From: John Popelish
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Subject: Re: connection between low currents and high phase shifts?
Date: Sun, 29 Sep 2002 03:44:38 GMT
NNTP-Posting-Date: Sat, 28 Sep 2002 23:44:38 EDT
"Ron J." wrote:
> Hi everyone...hopefully someone can clear up this for me. I've
> seen in a lot of place frequent references to high phase shifts
> made in conjunction with low levels of current. For example, darlingtons
> are supposedly prone to oscillations because in the input bjt of the
> pair, low emitter currents lead to high phase shifts (this confusing
> statement is taken out of the H&H book, btw). I also see frequent
> references to low currents/high impedances leading to low phase
> shifts when dealing with current feedback topologies. As usual, the
> sources where I see this never really get into any significant detail,
> so maybe someone here knows what the heck this is all about?
> Thanks in advance.
Think time constants.
When low currents are involved this implies high resistance, so
inductance has little involvement, because inductive time constant is
L/R. But capacitive time constants are R*C, so high impedances imply
that all capacitances produce longer time constants and capacitor
charging currents have a significant effect on node voltages. Thus,
high impedances imply more phase shift at any given frequency than if
the currents were larger (resistances lower). At very high currents,
the inverse effect takes place. Capacitive time constants and
currents become negligible, but tiny inductances like those caused by
component leads and board traces can produce significant phase shifts
and inductive EMFs.