probably get it pretty close.

Which in turn made me wonder: why isn't that current sink built right into
the opamp? Are there any opamps that do it that way, or is there some
reason it doesn't work?

I'm sure there are some examples. It sounds kinda familair. LM101 ? I expect
they date back several decades at least.
Then there were the 'super-beta' input op-amps !

Graham.

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

Ken Smith · Sep 26, 2004

In article <mngdl0p6uo20nakb1lp9jt7avu03mmm4rq@4ax.com>,
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

On Sat, 25 Sep 2004 21:33:49 -0700, the renowned "Walter Harley"
walterh@cafewalterNOSPAM.com> wrote:

Thanks for the leads. I had not realized that there were bipolar opamps
that had the other parameters I need, but also had low Ib. The AD8672 looks
like a remarkable component, although a bit pricey and...

Just out of curiosity, why the insistence on bipolar? What are the
"other problems" that using a JFET or CMOS input op-amp would cause?
Noise?

Getting 1nV/sqrt(Hz) out of JFETs requires that they be largish and run at
moderately high currents. Neither is easy to do in an IC op-amp.

JFETs have larger offset voltages than bipolars.

--
--
kensmith@rahul.net forging knowledge

Walter Harley · Sep 26, 2004

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10lcf9lob0ojod3@corp.supernews.com...

I assume you know of the technique to set the parallel equivalent of all
the resistances on the V+ input to the same on the V- input, so equal
offsets are generated?

Yes. Can't do it here, as the particular circuit has a varying impedance on
the V- input (simple audio mixer).

Winfield Hill · Sep 26, 2004

Walter Harley wrote...

Tim Wescott wrote ...
I assume you know of the technique to set the parallel equivalent
of all the resistances on the V+ input to the same on the V- input,
so equal offsets are generated?

Yes. Can't do it here, as the particular circuit has a varying
impedance on the V- input (simple audio mixer).

You want a high-quality low-noise audio opamp. I'm puzzled why
JFETs are out of consideration. For example the opa627 features
0.00003% distortion. It's 4.5nV noise level is about the same as
a 1.2k resistor, and is less than the noise from a 5k or 10k pot,
such as you'd use in a mixer, why is 1nV necessary?

The opa627's 0.25mV max offset voltage (for the cheaper A version)
is also very low, and shouldn't be an issue.

--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

Winfield Hill · Sep 27, 2004

Winfield Hill wrote...

... It's 4.5nV noise level ... Correction: Its ...

--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

Walter Harley · Sep 27, 2004

"Winfield Hill" <Winfield_member@newsguy.com> wrote in message
news:cj7gdp02pl5@drn.newsguy.com...

You want a high-quality low-noise audio opamp. I'm puzzled why
JFETs are out of consideration. For example the opa627 features
0.00003% distortion. It's 4.5nV noise level is about the same as
a 1.2k resistor, and is less than the noise from a 5k or 10k pot,
such as you'd use in a mixer, why is 1nV necessary?

See response to Spehro for particulars of my application... The OPA627
won't do in my case, because it's got unacceptably high supply consumption
(I think - TI datasheet summary says 7.5mA per channel, datasheet itself is
ambiguous, I haven't tried a sample). More importantly, it costs $16 a pop
even in 100 quantities - this goes into a unit with a retail price of $139,
so that won't quite work

There are a lot of opamps, more coming all the time. I'm sure there are
some that I'm missing just because I've not re-run the parametric searches
for all the manufacturers each time I've revised my expectations for the
chip! I'm always glad to learn about ones that are new to me.

John Woodgate · Sep 27, 2004

I read in sci.electronics.design that martin griffith
<martingriffith@yahoo.co.uk> wrote (in <r31gl05j23dqm9rf2gphh49lvsacm4sc
os@4ax.com&gt

about 'bias current', on Mon, 27 Sep 2004:

how about MC33078 from Onsemi, not quite as good, but ISTR 4ma/opamp
about half the consumption of a NE5532,

ST Micro LS204 is quite useful and has low consumption. There is also a
quad version.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Winfield Hill · Sep 26, 2004

Genome wrote...

Walter Harley wrote ...
| Bipolar opamps often have substantial input bias current - for instance,
| the LM833 has typical 0.5uA, which is presently causing me a design
| headache.
|
| As I was thinking about how to solve my problem (short of using a JFET
| amp, which introduces other problems), it occurred to me that one
| solution could be to put a current sink on each input of the opamp; it
| wouldn't be exact, but I could probably get it pretty close.
|
| Which in turn made me wonder: why isn't that current sink built right
| into the opamp? Are there any opamps that do it that way, or is there
| some reason it doesn't work?

It's done.

Indeed, it's a pretty popular technique.

AoE? talks somewhere about input bias current cancellation whereby current
mirrors sampling base current of a cascode transistor above the input stage
inject cancellation current into the actual input stage to counteract its
input bias current.

Pages 110, 111.

It must work because they do it.... on the other hand it sounds like a
wonderful opportunity for positive feedback.

Typically the datasheet lists the input bias current as +/- 1nA, etc.,
showing a possible over-correction. If an input is left floating, it'll
go to either the negative or positive rail, which could be considered
positive feedback, but for realistic source impedances it's not an issue.

Can't give any type numbers tho.

We mention the OP-07 and LT1012. The rest of the '07 contenders use
it, like the OP27, OP37 and TLE2027, plus others like NE5232, LT1001,
LT1024, LT6011, and LT6012, etc. There are likely many more opamps,
whose datasheets don't include schematics for us to analyze.

--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

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