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The KSC3503 might be "too good": it has a Cob under 5pF, and depending on other components, it could result in instability.
This has already been observed with other BFxyz video transistors, but the remedy is very straightforward: a small additional capacitor, like 3p3 or 4p7.
I have tested and used KSC3503/ KSA1381 and Philips BF series and also Sanyo Video transistors in both BJT and MOSFET version but MOSFET version donot require additional capacitor of 4.7pf but the BJT version need them to get clean output.
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