Operational Amplifier Design

Kevin Aylward · Nov 10, 2004

Ken Smith wrote:

In article <t29kd.10112$tV4.7591@fe2.news.blueyonder.co.uk>,
Kevin Aylward <salesEXTRACT@anasoft.co.uk> wrote:
Ken Smith wrote:
In article <cmp15a02tuc@drn.newsguy.com>,
Winfield Hill <whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
Jim Thompson wrote...

But I don't know of a single commercial amplifier that isn't set
up that way.

Doesn't adding a modest series resistor (along with an ESD network
after the resistor) give one a chance to both stabilize the loop,
and avoid ESD failure?

Yes, adding a resistor in series with the compensation cap can add
a zero to the transfer function. If this zero lines up with the
pole from, lets say, the input pair, then stability can be improved.

Ho humm.. never read my posts Ken?

Maybe I should stop reading them

Why?

I might be an arrogant know it all, but out of 10,000 posts, you would
be hard pressed to find more than a couple of technically
er...misleading posts, of mine on technical maters.

but for now I'll continue to read
some of them :

Actually try (i.e Spice it) this. The results are not promising at
all. If it actually worked in practise (op-amp design), you would
see it used.

The folks at Linear seem to disagree with you in that they include the
resistor in series with the compensation cap in several of their
designes.

Only several?

How about counting those that don't?

Look, heres the deal.

Consider a diff pair driving the high gain Miller stage. If the
Miller stage is well and trully clobering everthing else, all is
well. If it aint, like because one has included a resister, darkness
awaits. This is what happens.

1) The diff pair no longer has the same low value of impedance on its
collecter, becuse the input to the millor stage is no longer low
impedance. This means that there is HF gain at the first stage,
thefore creating a higher effective capacitance at the op-amp input.
This causus a roll off in the feedback loop.

Take a look at the LT1001 and you will see one of the simpler ways
this sort of thing is avoided. Linear used a cascode stage in that
design.

Yes, of course a cascode helps the input roll off, but it is not
completely effective. This cascode itself has Miller gain from its
emitter to collector. What saves one is that its the low impedance re
that drives this capacitor.

I think even the LM101, (IIRC) used a funny sort of cascode that
allowed the common mode range to include the plus rail and some sort
of a zero in the compensation section.

Several of the fast op-amps from Linear contain the resistor you
claim to dislike so much.

So what? So a in a few cases the numbers work out such that it *may* be
effective in some special cases. However, I would have to confirm the
sums myself. In many, many cases circuits have special topologies that
don't actually achieve anything with that topology.

As I said, go and do some typically spice simulations and it will be
very clear that this resister can only be used in special circumstances.

2) What was a low impedance at HF driving the load capacitance of the
Miller stage, is now a much higher impedance. This means that this
capacitive load will roll of the gain with a pole much closer than
what it was. i.e. it will bring what was a good pole split, i.e.
poles widely separated, much closer together.

This is why the Millar stage is so universally used.

Miller effect increases the effectively size of the capacitor. I
believe ,

Ahmmmm...

that without it the capacitor would have to be physically
larger that the chip makers would like it to be.

Oh dear...with all due respect, your phrasing here indicates that you
only have a basic understanding of all of this. *Of course* the Miller
effect is a capacitor multiplier, this is 101 electronics, but that is
not relevant to the idea of why Miller compensation is used. Even if one
had an enormous capacitor easily available one would still not simply
plonk it down at the input of the stage. The performance would be
dreadfull. So no, that is not why it is used. Unfortunately, you have
been reading too many layman descriptions.

The gain of the Miller stage is gm.Xc. This is what is important. No
mention of its Xc/Av is required for this fundamental feature.

However, it can't be greater than gm.ZL. If gm.ZL is limiting the gain
at HF, then an added resister in Ccomp might be trying to command a gain
that is not actually available open loop at that stage. The gain stage
is not an ideal amplifier. It is a current source with only a finite
gain. This is one reason why adding in the resister doesn't work in
general. The gain is simple not available.

