Operational Amplifier Design

[ameya]

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?
Thank You.
Ameya

 

[Dbowey]

ameya posted:
<< I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?
I suggested you explain to the person who gave you this task, that you lack the
knowledge to attempt it.

Don

 

[Ken Smith]

In article <5503c066.0411061758.7e490e0d@posting.google.com>,
ameya <ameya_purohit@yahoo.com> wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ?

First you have to decide on an amplifying technology. Op-amps can be made
with bipolar transistors, JFETS, MOSFETs, tubes, magnetic-amplifiers and
amplidynes or any combination of these.

Usually you base the selection on what devices you can obtain that have a
hope of meeting the specifications you are designing to.

Which
topology should I use ?

If you download the datasheet for an LT1028, you will see a good starting
point. I'd suggest you insert 2 extra gain stages and make it work rail
to rail. The LT1028 is a fairly old design so it doesn't swing rail to
rail.

How do I go about with all the design
constraints ?

It is usually best to take them in the following order:

(1) Crossover distortion.
(2) Maximum side fumbling.
(3) Underpass distortion.
(4) Output cross conduction.
(5) Supply voltage.
(6) Supply current.
(7) Transient image distortion
(8) Spectral purity.

Which constraints need more attention while designing ?
Thank You.

The one that says "you are supposed to design an op-amp".

--
--
kensmith@rahul.net forging knowledge

 

[John Popelish]

ameya wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?
Thank You.
Ameya

You study how others have done it.

Start here:
http://www.national.com/an/AN/AN-A.pdf

--
John Popelish

 

[Tim Wescott]

ameya wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?
Thank You.
Ameya

Really, this should be taught during the course of the course. I've
never actually taken that one, but I had friends who did when I was an
undergrad, and I TA'd the lab for it when I was getting my Master's
(best course I never took -- I had to know all the important stuff for
the lab, so I walked away with lots of good info for free).

At any rate, it was the same in two universities, 2500 miles and several
years apart. As the course unfolded the relevant bits and pieces of the
circuit were analyzed in class and built for the lab. I can't imagine
that a halfway decent instructor won't make sure that you have the
information you need when you need it.

--

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

 

[Kevin Aylward]

Ken Smith wrote:

In article <5503c066.0411061758.7e490e0d@posting.google.com>,
ameya <ameya_purohit@yahoo.com> wrote:
I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ?

First you have to decide on an amplifying technology. Op-amps can be
made with bipolar transistors, JFETS, MOSFETs, tubes,
magnetic-amplifiers and amplidynes or any combination of these.

Usually you base the selection on what devices you can obtain that
have a hope of meeting the specifications you are designing to.

Which
topology should I use ?

I would suggest a bog standard one. pnp diff input feeding a current
mirror load, feeding an npn stage with a current mirror load. e.g.
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm, the circuit
somewhere near the bottom of the page.

However, my preference is for the second stage also to be a diff pair
with a current mirror load.

If you download the datasheet for an LT1028, you will see a good
starting point.

Oh dear. I would suggest a simplar one.

I'd suggest you insert 2 extra gain stages

Oh dear.. Again not a good start. A *total* of 2 gain stages are a
better starting point for a novice. Its can be very difficult to get 3
stage and more to be stable.

and make
it work rail to rail.
The LT1028 is a fairly old design so it
doesn't swing rail to rail.

That is not why it doesn't swing rail to rail. Your pretty new at this
amplifier design aren't you

Rail to rail op-amp have a number of compromises that are better avoided
unless you actually require rail to rail output. For example, it is
usually much better to have push pull emitter followers as the output
devices. Why do you think this is?

How do I go about with all the design
constraints ?

It is usually best to take them in the following order:

Here we go again...

(1) Crossover distortion.
(2) Maximum side fumbling.

?

I see your fumbling here.

(3) Underpass distortion.

?

(4) Output cross conduction.
(5) Supply voltage.
(6) Supply current.
(7) Transient image distortion

Oh dear...why do I bother...

(8) Spectral purity.

Overall to the above, No.No.No.

