741 op-amps

[electro_Rookie]

I'm new to the 741 op-amp, and I'm trying to understand some basic concepts, like what will happen if the load resistance is low and approaches the output resistance of the 741 op-amp? (The output is feedback into the negative terminal with the positive terminal grounded creating an inverted output.)

My though is the low resistance will increase the current that is feed back into the negative terminal, but i don't how that will effect the 741 op-amp. Also with the low resistance and high current the input voltage will be greater at the input, and in fact decrease the gain of the op-amp, and the output voltage will be low. Does anything else happen or am I way off on everything???
Thank you everyone
Heath

 

[MikeMl]

Forget the 741. It was a poor excuse for an op amp when it was created 30years ago. It is the VW Beetle of opamps.

You a bit off on your understanding of how inverting opamps work. For an inverting opamp, the current in the feedback resistor is equal to the current in the input resistor (because the current into the inverting input is essentially zero).

Every opamp has a maximum current it can source or sink from its output pin. Some of the output current from the opamp flows through the two resistors back to the input source and the rest flows into an external load that the opamp is driving (if any). When picking the feedback resistor, you have to choose the resistors so that the max current of the opamp is not exceeded.

You need to understand the figure below, which came from this reference.

 

[indulis]

Simple (ideal) op-amp rules:

1) Both inputs have infinite input resistance and will not draw any current.
2) The output has zero impedance and can supply infinite current.
3) Whichever input has the higher potential “wins” and the output will swing in that direction. If the + pin is more positive than the – pin, the output will swing positive and if the – pin is more positive than the + pin, the output will swing negative.
4) Both inputs want to be at the same potential. If one is tied to ground, the other will want to be zero volts (virtual ground).
5) The output will do whatever it has to, to make both inputs the same potential.


Take the circuit Mike posted and put some values in there… Rg=Rf=1K and Vin=10V

The plus pin is tied to ground. This means (#4 above) the – (inverting input) will want to be at zero volts.

Since the inverting input want to be at zero volts, that means that Rg has Vin minus the inverting input voltage across it, but because the – pin is at zero, the voltage Vg appears across Rg, so Vin/Rg= I1 or 10V/1K=10mA

Since current can’t flow into either of the input pins (#1 above), and it has to go somewhere, the only other path is through Rf, so I1 = I2 =10mA

10mA through 1K will be 10V, but current flows from plus to minus and the “+” end of the resistor is at zero potential. That says that the “-“ end of the resistor has to be 10 volts below that potential, or the output of the opamp is at -10V (#5 above).


With “real world devices”, there is some input bias current, input offset voltage, max output current, etc. but for learning the basics, just apply the rules.

 

[audioguru]

Forget the 741. It was a poor excuse for an op amp when it was created 30 years ago.

No.
The lousy old Fairchild uA741 opamp was released in 1968 which is 42 years ago. Then most semiconductor manufacturers copied the thing. It was the first monolithic opamp with the frequency compensation capacitor built-in.

Most opamps cannot drive a load (or feedback) resistance that is less than 1000 ohms. Most opamp datasheets are spec'd with loads of 2k and 10k ohms.

 

[electro_Rookie]

Thank you everyone for the information. My knowledge of how the op-amps operates is much better now thanks to everyone. I have just started learning about op-amps so this will be a great help.
Heath

 

[crutschow]

Forget the 741. It was a poor excuse for an op amp when it was created 30years ago. It is the VW Beetle of opamps.

Well, I beg to differ (somewhat). It was an advanced monolithic op amp when it first came out, with built-in frequency compensation, input over-voltage protection, and short protection on the output, much of which the competition didn't have (see the old 709 for example). It was somewhat noisy but it was low enough for many applications other than high fidelity audio. And it was not rail-to-rail but that was not available on any op amps at the time. So, like the Beetle, it had its faults, but still was the most popular item if its kind for many years.