This explains why in the op-amps that contain the cascode stage or
other interstages
the compensation is still done with a Miller effect

No it doesn't. Not in the slightest. You simply don't understand what's
going on here. This is why:

POLE SPLIT LESSON 101:

The Miller capacitor is a *pole-splitter*. This is its *fundamental*
use. An amplifier usually has many poles. To ensure *stability* an
amplifier is usually designed such that there is only one main pole
dominating the loop gain. Without the Miller capactor, the main gain
stage has *two* fundermental poles, one at its input (Cin), on at its
output (CL). If the output of the gain stage has a low impedance, then
the effect of Cload will be minimised. For example:

Consider the Miller stage has a value of Ccomp. This negative feedback
results in an output impedance driving Cload. This results in the Miller
roll off and the Cload roll off. Now consider making Cmiller much
larger. This moves the Miller pole down in frequency, however, this also
results in more feedback reducing the output impedance. This means that
CL is going to have less effect on the output rolloff, i.e. it moves the
output pole further out.

FUNDAMENTAL POINT:

Therefore, the Miller stage converts what is two interacting poles into
two widely separated poles, where, ideally, the higher pole is well
above the unity gain frequency.

Again, the idea that the Miller cap makes for "a big capacitor" is just
layman's fluff.

It turns 3
potential poles into one dominant one.

Yes
This extra zero idea only works
in practice if one is using a gain stage that is much nearer the
ideal,

Or where the low impedance to the input section is otherwise assured.

No.

The diff pair input rolloff is not the main issue, it is just one of the
side issues. The main issue is that the main gain stage has *two*
inherent poles. One at is input (Cin), one at its output (CL). Using
Miller compensation converts them effectively, into one. End of story.

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.

Winfield Hill · Nov 10, 2004

Jim Thompson wrote...

Winfield Hill wrote:
Jim Thompson wrote...
Winfield Hill wrote:
Jim Thompson wrote...

Are you aware of the infamous "let's copy a 741 episode", where
the Fairchild published schematic was copied into silicon by
(IIRC) Raytheon, except that 'tain't the circuit ;-)

Do you know what the correct circuit is?

Yes ;-) At least I _think_ I remember it still.

OK, we're all eyes and ears!

I'll look thru my archives. I think that's one of the problem areas
for which I wrote an ICE report.

Hi Jim, we sure hope you find that old report, sounds like a fascinating
bit of detail we'd like to learn.

--
Thanks,
- Win

Jim Thompson · Nov 10, 2004

On 10 Nov 2004 02:32:23 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Jim Thompson wrote...

[snip]

I'll look thru my archives. I think that's one of the problem areas
for which I wrote an ICE report.

Hi Jim, we sure hope you find that old report, sounds like a fascinating
bit of detail we'd like to learn.

I'm still scratching my ... trying to remember what it was, might have
been something as obscure as positive feedback thru poor metal
routing, or was that the uA709, scratch, scratch ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

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

Winfield Hill · Nov 11, 2004

Jim Thompson wrote...

I couldn't locate the uA124QB. Could you send a copy? Thanks!

Jim, did I send it this morning - Sheesh, I don't remember!

--
Thanks,
- Win

Jim Thompson · Nov 11, 2004

On 10 Nov 2004 18:13:06 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Jim Thompson wrote...

I couldn't locate the uA124QB. Could you send a copy? Thanks!

Jim, did I send it this morning - Sheesh, I don't remember!

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

Terry Given · Nov 11, 2004

Ken Smith wrote:

In article <20kkd.14393$tV4.2310@fe2.news.blueyonder.co.uk>,
Kevin Aylward <salesEXTRACT@anasoft.co.uk> wrote:

[.. about ponk..]

Why?
I might be an arrogant know it all, but out of 10,000 posts, you would
be hard pressed to find more than a couple of technically
er...misleading posts, of mine on technical maters.

Consider this very thread. You said that placing a resistor in series
with the compensation capacitor of an op-amp was "bad design". You didn't
qualify it with "usually" or "almost always".

Do I get to call that 0.75 mistakes right here?

Take a look at the LT1001 and you will see one of the simpler ways
this sort of thing is avoided. Linear used a cascode stage in that
design.

Yes, of course a cascode helps the input roll off, but it is not
completely effective. This cascode itself has Miller gain from its
emitter to collector. What saves one is that its the low impedance re
that drives this capacitor.

