PSRR of 2SK1058/2SJ164

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U130421

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Joined 2015
These lateral MOSFETs are very commonly used on this forum, has anyone noticed the very poor power supply ripple rejection? Any solution? The feedback is bringing down to -90 db only if the gates are driven by high impedance.


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D

DF96

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Joined 2007
myweb.tiscali.co.uk/g8hqp/
nigelwright7557 said:
The output voltage is defined by the gate voltage not the power rails.
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Surely the output of any FET is defined by gate and drain voltages? FETs are not high output impedance devices like BJTs.

PS I thought Early effect applies to BJTs alone - it expresses how far short the transistor falls from being a perfect current source.
 
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Elvee

Elvee

Member
Joined 2006
kokoriantz said:
These lateral MOSFETs are very commonly used on this forum, has anyone noticed the very poor power supply ripple rejection? Any solution? The feedback is bringing down to -90 db only if the gates are driven by high impedance.
Click to expand...
Two distinct mechanism play a role here, and they shouldn't be mixed up: one is the equivalent of the Early effect for FET devices, the channel modulation, which happens under all conditions, including DC, and apart from massive feedback or cascoding has no solution, the other is the Miller capacitance effect between D and G, which is frequency-dependent and can easily be cured with a low drive impedance (the cascode also works in this case)
 
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