[time-nuts] Power Supply Noise Affects Thunderbolt 1 PPS

ed breya eb at telight.com
Tue Feb 28 04:47:21 UTC 2012


The PNP bipolar and P-channel MOSFET architectures do provide the 
best low-dropout performance, but as I understand, do not provide the 
best HF line rejection. Looking at the overall circuit - a high gain, 
band-limited amplifier driving a "P" pass device puts it in a 
common-base (or -gate) mode, with respect to the amplifier's output 
port. It provides excellent DC performance, but high frequency noise 
(beyond the control loop bandwidth) at the emitter (or source) has a 
large voltage gain available. This is of course for positive supplies.

An "N" device operates as a follower, and needs more overhead voltage 
to activate the gain as a common-collector (or -drain) amplifier, but 
is able to reject the input supply noise much more effectively, given 
the same band-limited control loop amplifier.

So, for best HF noise performance where the input noise may be large, 
it's best to use a follower or shunt regulator topology, despite the 
lower efficiency - unless efficiency is more important.

Ed





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