Please explain the term "Miller gain[1]" as you use it here. The
transistor has a capacitance from collector to emitter, but this does not
go via a gain to the emitter. There is also the collector-base
capacitance which does unless the base sees a very low impedance to
ground. For the collector emitter one, the low "re" does matter, for the
other it drops out of the issue.

[1] I usually take this to mean the increase in the effective capacitance
due to the far end of it being connected to an inverted signal.

Several of the fast op-amps from Linear contain the resistor you
claim to dislike so much.

So what? So a in a few cases the numbers work out such that it *may* be
effective in some special cases.

So, you were wrong, wrong, ever so wrong that's what. :> You overlooked
something, it happens.

As I said, go and do some typically spice simulations and it will be
very clear that this resister can only be used in special circumstances.

But, I don't need to do "some typically spice" simulations. I just need a
few weird examples to show that it works for those cases.

that without it the capacitor would have to be physically
larger that the chip makers would like it to be.

Oh dear...with all due respect, your phrasing here indicates that you
only have a basic understanding of all of this.

... and then there are lines like this. No way to win friends there
buddy. My phrasing is a bit odd in many situations for reasons I do not
wich to go into here.

*Of course* the Miller
effect is a capacitor multiplier, this is 101 electronics, but that is
not relevant to the idea of why Miller compensation is used. Even if one
had an enormous capacitor easily available one would still not simply
plonk it down at the input of the stage. The performance would be
dreadfull.

Yes that would be dreadful and I don't think anyone has suggested that.

There was more but I just deleted it. Your tone makes it not worth it,
but go back and check it and you will find you made an "oversimplifcation"
(error).

Really the "error" (and this is IMO the most common cause of flame wars
here) is making a definitive statement, rather than a suitably hedged
statement. We are (mostly) smart people, and all picky bastards too, so
the minute someone either forgets to qualify something, or makes an
over-bold assertion [eg "that never works in opamps" (not what KA said,
Im just too lazy to go back & look, so I'll paraphrase) vs "Usually that
doesnt help"] some picky bugger dashes off the minimum of 1 example that
technically disproves the "never" assertion.

mind you, if you try and hedge the things you say (to take into account
all these exceptions) then the verbiage quickly becomes unreadable.

Cheers
Terry

Cheers
Terry

Terry Given · Nov 11, 2004

Jim Thompson wrote:

On 10 Nov 2004 18:13:06 -0800, Winfield Hill
hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Jim Thompson wrote...

I couldn't locate the uA124QB. Could you send a copy? Thanks!

Jim, did I send it this morning - Sheesh, I don't remember!

Win, What do you sip on there in Massachusetts? We E-mailed back and
forth in regard to some typos ;-)

...Jim Thompson

Hash-a-chussetts?

ya better watch out if thats the case, 'cos Rich will be on his way

Cheers
Terry

Kevin Aylward · Nov 11, 2004

Ken Smith wrote:

In article <20kkd.14393$tV4.2310@fe2.news.blueyonder.co.uk>,
Kevin Aylward <salesEXTRACT@anasoft.co.uk> wrote:

[.. about ponk..]
Why?
I might be an arrogant know it all, but out of 10,000 posts, you
would be hard pressed to find more than a couple of technically
er...misleading posts, of mine on technical maters.

Consider this very thread. You said that placing a resistor in series
with the compensation capacitor of an op-amp was "bad design".

No I didn't.

You
didn't qualify it with "usually" or "almost always".

Yes I did, to wit, my exact words were:

"..If they do, its often a poor design..."

"Often" was my qualifier, as was "poor".

I, usually, always cover my arse. As indeed I do here wth the word
"usually".

Do I get to call that 0.75 mistakes right here?

Nope

Take a look at the LT1001 and you will see one of the simpler ways
this sort of thing is avoided. Linear used a cascode stage in that
design.

Yes, of course a cascode helps the input roll off, but it is not
completely effective. This cascode itself has Miller gain from its
emitter to collector. What saves one is that its the low impedance
re that drives this capacitor.

Please explain the term "Miller gain[1]" as you use it here.

Anywhere there is gain and an impedance connecting the gain output to
input.

