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OPA2613: can open-loop transfer function of amplifiers be used in large signal input to calculate output of amplifiers? and the relationalship between open-loop transfer function and SR(swing rate)

user4564936
Prodigy 20 points
Part Number: OPA2613
Other Parts Discussed in Thread: TINA-TI

Hello, It's a closed-loop transfer system with a 10V/V closed-loop gain. if my output signal now meets the limit of the SR, the SR is enough, and SR=2 * Pi * f * Vop. If the differential signal input is small signal such as sawtooth wave of 100Hz 100mV. We can calculate the output accurately according to the open-loop transfer function and negative feedback parameters. If the differential signal input is large signal such as sawtooth wave of 100Hz 2V, can I calculate it according to the transfer function????

The transfer function of the operational amplifier is the small signal transfer function. Taking the three-stage amplifier circuit as an example, the larger the differential signal input is, the larger the charging current to the second-stage miller capacitor will be. The charging current can be expressed by gm (transconductance) *vin (input differential voltage). If it is a large signal, the charging current of the second level miller capacitor will be saturated. The charging current of 100Hz 2V and 100Hz 4V of the second level miller capacitor will be the same (right?)., now that it's not linear, what's the transfer function? So is my understanding right?

  • 0
    TI__Genius 16725 points
    Hello,

    Today is a TI holiday. Due to the holidays our responses are delayed. We will get back to help you find an answer at the end of the holidays.
    Have you seen our Precision labs videos? training.ti.com/ti-precision-labs-op-amps
    You can run a TINA simulation to test out the output of the mode. The precision labs videos specify how to use TINA to calculate the output with your own schematic.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    TI__Genius 16725 points
    Hi,

    Were you able to find the answer to your question or do you still need help?

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 20 points

    i don't find answer and still need help

  • 0 in reply to user4564936
    Guru 140200 points
    Hi,

    all this Miller capacitance stuff is an idealization and will only tell half the truth. A true chip behaves much more complicated and far more issues beside the Miller capacitance have to be taken into consideration. I would run a simple TINA-TI simulation with the OPAmp in question. That's the best and fastest way to get the job done.

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 16725 points

    Hi,

    I agree with Kai, the best method to understand the opamp and its outputs would be to use the TINA model as I mentioned before.
    As for your transfer function. The total transfer function of an opamp is the superposition of its large signal and small signal models.
    In the large signal or DC model, all the caps are replaced by open circuits and inductors by short circuits. This makes the circuit linear. The small signal model of AC model takes into account the non linear components such as the capacitors. The superposition of both the AC and DC model combine to give total system response.

    Using your small signal transfer function to apply for larger AC signals to calculate your output should be fine. However to understand what the non-ideal output of the opamp will be, you will have to use the TINA Spice model.

    Thanks
    -Karan