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Voltage Divider

D

dbvanhorn

Jan 1, 1970
0
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.
Can anyone suggest a source? Seems like something that ought to be in
a NatSemi analog book somewhere.
 
L

linnix

Jan 1, 1970
0
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.
Can anyone suggest a source? Seems like something that ought to be in
a NatSemi analog book somewhere.

Google on summing junction.
 
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.
Can anyone suggest a source? Seems like something that ought to be in
a NatSemi analog book somewhere.

Probably is. While you're looking for it, and assuming discrete
parts, how about a log amp driven by each signal, those feeding a
subtraction circuit, and that followed by an anti-log amp. Four op
amps and a handful of discrete parts depending on input ranges,
accuracy, etc.

A nice bit on log/antilog amps:

http://www.electronics.dit.ie/staff/ypanarin/Lecture Notes/DT021-4/6LogAntiLogAmplifiers.pdf
 
T

Tim Shoppa

Jan 1, 1970
0
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.
Can anyone suggest a source?  Seems like something that ought to be in
a NatSemi analog book somewhere.

NatSemi was never very big in analog multipliers or dividers.

TI (formerly Burr-Brown) has the MPY634.

Analog devices has the AD633.

Those are nominally multipliers but with a feedback loop they become
dividers, see the data sheets for examples. The AD633 needs an
external op-amp to help out with division, but the MPY634 with its
extra pins doesn't need an external op-amp. Accuracy of X/Y becomes
worse and worse as Y gets smaller and smaller due to offset voltages,
some manual trimming can help out.

Tim.
 
E

Eeyore

Jan 1, 1970
0
dbvanhorn said:
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.
Can anyone suggest a source? Seems like something that ought to be in
a NatSemi analog book somewhere.

No, you need to learn basic Norton and Thevenin. What year are you in ?

Graham
 
E

Eeyore

Jan 1, 1970
0
Joel said:
Analog Device's big book, "Basic Linear Design" discusses this a fair
amount... the trick is to exploit the exponential relationship of a diode (or
transistor) in a feedback path of an op-amp to obtain log(a) and log(b), then
compute log(a)-log(b), and finally (using the same trick) exp(log(a)-log(b)) =
exp(log(a/b))=a/b.

Since when is that a 'voltage divider' ?

I think the OP chappie wants a circuit that outputs 2V when it has 2 inputs of
1.5V and 3V. I fear for his future. Cross-posted to basics.

More and more courses are getting kids who don't understand even basic
electricity.

Graham
 
E

Eeyore

Jan 1, 1970
0
Probably is. While you're looking for it, and assuming discrete
parts, how about a log amp driven by each signal, those feeding a
subtraction circuit, and that followed by an anti-log amp. Four op
amps and a handful of discrete parts depending on input ranges,
accuracy, etc.

A nice bit on log/antilog amps:

Where are you lot getting all these log amp ideas from ?

I think he just wants to actively sum some signals. It'll be in TI's "op-amps for everyone'
available from ti.com as a pdf file. They also do a similar (ex Burr Brown ?) basics throught to
not so basics book.

Graham
 
E

Eeyore

Jan 1, 1970
0
John said:
What does that mean?

I think he means you sum 3V and 1.5V - get 4.5V and multiply it by +0.44444
to get 2V.

In any case, he's not going to be an electronics designer if he can't even
ask the question intelligently.

Graham
 
S

Sylvia Else

Jan 1, 1970
0
Eeyore said:
I think he means you sum 3V and 1.5V - get 4.5V and multiply it by +0.44444
to get 2V.

Or maybe he wants to sum the squares, and divide by 5.625.

But the most natural interpretation of his admittedly unclear question
is that he wants a circuit that divides one voltage by the other (and
multiples the result by 1V).

Sylvia.
 
E

Eeyore

Jan 1, 1970
0
Sylvia said:
Or maybe he wants to sum the squares, and divide by 5.625.

But the most natural interpretation of his admittedly unclear question
is that he wants a circuit that divides one voltage by the other (and
multiples the result by 1V).

It didn't seem terribly natural to me. For one thing, that's not called a
'voltage divider'.

Graham
 
S

Sylvia Else

Jan 1, 1970
0
Eeyore said:
It didn't seem terribly natural to me. For one thing, that's not called a
'voltage divider'.

Graham

It may not be what is normally thought of when the expression "voltage
divider" is encountered, but by normal language usage, something that
divides one voltage by a nother is reasonably called a voltage divider.

Sylvia.
 
