LM741 connected to 5V/0V(gnd) supply outputs 4V, why?

[eranrund]

wierd behaviour of lm741

Hello people.

I'm connecting the 741 _only_ to a 5V/0V(gnd) supply (pins 4 and 7) and for some reason it outputs 4v on the output pin (6)
Why is that?

I've tried seveal 741's just to make sure its not fried or something
Am I missing something here?

Thanks,
Eran.

 

[nicleo]

wierd behaviour of lm741

Did you connect any signal to the input (inverting and non-inverting) terminals of the LM741?

 

[eranrund]

Re: wierd behaviour of lm741

Nope.
But in simulation, even if I do not connect anything, I do not get this stupid 4v..

 

[VVV]

Re: wierd behaviour of lm741

Simulation helps, but it cannot replace the actual circuit.
You can never leave the inputs open, otherwise the input stage transistors cannot be properly biased. Their bases are supposed to be biased through external circuit elements.

 

[eranrund]

Re: wierd behaviour of lm741

I want to amplify an audio signal. Does this mean that when there is no signal (i.e. nothing is connected to the line-in socket) my amplifier will output its maximum level?

 

[VVV]

Re: wierd behaviour of lm741

No.
You need to bias the inputs using resistors, for example. Use a divider for the + input: say 100kΩ / 100kΩ. The - input should get the feedback: a resistor from the output, let's call it R1 and a resistor in series with a capacitor to ground; let's call this resistor R2 and the cap C.

The + and - inputs will now be biased at about half the supply voltage, that is 2.5V. The + input is obviously biased by the divider. The - input is biased from the output through R1. C ensures R1 and R2 do not form a divider in DC, just AC. Hence, the - input will be at 2.5V, and so will the output.

Now you apply the signal to the + input, through another capacitor, which will prevent your signal source from shorting to ground the DC voltage established by the divider.

The gain of the circuit is, of course, G=1+R1/R2. So if you need a gain of 10, choose for example R1=100kΩ and R2=10kΩ.
The two capacitors should be chosen large enough for the lowest audio frequency to pass through them: 1 to 10µF should do. Their + side is towards the inputs of your opamp.
The output should most likely be AC coupled, so use another capacitor, say 10uF, connected with its + side to the output of your opamp.

So, to answer your question, when you have no audio signal, the output will be at 2.5V DC. The AC signal will be zero with no input, that is, you get no audio out.

Good luck!

Added after 23 minutes:

After a little thinking I decided a picture is worth a thousand words.

 

[XNOX_Rambo]

Re: wierd behaviour of lm741

I don't think the venerable 741 will be happy with a supply of only 5 V.
I belive the minimum is ±5 V, or +10 V in your case.

You can read about basic opamp stuff here: **broken link removed**

"Real men don't simulate." :wink:

/Rambo

 

[eranrund]

Re: wierd behaviour of lm741

it works now, thanks guys!

(still with 5v..)