First things first. Its always a trade off between accuracy, BW and
power.

What is the op-amp to do?

Input and output volts and current required?

For a novice, a good starting point is say 50ma out with +/- 15 volt
rails.

The fundamental real design issue is *stability*. The dc design is
usually straightforward. However, one often spends 95% of the time on
getting the bastard stable.

What unity gain frequency is required?

Better accuracy requires more feedback, hence high ugf. etc...

Which constraints need more attention while designing ?
Thank You.

The one that says "you are supposed to design an op-amp".

The one than says don't give advice unless you actually know what you
are talking about

And lastly, the best introductory tutorial I know of is:

http://www.national.com/an/AN/AN-A.pdf

My advice is anyone contemplating designing amplifiers understand this
paper in complete detail. Its a true goldmine.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

John Popelish wrote:

ameya wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?
Thank You.
Ameya

You study how others have done it.

Start here:
http://www.national.com/an/AN/AN-A.pdf

Dash...I made my post before I read this one. Its nice to know that
there are a few who are on the ball.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Paul Burridge]

On 6 Nov 2004 17:58:51 -0800, ameya_purohit@yahoo.com (ameya) wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?
Thank You.
Ameya
Oh hell, just copy a 714; no one will notice. ;-)

--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Winfield Hill]

Paul Burridge wrote...

ameya wrote:

I have to design an operational amplifier for a analog course.
Could some one please tell me how do I start with the design?
Which topology should I use?

Oh hell, just copy a 714; no one will notice. ;-)

If you meant the 741, with its unusual and distinctive input stage,
it certainly would be noticed. If you meant the uA714 precision
opamp, that's an awkard and overly-complicated starting design.


--
Thanks,
- Win

 

[Ken Smith]

In article <dOkjd.2496$hp4.531@fe1.news.blueyonder.co.uk>,
Kevin Aylward <salesEXTRACT@anasoft.co.uk> wrote:

Ken Smith wrote:
[..op-amp design ..]
made with bipolar transistors, JFETS, MOSFETs, tubes,
magnetic-amplifiers and amplidynes or any combination of these.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[....]

If you download the datasheet for an LT1028, you will see a good
starting point.

Oh dear. I would suggest a simplar one.

Oh dear, K.A. thinks the posting was serious even with the suggestions I
made and the fact that I was answering an obvious homework question and
the more suggestions below.

[... snippage ...]

(2) Maximum side fumbling.


?

Go search on "reto-encabulator" or "turbo-encabulator". There was a
thread mere month ago where we all giggled about that one.

--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <cmlev40178e@drn.newsguy.com>,
Winfield Hill <whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Paul Burridge wrote...

ameya wrote:

I have to design an operational amplifier for a analog course.
Could some one please tell me how do I start with the design?
Which topology should I use?

Oh hell, just copy a 714; no one will notice. ;-)

If you meant the 741, with its unusual and distinctive input stage,
it certainly would be noticed. If you meant the uA714 precision
opamp, that's an awkard and overly-complicated starting design.

The LM324 would be a much better example to start with. IIRC, it has a
darlington in the input that could be left out.

--
--
kensmith@rahul.net forging knowledge

 

[PaulCsouls]

On Sun, 7 Nov 2004 13:00:24 -0000, "Leon Heller"
<leon_heller@hotmail.com> wrote:

It's quite a popular project on degree courses; a former colleague of mine
had to do something similar. IIRC, his SPICE simulation worked OK, but the
actual circuit built from BJTs etc. didn't.

Leon

Op Amps require matched components. You'll have a lot trouble trying
to build it with discretes.

PaulC

 

[Pooh Bear]

Leon Heller wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:418DFB0C.4EF8E117@hotmail.com...

ameya wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ? Which
topology should I use ? How do I go about with all the design
constraints ? Which constraints need more attention while designing ?

Frankly, it sounds like a thankless task.

No less than thousands of designs of op-amps have been manufactured over
the years by many IC manufacturers in various attempts to optimise their
designs according to the semiconductor processes available to them at the
time and also to meet differing market requirements / sectors ( e.g low
power / low voltage / high voltage / load impedance / low distortion / DC
accuracy / HF bandwidth / low-high bias current - you name it ! ).


