op amp buffer and transistor buffer

Use an LM324.

Best regards, Spehro Pefhany

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"Output common-mode range"???

Right. Like LM7301.

John

What the OP is asking for is the Output Voltage Swing. He's violated the input common mode range specification in trying to measure it.

..

The LM324 has an input common mode range that does include the negative rail and can actually go up to 300mV or so below it without disaster, though the performace is not guaranteed below the rail

No amplifier has an output voltage swing that includes the negative rail. The LM324 output can typically get within 5mV of the rail - 20mV is guarnteed, when pulled down by 10k to the negative rail.

Jim Thompson correctly points out that CMOS rail-to-rail op amps can do better than bipolar rail-to-rail op amps but bipolar rail-to-rail op amps go back to Bob Widlar's LM10.

The input common mode range of the LM10 stopped about 1V below the positive rail, but it was pretty remarkable when it was first introduced in 1978, and apparently had no rival for the next decade.

get

sus

-- Bill Sloman, Nijmegen

1) Err..if the output is connected to an input (does not matter which one in this case) and then you ground that input (like you say), then you will be grounding the output and you damn well better measure zero volts there... House_some_other, if output is connected to I and NI is grounded,it is not too surprising you see around 2V due to internal current source circuitry for the input pairs. 2) Well, close, maybe..NPN with emitter resistor to ground will yield equivalent of just the resistor.

What about the class B '002 hybrid circuit? Fairly low in/out offset with near supply range at output.

On a sunny day (Tue, 26 Jul 2011 18:34:10 -0700 (PDT)) it happened panfilero wrote in :

Then you grounded the output too. You probably ment grounded the + input.

What has bipolar to do with it? Many CMOS opamps go close to the rail.

But in many cases it is easy to make a small negative supply. All you need is a 555 timer (;-) ), couple diodes and caps. LOL

Depending on load (and direction) that'll typically get you to ~50mV off of ground. ...Jim Thompson

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All the way and more if you drive an optoisolator and use the b-e junction as the op-amp's output load (courtesy of Williams or Pease, I can't remember which, rip^2.)

-- Cheers, James Arthur

I was in the middle of writing a "huh?" when I realised what you mean. Clever!

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zero, and remove the last word.

An LM741? "741" is _not_ synonymous with "op-amp!". Get some data sheets!

The number you are looking for is "output voltage swing". You also want to know about "input voltage range" -- it'll never get outside of the former, if you violate the latter then bad things will happen. The LM358 or LM324 are designed for single-supply use, with input range and output swing both going to the negative supply (which means you can conveniently use ground instead of needing a - supply), but not to the plus supply. So-called rail-rail op-amps will give you output and/or input swings almost to the rails.

Yes, but it has a humongous voltage shift.

LM358 or its ilk are cheap, plentiful, and will do what you want. Newer rail-rail parts will do it better, but the LM358 et all are often plenty good enough, and much less expensive.

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t

et

Sorry, I mis-typed, only the non-inverting input is tied to ground, inverting input is only tied to output

Inquiring minds want to know--how come all of National's op amps since about the LF356 are so stinking noisy? It seems like every time I see some tasty-looking part of theirs, I find out that it has 20 nV noise, or gross input capacitance, or something like that.

Seems like it takes talent to achieve a record like that.

Or are there good ones that I'm missing?

Cheers

Phil Hobbs

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Dunno. I don't even think about National for high-performance opamps.

But LMH6642 is cool. Admittedly it has 17 nv of noise, but it's a beast otherwise, not bad for 95 cents.

LM8261 is very nice. RRIO, high current, and happy with any capacitive load. That can be useful. And only 15 nV noise!

John

I use an LM7171 for a Howland current pump, and LM833s are our standard 30V opamp. Oh, and the LMV852 is useful in high EMI areas (ESD, too), like right next to the radio. It does have a 20nV of noise but that's better than the alternatives.

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Yup. It's just a cheesy isolated current source, but it might come in handy some day.

-- Cheers, James Arthur

Can you sketch something, 'coz I'm not in the middle of a huh:

Huuuuuuhhhhh?????

--
Thanks,
Fred.

:)

Like this AIUI

--

John Devereux

Or maybe this one, avoids zenering the transistor e-b junction for high outputs:

--

John Devereux

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Still don't get it, in both cases...

What you're doing is just injecting some photocurrent into a buffer output in the first one and in the second one you're still making a buffer, but that time with a serial output diode that you slightly modulate with photocurrent :-)

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Thanks,
Fred.