From: firstname.lastname@example.org (Ron J.)
Subject: Re: connection between low currents and high phase shifts?
Date: 29 Sep 2002 07:14:47 -0700
NNTP-Posting-Date: 29 Sep 2002 14:14:47 GMT
Thanks for the info, guys. I still can't see how darlington pairs
will oscillate though. There aren't enough capacitors in there to
cause this much phase shift. That statement from H&H about instability
in these compound transistors is really confusing.
> These statements are really referring to the relation between bandwidth
> and current. The BW is increased (usually) with higher current. Lower BW
> means more phase shift.
> Higher current cases increased BW principle for two reasons.
> For example, if a transistor stage requires say, 3V across its collector
> resistor, then at 100ua this would be 30k. If the current was 1ma the
> resistor would be 3k. Assuming that any capacitance on that node is
> constant the BW at that collector would be 1/2.pi.RC, which is obviously
> 10time worse for he 30k resistor compared to the 3k resistor.
> The second condition is the case where a transistor has a transistor as
> a current source load. In this case the "resistance" at the collector
> node is an active one is given by ro=VA/Ic, where VA is the early
> voltage. The gain remains constant with Ic, i.e. VA/Vt, where Vt is
> q/KT. However, it can be seen that increasing Ic reduces ro, hence its
> ro.Cload will result in a higher BW as in the resister load case.
> So, the summary is to get large BW and low phase shifts, one needs low
> resistances, and low resistances mean high current.
> The catch is that you cannot keep increasing current to get better BW.
> At some point the transistor performance degrades with two much current.
> This requires putting more devices in parallel to share the current.
> This of course increases the capacitance, which gets you back to where
> you started.
> Kevin Aylward
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.