another 741 question

[frosty12345]

hi guys, 1st post so please be gentle ive had a quick search but cant find a definate answer.

im trying to build a 741 opamp with a gain of 1k.

its wired up as per the diagram with both offset pins grounded, and supplies of +-15v

if i supply the device with +-10mV square wave, im expecting +-10V.

i get a steady -14V which suggests the device is at its supply limit, but it does not oscillate between +-14v.


the device also gets really really hot (hot enough that i burnt myself on it earlier).


does anyone have any suggestions?

thanks
alex

 

[Optikon]

hi guys, 1st post so please be gentle ive had a quick search but cant find a definate answer.

im trying to build a 741 opamp with a gain of 1k.

its wired up as per the diagram with both offset pins grounded, and supplies of +-15v

if i supply the device with +-10mV square wave, im expecting +-10V.

i get a steady -14V which suggests the device is at its supply limit, but it does not oscillate between +-14v.


the device also gets really really hot (hot enough that i burnt myself on it earlier).


does anyone have any suggestions?

thanks
alex

do not ground your offset pins. Looking at a 741 schematic from the datasheet. if you ground the offset pins, you are putting 15V across internal 1k resistors in 2 places. That makes about a 1/2 Watt power dissipation. Thats why the part is hot. This also screws up operation.

If you do not want to use offset pins - leave them OPEN circuit.

The part is too stressed, replace it and try again with offset pins open.

 

[Nigel Goodwin]

A 741 is also a pretty poor opamp, wanting a gain of 1000 makes it a very bad choice.

 

[frosty12345]

thanks for the help guys. ill give it a go tomorrow.

is there another opamp you would reccomend instead? (preferably just a quick swap).

thanks
frosty

 

[frosty12345]

btw, should i connect the 2 offsets together? or just leave them NC?

thanks again

 

[Nigel Goodwin]

btw, should i connect the 2 offsets together? or just leave them NC?

Just leave them disconnected, pretend they don't exist!.

Most opamps are drop-in replacements, as they use the same pinout, the TL071 or TL081 are good opamps at good prices and will drop straight in.

 

[audioguru]

You will probably need an offset adjustment to cope with the offset voltage of the squarewave or of the opamp. The offset pot's connections and value is different with a 741 or TL071 opamp, see their datasheets.

A 741 opamp won't have a very good squarewave output with a gain of 1k if the frequency is 50Hz or higher. A TL071 has a much better frequency response but even it will not have a very good squarewave output above 500Hz with its gain so high. Therefore it will be better to split the gain, two stages of 31.6 or three stages of 10 will have a much better high frequency response.

 

[akg]

A 741 is also a pretty poor opamp, wanting a gain of 1000 makes it a very bad choice.

may be he just want to play with an op-amp, in that case 741 is ok.

 

[frosty12345]

thanks for all the advice, seems to be working quite well for this task.