The
transistor has a capacitance from collector to emitter, but this does
not go via a gain to the emitter. There is also the collector-base
capacitance which does unless the base sees a very low impedance to
ground.

Not at issue.

For the collector emitter one, the low "re" does matter, for
the other it drops out of the issue.

As I said, re is low so its effect is nothing like as significant as
when driving from the base. Of course it has some effect. The Miller
effect is, esentially, the same whether the input is from the base or
emitter, but one is Rs.Ccb/Av the other is re.Cce/Av. Both re and Cce
are usually much lower.

Side point. Miller gain has nothing to do with negative feedback. Its
gain that is important not the polarity. The gain from emitter to
collector is positive, but the magnitude of the impedance is still
reduced by (1+Av), in this case (1-Av).

[1] I usually take this to mean the increase in the effective
capacitance due to the far end of it being connected to an inverted
signal.

See above. Common misconception but the inversion is not necessary. It
the output swings many times more than the input, it will source/sink
additional current through the coupling impedance, whatever its sign.

Several of the fast op-amps from Linear contain the resistor you
claim to dislike so much.

So what? So a in a few cases the numbers work out such that it *may*
be effective in some special cases.

So, you were wrong, wrong, ever so wrong that's what. :> You
overlooked something, it happens.

Nope. What part of "often" did you miss?

As I said, go and do some typically spice simulations and it will be
very clear that this resister can only be used in special
circumstances.

But, I don't need to do "some typically spice" simulations. I just
need a few weird examples to show that it works for those cases.

Experienced designers know that that it is usually not effective to use
such a technique, thats why we dismiss it it general. As I explained,
the Miller cap does a pole split. If one tries to insert a resister to
compensate for another HF pole, it reintroduces the parasitic poles at
the Miller stage that were previously clobbered. One negates the main
feature of the Miller stage, and gets one back to where one was. For the
technique to work it has to be compensating a pole that is much lower
then when the Mirrrr stage runs out of steam. In most/many cases the
extra poles are all at HF, so the technique has limited value.
Everything in an op-amp all tends to crap out at the same point, i.e.
related to the basic Ft of the process.

that without it the capacitor would have to be physically
larger that the chip makers would like it to be.

Oh dear...with all due respect, your phrasing here indicates that you
only have a basic understanding of all of this.

... and then there are lines like this. No way to win friends there
buddy. My phrasing is a bit odd in many situations for reasons I do
not wich to go into here.

You came across as someone who actually does not do this stuff 10,000's
of hours a year, and I mean 10,000's hours, i.e. hard simulations, as I
do. How can one phrase something that can be taken as personal, but
isn't? Again, with all due respect, your comments in this post, while
showing that you have a good basic understanding, show that you have
significant gaps in your knowledge. Its just the way it is. We all have
to learn sometimes.

*Of course* the Miller
effect is a capacitor multiplier, this is 101 electronics, but that
is not relevant to the idea of why Miller compensation is used. Even
if one had an enormous capacitor easily available one would still
not simply plonk it down at the input of the stage. The performance
would be dreadfull.

Yes that would be dreadful and I don't think anyone has suggested
that.

There was more but I just deleted it. Your tone makes it not worth
it, but go back and check it and you will find you made an
"oversimplifcation" (error).

Not in my view. Were would that have been?

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.

Winfield Hill · Nov 11, 2004

Jim Thompson wrote...

Winfield Hill wrote:

Jim Thompson wrote...

I couldn't locate the uA124QB. Could you send a copy? Thanks!

Jim, did I send it this morning - Sheesh, I don't remember!

Win, What do you sip on there in Massachusetts? We E-mailed
back and forth in regard to some typos ;-)

Oh yeah. I've not had email at home for almost a year now, and
I have been enjoying the peace. But trying to remember if I did
something after I went to work or not sometimes gets me confused.
<sigh> I must be working too hard.

--
Thanks,
- Win

Clifford Heath · Nov 12, 2004

Kevin Aylward wrote:

I, usually, always cover my arse. As indeed I do here wth the word
"usually".

You should afford others the right to cover themselves in the same way.

