Op Amp circuit - I think..?

[The Grim Reaper]

Hi all,

I'm wanting to get a voltage amplified in a negative way... the opposite of
amplified (!!??!).
For instance, if I put between 0 and 5 volts into the amp, I want a
corresponding 1 to 4 volts out - does that make sense??
In practice, I need to get between 0.1 and 0.35 volts from the 0 to 5 volt
input.

I've been googling for hours, re-learning everything about 741's, gain and
all that stuff I thought I'd remember from school... but no luck!
Perhaps I've been looking for the wrong thing?!? If anyone knows the
correct phrase to describe this type of circuit, it may point me in the
right direction!!

Thanks for any suggestions.
__________________________________
The Christmas Grimch

 

[Anders N. Vinje]

Hello.
Take a look at this.
http://home.online.no/~annlaune/Electronics/Opampcircuit.bmp

I made two resistor divider networks. On making 1 V from a 12 V source. You
may need to change these if you have another powersupply with a diffrent
voltage. The best soulution would be if it is a voltage ref source.
The other divider makes the 5V go from 0V - 3V instead.
These er feed into buffer amplifiers so loading won`t be a problem. Then
these to voltages are added, and then inverted by the last opamp. So now u
got a input voltage going from 0-5V and a output going from 1-4V
This may not be the best soultion but it will work....


http://home.online.no/~annlaune/Electronics/Opampcircuit.bmp

Cheers

Anders n. Vinje

 

[Kingcosmos]

Hi all,

I'm wanting to get a voltage amplified in a negative way... the opposite of
amplified (!!??!).
For instance, if I put between 0 and 5 volts into the amp, I want a
corresponding 1 to 4 volts out - does that make sense??
In practice, I need to get between 0.1 and 0.35 volts from the 0 to 5 volt
input.

When you state 'amplification in a negative way' you must mean
attenuation. Op-amps can be used as an attenuator. However I'm
confused...do you need a 1V to 4V or a 0.1V to 0.35V output signal from
a 0V to 5V input.

 

[The Grim Reaper]

When you state 'amplification in a negative way' you must mean
attenuation. Op-amps can be used as an attenuator. However I'm
confused...do you need a 1V to 4V or a 0.1V to 0.35V output signal from
a 0V to 5V input.

I need the 0.1v to 0.35v from a 0v to 5v input. The 1v to 4v was just an
example.
If it can be done with some sort of resistor divider arrangement, then I'd
be able to calculate slightly different voltages - for manual calibration.

I've had another idea - scrap all the fiddling about and use a PIC chip....

_______________________
The Grim Reaper

 

[AJ]

Hi all,

I'm wanting to get a voltage amplified in a negative way... the opposite
of amplified (!!??!).
For instance, if I put between 0 and 5 volts into the amp, I want a
corresponding 1 to 4 volts out - does that make sense??
In practice, I need to get between 0.1 and 0.35 volts from the 0 to 5 volt
input.

I've been googling for hours, re-learning everything about 741's, gain and
all that stuff I thought I'd remember from school... but no luck!
Perhaps I've been looking for the wrong thing?!? If anyone knows the
correct phrase to describe this type of circuit, it may point me in the
right direction!!

Thanks for any suggestions.
__________________________________
The Christmas Grimch

Could you use a resistor divider buffered with an OP AMP configured as a
"voltage follower"?

Regards

AJ

 

[Chris]

Hi all,

I'm wanting to get a voltage amplified in a negative way... the opposite of
amplified (!!??!).
For instance, if I put between 0 and 5 volts into the amp, I want a
corresponding 1 to 4 volts out - does that make sense??
In practice, I need to get between 0.1 and 0.35 volts from the 0 to 5 volt
input.

I've been googling for hours, re-learning everything about 741's, gain and
all that stuff I thought I'd remember from school... but no luck!
Perhaps I've been looking for the wrong thing?!? If anyone knows the
correct phrase to describe this type of circuit, it may point me in the
right direction!!

Thanks for any suggestions.
__________________________________
The Christmas Grimch

Hi, Grim. The key word you're looking for is "Non-inverting summing
amplifier". Google that and you'll be OK.

