741 Newbie

[miscreant]

hey i'm new to 741n and most other stuff. i trying to make an adder, subtractor and comparator circuits. how much maximum voltage can their be at + terminal.

if voltage is 12v at + terminal and Rf/Ri=10 then putput voltage will be 120 V ??

what resistor value should i use if i want to make an inverter with +10 V input and -10v for output. Should it be Rf=Ri=10Mohm or 10kohm ??

please help...

 

[miscreant]

resistor values

what resistor values should i be using ? some circuits use 10k some 4.7Mohm etc. What range should the values be ? because datasheet says maximum current is 250nA. To be on the safe side it should be 200nA.

If i input 10volts then resistor to use will be 50 Mohms !!!!

 

[Optikon]

hey i'm new to 741n and most other stuff. i trying to make an adder, subtractor and comparator circuits. how much maximum voltage can their be at + terminal.

if voltage is 12v at + terminal and Rf/Ri=10 then putput voltage will be 120 V ??

what resistor value should i use if i want to make an inverter with +10 V input and -10v for output. Should it be Rf=Ri=10Mohm or 10kohm ??

please help...

Not quite. Keep in mind, you will not be able to output a voltage larger in magnitude than the + supply rail nor more negative than the - supply rail. In many cases (such as the 741) you won't even be able to get within a volt of them. If you need to output very closely to the supply rails, look for rail-to-rail output type opamps.
I assume for your inverter, you are trying to build a unity-gain inverting amplifier. Whether or not you use Rf=Ri=10M or 10k depends on what other characteristics you want. If you use the 10M, the input impedance will be higher so it will not load the previous stage (or source), whereas the 10K it will load somewhat.

On the otherhand, the 10M resistors will cause a voltage output error due to the input current flowing through that resistor. Noise is another issue.. but I get the feeling you just want it to work and 10M or 10K will work.

 

[Optikon]

Re: resistor values

what resistor values should i be using ? some circuits use 10k some 4.7Mohm etc. What range should the values be ? because datasheet says maximum current is 250nA. To be on the safe side it should be 200nA.

If i input 10volts then resistor to use will be 50 Mohms !!!!

The resistors you should be using are the ones that will make your design work. This is why you see some circuits using different values... it depends on what you want to do! ...as well as the performance you need.

250nA is what on the datasheet? Iib? input bias current? You need to be a bit more clear. The datasheet will spec maximum bias current and you just need to know this. Then, based on whatever resistor this flows through, that current will cause a voltage drop (V=IR). IF that voltage drop bothers you, there are ways to avoid / fix it.. but maybe you dont care! If you have 10K on input, thats only a 2mV drop and from a 10V signal thats only.02%

Ask more questions if you are still unclear..

 

[miscreant]

thank you. i'll make it more clear.

Input bias current is max 250nA. Now, like i said when using as inverter, i'll aplly +10v at - input. to get 250na, i'll need 10v/250na=50 Mohms ? is that true ? or because of the Rf resistor current will be too small after subtracting from input.

also there is another comparator LM311. will the same resistors give the same output with both 741 and lm311 when used as inverter because lm311 has lower bias current.

 

[mstechca]

I can partially help you.

First, we must determine your power supply. Since you are using negatives, take the highest voltage that you are applying to the circuit, take the lowest voltage, and subtract the two numbers. Ignore the negative sign if you get it.

Now, the easiest way to pick a voltage is to use a voltage divider, and your knowledge of fractions.

Let's assume that your supply is 9V, and the lowest number is ground (0v). the number is now 9.

and lets say you wanted 6V at the input.

so you need a resistor pair that matches the fraction 6/9.

Of course, 6 ohms from + to input and 9 ohms from input to ground (or -ve in your case) will work, but the major problem is that you will not only waste power, your resistors may get over heated. why? because 6 + 9 = 15, and current = voltage / resistance, and 9 volts over 15 ohms is half an amp (500ma) and for watts, 9 * 0.5 = 4 1/2 watts!

but, we must understand how much current the amplifier requires "Just" to turn it on. Once we obtain that current, and not go too much over, you will be using the same battery over and over without throwing it away or recharging it.

Now if we add a couple of 0's, we can try 6 megaohms over 9 megaohms.
and now if you use the above equation, you will get between 5 and 500na.

I still think thats a small amount. I would go between 1ua and 100ma.

good luck

 

[Optikon]

thank you. i'll make it more clear.

Input bias current is max 250nA. Now, like i said when using as inverter, i'll aplly +10v at - input. to get 250na, i'll need 10v/250na=50 Mohms ? is that true ? or because of the Rf resistor current will be too small after subtracting from input.

/quote]

There are two currents to consider. The current that flows through your Rf & Ri (which you can control by selecting resistors)
The second current is the one that the opamp will sink into its input terminals (250nA in your case). You dont need 50Mohms..

For example, consider inverting unity gain opamp.
Ri = 1K
Rf = 1K

If input = 10V, then current flow through Ri will be 10/Ri = 10mA.
If we assume opamp input current is small (like 250nA) then this 10mA
also flows through Rf causing a voltage drop of -(10mA * 1K) = -10V
which is the output you wanted. +10V in makes -10V out. You have an inverter of sorts.

Now seprately, consider that 250nA flowing through the 1K resistor, this will cause a voltage drop of 250uV through the 1K resistor and is a source of error. But this voltage error is so small, you might not care and so in that case, the input bias current didnt matter much to you.

The datasheets are not trying to tell you what resistors to use. Use whatever ones you want and then look at the datasheet to help calculate the effects.

As a completely seperate topic, yes you can use the Lm311 to make an inverter if you want.