You can choose whatever constraints you choose. Since you'll be designing
a discrete op-amp no doubt - at least you won't have certain
semiconductor *processes* and their constraints to worry about.


Sounds like a poorly conceived project to me. Is the lecturer a bit
clueless ? Seen that before.

It's quite a popular project on degree courses; a former colleague of mine
had to do something similar. IIRC, his SPICE simulation worked OK, but the
actual circuit built from BJTs etc. didn't.

There's a lesson there. Learn how to design practical circuits that work before
using simulation. Too many graduates know loads of theory and precious little
practice about making things ( that work ).


Graham

 

[Tim Wescott]

PaulCsouls wrote:

On Sun, 7 Nov 2004 13:00:24 -0000, "Leon Heller"
leon_heller@hotmail.com> wrote:


It's quite a popular project on degree courses; a former colleague of mine
had to do something similar. IIRC, his SPICE simulation worked OK, but the
actual circuit built from BJTs etc. didn't.

Leon



Op Amps require matched components. You'll have a lot trouble trying
to build it with discretes.

PaulC

You get matched transistors -- there used to be some RCA chips
available; if they aren't still out there Zetex has some matched pairs
they sell for current mirrors.

--

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

 

[Winfield Hill]

Ken Smith wrote...

The LM324 would be a much better example to start with.
IIRC, it has a darlington in the input that could be left out.

If you leave that out it won't operate to the negative rail,
which is its most important feature. The LM324 output stage
is severely in need of improvement, but it'll still need to
go down to 0V. ...


--
Thanks,
- Win

 

[Ken Smith]

In article <cotjd.138122$b5.6863837@news3.tin.it>, Ban <bansuri@web.de> wrote:

Ken Smith wrote:
[...]
Ken, this information is available all over the net, and in every book. I
can see, you never visited a course like this.

In that case you really-really need glasses.

Go back and read my post. I was explaining why it is bad to simple answer
the poster's question with a complete solution. There is no case in which
it is a good thing to do in the situation as it is.




--
--
kensmith@rahul.net forging knowledge

 

[Ken Smith]

In article <cmlp9n0248p@drn.newsguy.com>,
Winfield Hill <whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Ken Smith wrote...

The LM324 would be a much better example to start with.
IIRC, it has a darlington in the input that could be left out.

If you leave that out it won't operate to the negative rail,
which is its most important feature. The LM324 output stage
is severely in need of improvement, but it'll still need to
go down to 0V. ...

If the OP is just trying to get a starting point, operation down to the
negative rail can be set aside. Yes the output stage sucks but it is
simple.


--
--
kensmith@rahul.net forging knowledge

 

[Rich Grise]

On Sat, 06 Nov 2004 17:58:51 -0800, ameya wrote:

I have to design an operational amplifier for a analog course. Could
some one please tell me how do I start with the design ?

The teacher, maybe?

Which
topology should I use ?

How the hell are we supposed to know? We weren't sitting in on
the lectures you skipped any more than you apparently were. And
we don't have your textbook.

How do I go about with all the design
constraints ?

????

Which constraints need more attention while designing ?

The ones that the teacher is going to grade your score on.

Thank You.

You're welcome.

Ameya

Rich

 

[Winfield Hill]

Ken Smith wrote...

Winfield Hill
The LM324 output stage is severely in need of improvement...

If the OP is just trying to get a starting point, operation down
to the negative rail can be set aside. Yes the output stage sucks
but it is simple.

Simple, but pedagogically best as an example of how not to do it.


--
Thanks,
- Win

 

[Daniel Haude]

On Sun, 07 Nov 2004 16:58:31 -0700,
Jim Thompson <thegreatone@example.com> wrote
in Msg. <3gdto0pp6b452if8gp06t00nk5rs4muecr@4ax.com>

See "MC1530-TeachingExercise.pdf" on the SED/Schematics page of my
website.

Nice! What's a quadrille pad? What's Lubyrith?

--Daniel