To wit, when I recently said "XYZ is surprisingly strongly affected by
ABC", what I obviously meant was that the effect was large enough to
surprise me, not that it was large in absolute terms.

Nevertheless you took me to task until I was tired of answering you,
even though I asked you to show your studies of the "degree of surprise"
in average subjects when confronted with whatever idea it was. I had
a factual example to back me up, but the discussion grew tiresome before
I offered it. Your loss, not mine.

Clifford Heath · Nov 12, 2004

Kevin Aylward wrote:

Your particular meaning of this phrase is not obvious to me.

Ahh, I see. It might have been a language issue. Subtle
difference there I guess, when adverbs are appended like
that, as to the expected precedence

. Never mind.
I would have read "Surprisingly, XYZ is strongly affected..."
quite differently, perhaps in the way you understood what I
actually wrote. Maybe a US/Oz thing. Still, we need to try
to ask what was meant rather than flatly contradict.

Kevin Aylward · Nov 12, 2004

Clifford Heath wrote:

Kevin Aylward wrote:
Your particular meaning of this phrase is not obvious to me.

Ahh, I see. It might have been a language issue. Subtle
difference there I guess, when adverbs are appended like
that, as to the expected precedence . Never mind.
I would have read "Surprisingly, XYZ is strongly affected..."
quite differently, perhaps in the way you understood what I
actually wrote. Maybe a US/Oz thing. Still, we need to try
to ask what was meant rather than flatly contradict.

I agree. But often, its simply not obvious that one is discussing
different things. Its a main stay of many comedy routines. One recent
one I saw was where Frasier (the us sit comm) was discussing moving to
San Francisco, whilst the family all thought he was discussing his
presumed impeding death, like "...when I go through that Golden Gate..."

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 · Nov 9, 2004

Jim Thompson wrote:

11 resistors, 4 caps and one JFET. BTW, this happens to exactly
match the number of parts claimed by the TI datasheets to be in
their version.

Despite the high degree of detail (one even shows which resistors are
pinch resistors), none of the schematics shows class-AB biasing
diodes, or a collector diode associated with the current-limit
transistor.

IIRC this is one of Widlar's jewels. He was fond of leaving critical
components off of schematics, or using diffused structures that just
happened to create additional, invisible components.

I assume the reason for this is *ONLY* to keep things secret from the
competitor. Any other reason for using obtuse, hard to uanderstand
designs should result in the designer being burned alive in oil.

Even the abbreviated drawings do show considerable difference between
various manufacturer's parts. For example, the Fairchild types use
two current sources to increase bias current for the input
emitter-follower of the input Darlington pairs, while the Motorola
types use a collector tap on the differential transistors to get
additional current for the input EF transistors. This creates a
defined beta for the differential transistors, and either approach
reduces the offset-voltage error from the input EF transistors, by
more accurately defining and matching their two collector currents.
The Fairchild types also have collector taps on the differential
transistors, going to ground. Both approaches rob much of the
differential transistor pair's collector current, thereby reducing
the transconductance of the input pair,

Yes, but to no avail for the gm/c GBW/slew limit.

and allowing the use of a
smaller comp cap (5pF) than would otherwise be possible, according
to Motorola.

Ahmmm. Its a bit more subtle that that. The high current and then thrown
away again is used because the *Ft* is higher at higher current. That
is, FT is a non-linear function of current. You don't get any
*effective* gm/C advantage by throwing away the current. If you throw
away some of the current it cant go into the capacitor. Dah!!! i.e. both
current and gm are thrown away, which by itself, is useless. Its only
because the Ft gets better at higher current that this can work, i.e.
the whole amplifier gets faster so the parasitic poles are less bother
allowing for a smaller Ccomp.

Yes, that always surprises everyone... low gm is desirable in the
frontend for best GBW product _and_ slew rate.

Yes, within reason!

Emitter resisters of 100k aint gonnna get one much BW.

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.

Rich The Philosophizer · Nov 12, 2004

On Fri, 12 Nov 2004 07:53:52 +0000, Kevin Aylward wrote:

Clifford Heath wrote:
Kevin Aylward wrote:
I, usually, always cover my arse. As indeed I do here wth the word
"usually".

You should afford others the right to cover themselves in the same
way.