Since your gain is actually less than 1, you can use the input
resistors as an attenuator, and use the special case of the
non-inverting summer with a gain of 1 (also called a voltage follower),
like this (view in fixed font or M$ Notepad):

|
| +5V
| +
| | Vin
| .-. 0 to 5V ___
| R1| | o-----|___|--.
| | | 200K | 1/2LM358
| '-' 1/2LM358 | |\ Vo - approx.
| | +5V o----|+\ 0.1-0.35V
| .-. |\| | | >--o----o
| P1| |<---|+\ 0.1V___ | .--|-/ |
| | | | >--o-|___|--' | |/ |
| '-' .--|-/ | 10K | |
| | | |/| | | |
| === | === | '--------'
| GND | GND |
| '--------'
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Choose R1 to go with your P1 such that you can easily set it for 0.1V
at the output of the first voltage follower. Them the two resistors
give you an approximately 20:1 attenuation of the input signal, which
is what you need to get 0.25V span (0.35V - 0.1V).

1% resistors will get you in closer, but you get the idea, I guess.

Have a Grim holiday, sir.
Chris

 

[[email protected]]

2 resistors and Bobs your uncle..

 

[Chris]

Hi, Grim. The key word you're looking for is "Non-inverting summing
amplifier". Google that and you'll be OK.

Since your gain is actually less than 1, you can use the input
resistors as an attenuator, and use the special case of the
non-inverting summer with a gain of 1 (also called a voltage follower),
like this (view in fixed font or M$ Notepad):

|
| +5V
| +
| | Vin
| .-. 0 to 5V ___
| R1| | o-----|___|--.
| | | 200K | 1/2LM358
| '-' 1/2LM358 | |\ Vo - approx.
| | +5V o----|+\ 0.1-0.35V
| .-. |\| | | >--o----o
| P1| |<---|+\ 0.1V___ | .--|-/ |
| | | | >--o-|___|--' | |/ |
| '-' .--|-/ | 10K | |
| | | |/| | | |
| === | === | '--------'
| GND | GND |
| '--------'
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Choose R1 to go with your P1 such that you can easily set it for 0.1V
at the output of the first voltage follower. Them the two resistors
give you an approximately 20:1 attenuation of the input signal, which
is what you need to get 0.25V span (0.35V - 0.1V).

1% resistors will get you in closer, but you get the idea, I guess.

Have a Grim holiday, sir.
Chris

Sorry -- I should have proofed this, but got busy on a call. A couple
of tweaks:

1) You adjust P1 so that there's 0.1V at the output of the second
voltage follower with a 0V input.

2) The 200K/10K divider is a 21:1 divider. A little sloppy there. Try
82K/4.3K for best 5% match, or 215K/11.3K for best 1% match.

3) The LM358 is a nice cheapie dual op amp, but you might have a little
trouble getting down to 0.1V without placing a 3.3K load resistor from
the outputs of the first and second voltage followers to GND. This is
in addition to the rest of the circuitry.

Cheers
Chris

 

[The Grim Reaper]

<snipped>

You've put far too much effort in Chris! So as to not have wasted your
time, I'll build your suggestion at the weekend!!
(Need to wait til Friday to pick up some more pots anyway...)

The reason I mentioned a PIC, is that this whole thing needs to be
controlled from a PC anyway, and obviously interfacing to a PIC would be
quicker and more flexible. The system will also drive a number of servos -
4 in the original design - so it's got to be easy to make 4 off!!

Thanks for you help so far - it's appreciated.
___________________________________
Grim

 

[The Grim Reaper]

2 resistors and Bobs your uncle..

I had a feeling that simple answer would come up sooner or later!! The
trouble is, I'm so rusty, I've having to go back to simple voltage dividers
to practice the maths and stuff!!

Would I be right in thinking I need to calculate a pull-down resistor for
the (5v - 0.35v) 4.65v, and a pull-up resistor for the 0.1v??
Then shove the 0v to 5v signal in the middle - simple as that??
___________________________
The Grimch Reaper

 

[[email protected]]

actually, im wrong, i didnt read the post close enough. the 4volts from
5 is easy enough, but to have 1 volt at a 0 volt input is the trick and
to make it variable up to the 5 volt mark.
workin on it in spice simulator.

--eric