You say nothing about your circuit, so we don't know. Your drive current is insufficient to charge this capacitor quickly, so the result is similar to an RC delay. The 6n137 seems designed to pull down, not up. Is this a P channel Mosfet? If not the 6n137 seems a poor choice. If you want us to figure, more details are needed. All your current into the ~15pF is needed instantaneously on the gate if you want a square wave. The ideal switching circuit has a LV rail with current behind it, and switching is done by a fast device, e.g. Mostly agree with Declan (and, especially, more details are needed!).Is this switch high side or low side? (Answer by Declan Moriarty) I have also seen pulse transformers driven by oscillators making a supply to switch bipolars when required. But, depending on the transistor you're driving, its gate capacitance may be as much as several nF (as opposed to pF). The limited current capability of the 6N137 (along with the related pullup current capability) may be far too low to obtain square wave gate drive of your transistor. Your mention of a ramped gate voltage tends to confirm this. Investigate the gate drive requirements for your transistor in detail. You can use 6N137 together with driver."Gate charge" datasheet plots can be very helpful for this.Additional comment: to summarize, you may need a much higher current capability gate drive circuit (both sourcing and sinking) in order to obtain reasonably fast switching and gate drive waveforms. It provides 9A peak current and is capable to operate in quite high frequency range. I assume the 6N137 has an open collector output.There is a modification with 2-fet output cascade. What's the value of the pull-up resistor? If the gate resistor is too large - or maybe missing! - you will have a large time constant together with the gate capacitance and that could result in a ramp voltage at turn on.
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