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On the left is a simple MOSFET switching circuit as may be used in, for example, a class-D amplifier, or as one half of an H-bridge. However, as can be seen, it struggles to switch at the 40 kHz frequency of the driving signal. The issue at hand is that it has too high of an effective input capacitance due to the miller effect. The parasitic capacitance responsible for this is represented by a 1 nF capacitor between the gate and drain of every MOSFET. The 100 ohm output resistance of the MOSFET driver (represented here as a 40 kHz square wave source) limits how much current it can deliver, resulting in an unacceptably slow switching speed, necessitating that a much more powerful driver be used with a correspondingly reduced output resistance (10 ohms would suffice in this case). However, by using the cascode configuration on the right, the miller effect is eliminated, allowing high speed switching despite the 100 ohm output resistance of the driver, removing the need to use a more powerful driver. This also considerably reduces the energy expended by the driver while switching. The circuit on the right demands that only one tenth as much energy be expended by the driver to switch it as the driver would expend switching the circuit on the left.
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