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From: Phil Allison on 17 Oct 2009 09:46

"Bob Elderly"
> "Phil Allison"
>> "Bob is too Eld"
>>
>> > At 100 Hz the open loop gain should be about 100 times and, 5mV should
>> > give about 500mV out, open loop.
>>
>>
>> ** Err - the open loop gain of a 741 is 10,000 times at 100Hz.
>>
>>
>> > Connect the amp as a gain of 10 with a 1k input resistor and a 10k
>> > feedback resistor to the neg input, ground the pos input and be sure
>> > there is a DC path to ground through your source and try again.
>>
>>
>> ** No need for a DC path exists when the pos input is grounded cos the
>> feedback loop will make sure the neg input is within 1 mV of the pos one.
>>
>>
>
> Your right....I had a brain-fart and figured it wrong. For some reason I
> can't seem to divide a million by 100. Oh well, the fact remains that the
> OP must be trying to run the amp open loop.


** The OP is probably a congenital half wit making the dumbest of errors
with a basic simulator.

Die of fright if he ever saw a * REAL * uA741 ....




...... Phil


From: Jamie on 17 Oct 2009 10:56
cc wrote:
> I'm aware of uA741 slew rate issues and limited GBW, but I can't even
> get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV
> peak at only 100Hz (yup, one hundred Hz). What?
It would be nice if you can show us your attempted circuit?


From: default on 17 Oct 2009 11:15
On Fri, 16 Oct 2009 18:02:16 -0700 (PDT), cc <pccmd(a)comcast.net>
wrote:

>I'm aware of uA741 slew rate issues and limited GBW, but I can't even
>get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV
>peak at only 100Hz (yup, one hundred Hz). What?

Open loop gain of a 741 would give you a rail to rail square wave on
the output. You have to be limiting the gain external to the op amp
or your measuring method or instrument is wrong.
--
From: Jamie on 17 Oct 2009 12:12
default wrote:

> On Fri, 16 Oct 2009 18:02:16 -0700 (PDT), cc <pccmd(a)comcast.net>
> wrote:
>
>
>>I'm aware of uA741 slew rate issues and limited GBW, but I can't even
>>get above gain of 2x with Vcc of +/- 12v and input sine wave of 5mV
>>peak at only 100Hz (yup, one hundred Hz). What?
>
>
> Open loop gain of a 741 would give you a rail to rail square wave on
> the output. You have to be limiting the gain external to the op amp
> or your measuring method or instrument is wrong.
A 741 won't do Rail TO Rail. I think the fact it does square wave
in an open loop is what you're trying to convey in this situation.

To be honest, I haven't really found a spec anywhere that signifies
what Rail to Rail is. Every chip i've spec'ed as being rail to
rail, never reached it's actual rail voltages in the data sheet.
So some where, some one has decided what constitutes a rail level
output and what does not. Compared to what RR types do that are
currently out there, the 741 does not fall into that category.



From: cc on 17 Oct 2009 23:30
On Oct 16, 7:57 pm, "Bob Eld" <nsmontas...(a)yahoo.com> wrote:
> "cc" <pc...(a)comcast.net> wrote in message
>
> news:37514530-3118-4123-bc52-ff93af7461f1(a)i12g2000prg.googlegroups.com...
>
> > I'm aware of uA741 slew rate issues and limited GBW, but I can't even
> > get above gain of 2x  with Vcc of +/- 12v and input sine wave of 5mV
> > peak at only 100Hz (yup, one hundred Hz).  What?
>
> Are you applying any feedback? It sounds like you are trying to operate it
> open loop.
>
> At 100 Hz the open loop gain should be about 100 times and, 5mV should give
> about 500mV out, open loop.
>
> However, the DC input offset voltage and offset current could easily drive
> the open loop amp output into one of its rails causing virtually no AC out.
>
> Connect the amp as a gain of 10 with a 1k input resistor and a 10k feedback
> resistor to the neg input, ground the pos input and be sure there is a DC
> path to ground through your source and try again.


Thank you all.
It's set up as simple inverter, with (+) pin to ground via same R as
the input R and only 100Hz.

Still working on it, but I replaced the F/B circuit with only 10ohms
(R) between signal and (-) pin, and 1000k between (-) pin and output,
for closed loop gain 100x and it worked. So it seems that I need low
R at (-) input terminal. You see, I had been using R=1k and the
feedback R=100k.


Now, my fcn generator has about 700 o/p impedance. Does that matter?


I had similar problem with AD8620 and CA3140...still working on them.


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