differential amplifier

As I recall, the TL082 does NOT like being run near the rails, top or bottom. I'd find a cmos device like the MC34074 that doesn't mind being "railed".

Jim

Hi Kell,

Forget the use of TL08x here. This OP can not be used in applications near rail, neither input nor output. Use R2R OP instead ...

I wold prefer a OP with a offset tha is good enough. Every trimming will be unstablevs temp & time, expensive in adjusting and not very robust. One mechanical Stress moment and your adjustment may be gone...

Marte

This looks like about the third iteration of this same homework question.

Apparently, all three of them are in the same class, with a lame prof.

Cheers! Rich

Please bottom post. The TL082 has a common mode that includes the top rail, which this circuit calls for. A quick glance at the MC34074 suggests its common mode includes ground, but not the top rail. The common mode in my circuit will certainly never get anywhere near ground.

Thanks, kell

Thanks for the advice. I hear the same thing all the time. And I may use trimming in spite of what everyone says, for sufficient reasons.

Thanks, kell

Homework?

Professor?

Explain.

Since the voltages are near the upper rail, and you have the amps configured as followers, their outputs will be near the upper rail. Make sure your amplifier of choice will handle positive rail input AND output gracefully. And for offset adjustment, I would recommend NOT tying the pot center to ground, unless the voltages are pretty stable. Even then, the pot also adjusts gain where you have it shown. Also, note that the current in the upper section of the pot works to drive the upper op amp output more positive: be careful that it doesn't try to drive it beyond the rail. Why not apply the offset to the output side? Since you want a lot of gain, and the gain will be proportional to the ratio R4:R3, presumably R4 will be fairly large, and you may be able to just use a 100-ohm pot tied between, say, +1 and -1 volts, and the +/- 12.5 ohms variability in the net value of R4 as you adjust the pot might be insignificant. (Can't tell; you didn't post enough info about it.)

Cheers, Tom

Correct! I was going to comment when the OP said TL082 would work... inputs will go to +rail, but output won't.

...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:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

It's finals week. All of the kiddies who have been drinking instead of doing their lessons are now panicking as they cram for the exams, hoping they can get somebody else to do their homework for them.

Hope This Helps! Rich

martin

And this applies to me how?

Within a matter of days, you and two other people asked the exact same question, and you're posting from googlegroups. That's two of the three major warning signs right there! ;-)

Cheers! RIch

I don't quite follow your description of the pot on the output. Thanks, Kell

Hi kell,

never design with typical values! TI specifies min +-11 V input common mode voltage range for +-15 V supply.

Marte

Hi Jim,

even with R2R Out OP there are usually a few ten to hundred mV left.

Marte

Jim, what do you think of the LMC6484?

Kell

Hmmm...if you don't quite follow it, why do you thank me? ;-)

Here's a second try: The way you drew the diagram, R4 is grounded. The current through it causes a voltage drop that (you expect) is proportional to the differential voltage at the high side rail, but there is an offset voltage. If you return R4 to a voltage other than ground, you can null out the offset voltage. One way to do that is to simply tie the "bottom" of R4 to the wiper of a low-value potentiometer (much lower value than R4). The ends of the potentiometer might go to

+1V and -1V, for example, giving you the ability to adjust up to +/- 1V of OUTPUT offset out of the circuit.

If you insist on nulling the offset with a circuit attached to the op amps, I suggest you look for a way to do it that lowers the output voltages of the op amps, and also does not change the gain of those op amps. The circuit you drew raises the output voltage of the amps, and does change the gain of at least the upper one.

Cheers, Tom

Output is still 20mV from rail with 100K load.

What is the range of interest for VX?

Why does output have to get so close to rail?

...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:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

[snip]

Kell,

Where do the OpAmps get their power? From the + end of VX ??

...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:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.