[SI-LIST] Re: Bypass vs Decoupling capacitors
- From: "Brown, Mike (Austin, TX)" <mibrown@xxxxxx>
- To: <weirsp@xxxxxxxxxx>, <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 25 Aug 2004 13:21:01 -0500
A history lesson, as I remember it:
Back in the bad old days of vacuum-state electronics, the power supply
source impedance was not as low as we are accustomed today. Regulated
supplies were the exception, rather than the rule. The flow of, say,
power amplifier (PA) load current in the power Z would modulate VCC to
low level stages, providing a feedback coupling mechanism that often led
to oscillation. The solution was to put an RC (occasionally LC) LPF in
the VCC feed to the low level stage(s) plate circuit, thus "decoupling"
it from the feedback caused by the PA I*Z on the power supply output.
In practice, each stage except the PA usually had its own LPF network.
The cap of the RC network was known as the decoupling cap, for obvious
reasons.
That cap kept the low-level AC current loop local to that stage, and
provided attenuation to the AC from the PA I*Z, which was still present
on the VCC terminal of the power supply.
With the advent of low-Z (regulated) power supplies, the series R of the
LPF has been reduced to near zero, and the series element of the LPF
becomes the distribution impedance of the power distribution system
feeding the capacitor. In effect, the decoupling capacitor causes AC
current to "bypass" the PDS impedance.
To me, bypass and decoupling caps are two different names for the same
circuit function when used in the PDS. =20
The cap across the emitter/cathode resistor to increase AC gain of a
stage is, in my mind, always a bypass cap - more correctly a cathode or
emitter bypass. (Or a screen bypass, in the multi-junction vacuum state
devices having same).
Regards
Mike=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Wednesday, August 25, 2004 11:11 AM
To: a.ingraham@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Bypass vs Decoupling capacitors
Andy,
Decoupling, as in anti-coupling, is used to isolate circuits. Back in
the=20
DTL / RTL days with point to point, or two-sided board wiring, that=20
isolation was important, and we had ferrite beads on boards and at the=20
power entry to boards, and between the analog and digital power feeds.
The=20
decoupling network was a LPF, much as Ray described.
The advent of low-impedance planes did away with the need for series=20
isolation in most digital circuits, so with the series element gone, all
that was left of the decoupling network was the shunt capacitor on the
load=20
side which looks and acts like a bypass capacitor, because it is one.
But=20
the decoupling term got carried forward.
Since, I am a fan of putting the series impedance back in decoupling=20
networks as a way to dramatically reduce cost and improve EMC
performance,=20
I like to use the terms the way they were 30 - 35 years ago.
Regards,
Steve.
At 11:42 AM 8/25/2004 -0400, Andrew Ingraham wrote:
> > Bypass caps are used to eliminate (short out) resistors during ac=20
> > operation. An example would be to bypass an emitter resistor in=20
> > order to increase the voltage gain of an amplifier.
> >
> > Coupling caps are used to block the direct current, but still allow=20
> > ac signal to pass. An example would be to couple multiple stages of
> > an amplifier.
>
>Spoken by what must be an analog guy! Who else would have remembered=20
>bypassing cathode or emitter resistors, and inter-stage coupling=20
>capacitors?
>
>Somewhere along the way, the term "decoupling" (as in decoupling=20
>capacitors) seems to have taken on some of the meaning of "coupling",=20
>in that decoupling capacitors actually couple some current (usually to=20
>ground in modern digital
>use) ... thereby doing something similar to what Steve Weir said, which
is
>to isloate one device from another, or a load from a power source. I
used
>to wonder why a capacitor, whose function is to couple signal, would be
>called a decoupling device when it itself does nothing of the sort.
But
>that's what we call it these days.
>
>Regards,
>Andy
>
>
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