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LM741: BALANCING SIGNAL

Prodigy 165 points

Replies: 22

Views: 1287

Part Number: LM741

To hom it may concern

I'm using two LM741 to make a triangle signal, but the signal is umbalanced respect zero. Positive part is 1.8 V and negative part is 2.8 V in the output of the first ampop and simmilar to the output of the second.   I rode about 1,5 pins offset balance, but as I understand it is just to balance feeding.  I'm using a PC's power supply where I get 12 V and -11.5 V.   The feeding balance is opposite of what I'm getting.  What do you suggest to do to  get a balanced output (simmilar positive and negative parts of the signal) respect zero?

Thank you for your valuable time

Luis

  • In reply to kai klaas69:

    I noted the feedback trough a 47 K resistance.  I'll make some tests.  So to shift down output I'd need to feed V5 with a lower than zero voltaje?  Would it work to shift it in the original circuit?

  • In reply to user5866636:

    Hi Luis,

    in my last simulation I have set V5=1V. This shifts up the triangle wave. A negative V5 would shift it down:

    Please notice that I have taken a supply voltage of +/-12V. When using an unbalanced supply voltage you might need V5 to recover the balance of output signal, like shown in this example:

    But I would not use an unbalanced supply voltage, because this will make the triangle become asymmetrical, as shown in this example:

    It's best to have a well regulated and stable bipolar supply voltage which is balanced. Take care, PC supply voltages can be very noisy and full of ripple. I wouldn't use them for this circuit.

    In your original circuit the triangle wave is unbalanced because the LM741 shows different slew rates for the rise and the fall of output voltage. The LM741 is totally overstrained in this circuit and will not show a stable performance. Slew rates depend on several factors like temperature, etc. . So, you might need to adjust the shifting all the time when using this circuit. And the frequency will not be stable either. I wouldn't use your original circuit.

    Kai

  • In reply to kai klaas69:

    Thank you very much for your advices, I'll make some tests. 

    I'd like to know which are the 'overstrain' conditions in a lm741. 

  • In reply to user5866636:

    Hi Luis,

    by "overstrained" I mean that the LM741 is much too slow for this circuit. Have a look at your original circuit again: U6 is actually meant as a square wave oscillator. With a proper chosen OPAmp, which is fast enough, the output signal of U6 would look like this:

    You can see that the output signal is a square wave. But now see what the LM741 is doing in the same circuit:

    The edges are so slow that the output signal looks like a triangle wave and not a square wave as it should. So, the LM741 is much much too slow for this circuit.

    For such a square wave oscillator an OPAmp with a high slew rate must be chosen. The LM741 has a slew rate of 0.5V/µs. But the TL082 has a slew rate of 13V/µs. So, the TL082 is 26 times faster!

    Kai

  • In reply to user5866636:

    Hello Luis,

    I would just like to add that TI has a very helpful resource on the topic of slew rate, in case you have any confusion regarding this topic. The linked videos and slides explain slew rate, the effect of temperature on slew rate, how slew rate distortion changes with bandwidth (see full power bandwidth), and the effect of common mode voltage changes on slew rate. The first and second videos/slides are most relevant to your issue.

    training.ti.com/ti-precision-labs-op-amps-slew-rate-1

    Please let us know if you have any more issues.

    Regards,
    Daniel
  • In reply to kai klaas69:

    So if I designed with LM741 or even UA741, if I change to TL082 the circuit would do another thing?. The problem is that I didn't see locally this device (I buy to Steren and other local stores), so I prototype with local components. When prototype be ready (a modular inversor for high capacity), I want to do a lot of this circuits, and I know that surely I can get TL082 from Mouser or Arrow, but now I have not this components. What is the average price relation between LM741 and TL082? In other question, do you handle competitive IGBTs?
  • In reply to user5866636:

    Luis,

    If you want to get an idea of the price relationship between parts, go to "ti.com" and then "Order Now" followed by "Buy ICs." You can then enter your part number in the search bar and get a price.

    If you would like to test your part(s) before implementing your design, you can request samples. To do this, go to "ti.com" and "Order Now" again. But, this time click on "Request Samples." You will have to fill out a request form and have it approved.

    Regards,
    Daniel
  • In reply to Daniel Miller56:

    It says that my account (new) is not ellegible for samples, and if I want to know why to do click in a link, which is not available.

    If you know what are the requisites for it, I'll consider TI components for the design.   Indeed, I have a provider in China for printed circuit boards, I'd like them to build the complete circuit, but I suppose then you would provide them.  Have you a dealer in China?

  • In reply to Daniel Miller56:

    I saw your catalog and I didn't see dip packages with long pins, just surface mounting, or have you the option or converters from smd to dip? The technology I have lacks about it (needed a robot to smd) and it would be the way of not extrictly depending of another providers just in case there were problems importing.
  • In reply to Daniel Miller56:

    I have another project who involves 16F628A from microchip. Have you a microcontroller which price - advantage be better than this one? Indeed, for the inversor I'm doing I'll make the digital version based in a uController, and I have thought in the same model of microchip, but maybe you have something competitive.

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