The LM324n has a gain-bandwidth product of 1 MHz, so I guess it works from 0 Hz to 1 MHz, is what I understand true?

Its slew rate is 0.5V/us which means that it can generate a 500 kHz output signal, is that true too?

If the above are both true, why when I apply a 75 kHz 12V peak signal to Vin+ with +12V DC supplies and Vin- is connected to Vout so it should work as a buffer, is the output signal destroyed?

  • 1
    \$\begingroup\$ Unity Gain Bandwidth Product is the maximum bandwith at 1:1 (i.e., unity) gain. At 10x gain you get a 10th of the bandwidth, at 100x gain, 100th of the bandwidth. \$\endgroup\$ – Majenko Oct 10 '14 at 19:00
  • \$\begingroup\$ yes thank u ,so the buffer connection is a unity gain i think no amplifying at all what i feed in , i get it as it is in feed out so i guess the maximum input signal frequency in that case should be the GBW 1 MHZ , am i right? \$\endgroup\$ – Ahmed Zain El Dein Oct 10 '14 at 19:04

There are a couple of problems with what you are trying to do:

  1. The slew rate is not sufficient to pass a 75kHz 12Vp signal. The max rate of change of your input signal is the max of the derivative of 12*sin(wt) or max(12*w*cos(wt)), which is 12*w or 5.6V/us. Look at Fig 11 in the datasheet.

  2. The max output signal swing is (typically) the supply rail minus 1.5V, so a buffer with +/-12V supplies will not be able to reproduce a 12Vp sinewave.

  • \$\begingroup\$ thanks i got it , just one question supply rail minus what does it mean? even if the 75KHZ is inside its bandwidth 1MHZ GWB but due to being outside its slew rate that is why it doesn't work . i understand u right ? \$\endgroup\$ – Ahmed Zain El Dein Oct 10 '14 at 20:24
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    \$\begingroup\$ @AhmedZainElDein If I understand your question (the supply rail minus 1.5 statement) means that if you give the amplifier a 12V supply, the maximum signal amplitude at the output is 12V-1.5V or 10.5V peak. Other than that your understanding is correct. A slew rate limited sine wave looks like a triangle wave, possibly with much smaller than expected amplitude. \$\endgroup\$ – John D Oct 10 '14 at 21:08
  • \$\begingroup\$ this 1.5 is due to 0.7*2 to operate the amplifiers inside ? \$\endgroup\$ – Ahmed Zain El Dein Oct 10 '14 at 22:13
  • \$\begingroup\$ It can drive the output only to 1.5 Volts less than the +ve rail. It can drive the out almost to the -ve rail. How close depends on load. The datasheet tells you how close for various load and supply voltage values. you \$\endgroup\$ – Russell McMahon Oct 10 '14 at 22:24
  • \$\begingroup\$ THANK U :) one more thing GBW which is 1MHZ , does it means at unity gain this op amp can generate signal whose frequency varies from 0 to 1 MHZ? \$\endgroup\$ – Ahmed Zain El Dein Oct 11 '14 at 0:19

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