I usually do

When I read anything, I have "Yes-Minister" to mind. Is what is being
said in code.

You are aware, of course, (aren't you?) that "Is what is being said in
code" not only is not a sentence, but has no referent in context, and
wouldn't mean anything even if it were/did?

Thanks,
Rich - smugly superior vocabularian, etc., esq.,
Self-Appointed Chief, apostrophe/grammar/syntax/'makes sense' Police.
;^j

Rich The Philosophizer · Nov 12, 2004

On Fri, 12 Nov 2004 19:26:28 +1100, Clifford Heath wrote:

Kevin Aylward wrote:
Your particular meaning of this phrase is not obvious to me.

Ahh, I see. It might have been a language issue. Subtle
difference there I guess, when adverbs are appended like
that, as to the expected precedence . Never mind.
I would have read "Surprisingly, XYZ is strongly affected..."
quite differently, perhaps in the way you understood what I
actually wrote. Maybe a US/Oz thing. Still, we need to try
to ask what was meant rather than flatly contradict.

Well, don't look now, but there is a non-zero probability that
it's a Kevin Ale-Ward/Reality thing. ;-p ;-p ;-p ;-p

;^j
Rich

Kevin Aylward · Nov 12, 2004

Rich The Philosophizer wrote:

On Fri, 12 Nov 2004 07:53:52 +0000, Kevin Aylward wrote:

Clifford Heath wrote:
Kevin Aylward wrote:
I, usually, always cover my arse. As indeed I do here wth the word
"usually".

You should afford others the right to cover themselves in the same
way.

I usually do

When I read anything, I have "Yes-Minister" to mind. Is what is being
said in code.

You are aware, of course, (aren't you?) that "Is what is being said in
code" not only is not a sentence, but has no referent in context, and
wouldn't mean anything even if it were/did?

Yes it does, its in code

Put a , after the said.

Remeber the "Oscillators don't, amplifiers do"?

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.

Jim Thompson · Nov 8, 2004

On Mon, 8 Nov 2004 21:48:46 +0000 (UTC), kensmith@green.rahul.net (Ken
Smith) wrote:

In article <kuuuo055ldqf0oam7oit783ljcas7t85os@4ax.com>,
Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote:
On 7 Nov 2004 18:26:07 -0800, Winfield Hill
whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

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.

It can't be *that* bad. In that poll of "classic chips" we had last
year, the 324 was one of the most popular, ISTR.

The output section has about 3 diodes worth of crossover distortion and
the compensation does not enclose that distortion so at high frequencies,
it appears in the output. It also makes the output impedance rise quite a
bit for increasing frequencies and vary a lot with load current. This can
lead to trouble when you use them to drive a transistor buffer.

The "simplified schematic" is flawed... it can't be as-drawn... there
is a fundamental error. I tried to tease Win into _stepping_in_it_,
but he won't take the bait ;-)

And what's this statement, "...compensation does not enclose that
distortion..."?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

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

Pooh Bear · Nov 7, 2004

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.

Graham

Ken Smith · Nov 7, 2004

In article <ffijd.135932$b5.6714097@news3.tin.it>, Ban <bansuri@web.de> 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 need a differential input stage, for example a differential amplifier

You are answering someone's homework question for them. This person
either (a) skipped a lot of classes (b) has the worst instructor in town
or (c) is unable to absorb the information from class.

If it is (a) you are encouraging him to continue to skip classes and not
learn. This may result in him having the paper but not the knowledge that
goes with it. It is no service to him.

If the situation is (b) I think we would be much better off explaining why
things should be done rather than what should be done. We could offer him
a complete solution but I don't think he would learn much from it.

If the answer is (c), it is best that the person fail now before he has
invested too much time in the effort. He may have abilities in other
areas. If he fails his op-amp design question but does well in optics or
something, he is likely to switch to a better option.

--
--
kensmith@rahul.net forging knowledge

PaulCsouls · Nov 7, 2004

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

If you are serious about learning how to do it but are having trouble
understanding your teacher and textbook, I suggest getting Analysis
and Design of Analog Integrated Circuits by Paul A Gray and Rober G.
Meyer. I bought a used copy for 7 dollars.

Paul C

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