E

Eeyore

Jan 1, 1970
0
Sylvia said:
It may not be what is normally thought of when the expression "voltage
divider" is encountered, but by normal language usage, something that
divides one voltage by a nother is reasonably called a voltage divider.

That's actually a voltage multiplier with scaled factors.

Graham
 
N

Nobody

Jan 1, 1970
0
Calling it a "voltage divider" is reasonable if you've never been told what
the "traditional" voltage divider is -- which I suspect is the case here: As
you suggested, he's probably a beginner, so his terminology is a little
off at this point.

Well, he knows enough to have said:
This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.

IOW, he knows what the term "voltage divider" normally means, but not the
correct term for the part in question.

FWIW, what *is* the correct term? I don't buy "multiplier, with scale
factors", because it's not multiplying, with or without scale factors.
"Inverse multiplier"? "Reciprocal multiplier"?
 
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.

Dear Mr/Ms Van Horn (guessing from your username, sorry if it's
inaccurate),

Before this thread degenerates any further into the usual
sci.electronics.design slag-fest, do you suppose you could enlighten
us further about what you're after? And whether or not any of the
replies you've seen have been of any use to you?
 
D

Daniel

Jan 1, 1970
0
Analog divider, perhaps?  "Analog multiplier" is certainly a well-known
term...

Anyone know where analog dividers are (or were) typically used (other than in
general-purpose analog computers)?  The only application I know of is in
demodulating the old Motorola CQUAM ("AM stereo") format.

---Joel

i used an analog devices 'divider' once. it was used in an x-ray kV
measuring unit that i designed and it depended on the ration between 2
signals. the first signal was the output of a set of diodes, whilst
the second was the output of another set of diodes placed under a rare
earth and copper attenuating screen. the output apparently of this
ratio was proportional to the beam energy. i did that 30 years ago so
please don't press me on the details...
 
D

dbvanhorn

Jan 1, 1970
0
I just got handed a requirement for a circuit that I've not seen yet,
an op-amp based voltage divider.
Didn't see it in Horowitz and Hill either.

This is not a resistive divider, for example the output of 3V and 1.5V
would be 2V.
Can anyone suggest a source?  Seems like something that ought to be in
a NatSemi analog book somewhere.


Wow.. So much argument over terminology.
FWIW I'm no "beginner". I know what I'm after is NOT a resistive
divider, and I didn't want to fend off 1000 replies telling me to use
two resistors.
That's why I included the part illustrating that what I wanted was the
division of voltage A by voltage B, and not the division of voltage A
by some constant.

The end result is a circuit to do monopulse resolution enhancement of
two sensors with overlapping detection fields.

The circuit needs to take two inputs, A and B, and give the result of
(A+B)/abs(A-B)
I had everything worked out at that point except for the analog
divider.

I've found several approaches using log-antilog now, and I'm satisfied
with that at this point.
I didn't know what to call it when I asked, as I'd not seen such a
circuit in quite some time, and looking thru H+H I also came up blank.
It's hard to ask for something precisely when you don't yet know what
the name of it is..

So thanks for the helpful replies, but I think I'll pass on the
"attitude" replies.
 
D

dbvanhorn

Jan 1, 1970
0
Analog Device's big book, "Basic Linear Design" discusses this a fair
amount... the trick is to exploit the exponential relationship of a diode(or
transistor) in a feedback path of an op-amp to obtain log(a) and log(b), then
compute log(a)-log(b), and finally (using the same trick) exp(log(a)-log(b)) =
exp(log(a/b))=a/b.

Right, I don't have that one handy, but I have found some examples in
NS app notes.
Actually doing this accurately requires a bit of experience, I suspect --I'm
not surprised those Analog Devices ICs aren't exactly cheap.  (Granted,not
all applications need *that* much accuracy.  What are you doing?)

Monopulse resolution enhancement.

Think of two sensors, could be phototransistors, CdS cells, etc
looking out with their detection fields overlapping.
If you plot the response of either, you get a broad hump.
If you sum them, you get a broader hump with a dip in the middle, but
still indistinct.
Difference is similarly unhelpful.
But if you sum them, and divide that by the absolute value of the
difference, you get a very sharp peak.
 
D

dbvanhorn

Jan 1, 1970
0
If this isn't an analogue computing application (which would be rare and
interesting these days),

Only in the strictest sense..
it's frequently enough to use a diff pair in a
feedback loop--you get the log ratio instead of the actual ratio, and
the simple method requires that the voltages have the same polarity and
that one can be made to be always bigger than the other.

I can satisfy those requirements. Thanks for the pointer.
 
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