# 741 IC issue. Still a beginner.

#### sparky 1

Joined Nov 3, 2018
306
@ audioguru,
No. If R1 is 4k and R2 is 1k then the voltage gain is 5.0, not 10.
No again. If VRMS is 1.0V then V P-P is 2.828V. If Vpeak is 1.0V then VRMS is 0.707V.
I am sorry, yes I made an error. As per the schematic given
the sum of the absolute value of + .27V and -.27V equals 540mVp and not 540mVp-p

I set the function generator to 540 mVp which is 1.08Vp-p which is not correct.
I should have set it to 270mVp (millivolt peak) and measured that in Vpp units.

I ran the simulation again at 8 volts (because of the clipping at 5V) the output is
5.94Vpp the gain is more than 10. I expected 5.40Vpp an error of 0.54Vpp

So I compensated for the error usng the value for R1 = 9.05K which gives an output of 5.40Vpp
this will help in the future, Measure Vpp input and measure Vpp output then compensate.
thanks for pointing that out.

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#### atferrari

Joined Jan 6, 2004
4,071
Three mistakes by the OP made for this long thread and a lot of time and effort wasted:

He did not read a data sheet.

He did not bother to draw a simple schematic.

He confused the inputs (that should be called "inverting" and "non inverting") with the pins where "positive" and "negative" voltages from a PSU should be applied.

#### Audioguru again

Joined Oct 21, 2019
1,785
Without seeing his schematic I do not know if his opamp is inverting or is not inverting.
I also do not know if he has a single positive supply or has a positive and negative supply.
I do not know if he is still using an antique 741 opamp or a modern opamp.

#### andrewmm

Joined Feb 25, 2011
538
Just a comment re the gain = 1+R1/r2

This is an approximation, assuming the opamp has zero bias current, zero offset, in finite gain, and infinite power supply range.

For 99.9999 % of the cases this is more than good enough, one of the reasons op amps became so popular,
but it is an approximation,

so if your down in to the mV range of looking for differences, then you need to look at a more detailed equation, or like the rest of us , simulate. ...

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,157
there are a few things I'd lie to add. Every component no matter what is it has residuals or parasitics. They are not ideal. Three 1K resistors in series may not be equivalent to one 3K resistor. Hard to imagine, but true.

Wires have inductance, so bypass capacitors are used on the power supply pins LOCATED near the power supply pins

The OP amp cant swing to either rail. Some might be able to get within 200 mV of ground and 3V of the +supply. LM334.

Rail to rail OP-amps don't go rail to rail either, but they get pretty close. A 0-5V sensor might look like 0-5 on the surface, but it may be a ratiometric sensor that does use the current value of Vdd/2 as it's reference. It's OK and even desireable to do this. Automotive sensors, e.g. Mass Air Flow (MAF) are a good example.

There is an input bias current. be careful, a few pA across a short is a large current, so sometimes a resistor is used in series with the inputs.

There is typically a diode like behavior when the inputs exceed the supply voltage. When the unit is off, you can;t apply vltage except maybe -0.3V if you have a single supply OP-amp. This is another reason why sometimes there is a resistor in series with the inputs.

There are exceptions to the rule with "Over the Top" Op amps.

The things you measure with, like DMM's have internal resistances. So, don't insert a DMM in current mode and then take it out of the circuit and measure the voltage. it generally won't work.

There are such things as a 4-terminal resistor.

What can get you sometimes is the common mode voltage. What voltage can be common to both inputs at the same time.

The offset voltage gets amplified as DC.

Ib or the bias current varies with temperature.

Then there is, you generally have to drive say a summing input with a low impedance source.

The circuits that I designed and worked on, fingerprints matter.

Oh, and BTW, a wiggling wire generates a current. Why? Physics tells us that a wire in a magnetic field (the Earth's) generates a current.

RF power is better transmitted with a silver plated copper tube, not a wire.

A wire to a screw terminal is a battery. It's really a thermocouple.

A 741 OP-amp is ancient history.

#### dl324

Joined Mar 30, 2015
11,317
A 741 OP-amp is ancient history.
If you know how to use it, it's still useful.

The only problem I see is when someone who can't, or won't, read the datasheet tries to use them with inappropriate power supplies.

#### Papabravo

Joined Feb 24, 2006
14,266
It is right up there with the three greatest sketchy statements:
1. The check is in the mail
2. I love you
3. I promise not to ...