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Original question:

OPA564: Output Stage

OPA452: Voltage Peaks OPA445 driving Capacitive Load, Feedback Loop Compensation

Daniel Wagner
Prodigy 60 points
Part Number: OPA452
Other Parts Discussed in Thread: TINA-TI

Hello,

I need to drive a 10uF capacitve load which acts as a low pass filter to block the dc current.

The OPA452 amplifies the voltage of a rectangular signal and 2 transistors in a push pull configuration amplify the current. The circuit is also stated in the datasheet on page 12: https://www.ti.com/lit/ds/symlink/opa452.pdf?ts=1674827094813&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA452

I chose a value for the capacitor Cf so that there are no interefrences in the voltage when driving a light load with high currents. But if the load is for example 30kOhms you can still measure peaks in the Signal. If I increase the value for Cf the signal is not rectangular anymore and gets more round as the high frequencies are not amplified enough anymore.

Here is a picture of the problem:


What can I do to compensate that better and still having a rectangular looking signal?

I hope somebody can help me.

  • 0
    Guru 140200 points

    Hi Daniel,

    I see two issues:

    1. A fast slew rate into a big capacitance may mean a short circuit condition with a huge load current spike.

    2. The huge capacitive load of 10µF very probably destabilizes the circuit and causes oscillation (-> scope plot).

    A tipp: The scope plot shows aliasing artefacts because your time base is set too slow. Choose a deflection time of 1µs per division and you will see the oscillation.

    I would run a "phase stability analysis" in TINA-TI with the

    OPA452 TINA-TI Reference Design (Rev. B)

    to see how the circuit can be stabilized.

    Can you show a schematic?

    Kai

  • 0
    TI__Mastermind 48026 points

    Hi Daniel, 

    Daniel Wagner said:
    What can I do to compensate that better and still having a rectangular looking signal?

    Could you provide us the design requirements or schematic so that we can recommend the best approach? We need to know the supply voltage, input signal, input signal frequency, rising/falling edge timing requirements, output swing, load currents, complementary current boost stage components etc. for the simulation and compensation. 

    As Kai pointed out, the existing circuit is likely oscillating due to 10uf capacitive load at the op amp's output.  We also have OPA593 which it may meet your requirements without the extra current boost stage. Please let us know 

    Best,

    Raymond