[SI-LIST] : Miller Capacitance

hanna@dalsa.com
Thu, 17 Sep 1998 09:02:08 -0400

Hello All;

I currently have a situation in which Miller capacitance is significantly
degrading signal performance. The situation is as follows:

I have a BJT preamp configured in a push-pull configuration. This preamp
(two of them, actually) drives a N-channel MOSFET charge-drain circuit
(i.e. one BJT push-pull for the drain, one for the charge MOSFET). The
BJTs are used to supply sufficient voltage to overcome the MOSFET Vth (3.5V
typ, no max spec), and to turn the MOSFETs fully on, hard. There is
currently no issue with the charge MOSFET (drain is a constant voltage wrt
gnd). However, due to the changing voltages on both the drain and the gate
of the drain MOSFET, the Miller capacitance effect is essentially damping
the gate signal on the drain MOSFET by a factor of 2x. Raising the source
voltage for the drain BJT-preamp is not a solution, as I am now at 12V and
can only go up to 15V (assuming the 2x factor remains constant, that will
only give me an extra 1.5V, not enough).

My question: how can I overcome this, if at all? I need to get about
another 3V out of the BJT preamp. Please advise.

Regards,
D. J. Hanna

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