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Question about op amps?

Discussion in 'Electronic Basics' started by Peyter, Nov 17, 2003.

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  1. Peyter

    Peyter Guest

    I have a few questions about the LM741CN, please answer
    even if you don't know all the answers.

    1. In my circuit which I am checking this 741 has a 10nF cap
    between offset-null (pin5) and output (pin6), so what is the
    purpose of this? (the other offset-null is unconnected)

    2. The positive supply is a stable 12v from an L7812, but the
    minus supply is very strange, it is connected to 0v through
    a diode, and to the negative supply (which is not regulated
    it seems) through a resistor of 560 ohms, what is going on?

    3. On the same circuit board the is an LM324N with 4 op
    amps, two of which are left unused. Is there any reason
    why a 741 would be used when there are 2 free op amps
    near by? what could be the reason.
     
  2. 1. The purpose of that cap is frequency compensation. It prevents
    oscillation.

    2. Most likely, that diode is a zener diode. In that case the cathode must
    be connected to ground (0V), and the anode to the minus supply of the opamp
    and the 560ohms resistor. It stabalizes the minus supply voltage of the
    opamp to the zener voltage.

    3. Maybe the compensation in question 1 (although there are other ways to
    do this)? The opamps in an LM324 don't have offset input terminals.

    HTH,
     
  3. Peyter

    Peyter Guest

    But isn't the whole point about the new generation of op-amps such as
    LM324 is that they do not oscillate anyway because they are *better*
    designed ?
     
  4. There are many problems to solve in designing the perfect opamp. The
    LM324 approaches perfection in only a couple of these ways, compared
    to the 741. It uses less supply current per section, its input bias
    current is lower, and its offset voltage may be a bit lower. Its
    input common mode range is larger, and its output swing is also
    larger. However, its output distortion is worse (this is where the
    tradeoff to save power was made), and its speed is no better. Its
    stability margin is about the same.
     
  5. Peyter

    Peyter Guest

    you were right, these diodes where all zener ones, they look like
    normal diodes (transparent orange) which confused me, but I
    now confirmed it with a voltmeter. Maybee I can now start to
    make some more sense of this mess.

    Thanks!!
     
  6. Compensation comes with loss of bandwidth.
    The LM324 for instance has an open-loop cut-off frequency of 1 or 2Hz!
    These opamps are unconditioanlly stable, but have a limitted bandwidth.

    Other opamps are not compensated, or compensated for a certain minumum
    gain. If you want to use the opamp for lower gains, you'll have to
    compensate externally.

    According to the datasheet, the LM741 is also unconditioanlly stable, just
    like the LM324. The only reason for the 10nF cap I can think of now, is
    narrowing the bandwidth to suppress unwanted frequencies.
     
  7. Peyter

    Peyter Guest

    I don't know if it makes any difference but it was a 0.1 nf cap, sorry I interpreted
    '101' in the wrong way, i.e. it sould be 10 + one zero = 100pF = 0.1nF
    What I still don't understand is why they are using the LM741 on the pcb and then
    leaving two unused op-amps in the LM324, when all that it is is a circuit for
    a 15V 3A regulated variable voltage/current psu.
     
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