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History of decoupling capacitors

T

Theo Markettos

Jan 1, 1970
0
I was just wondering...

Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design? I'm thinking mostly logic circuits here, though I can
see that audio/RF circuits might have needed them too. I suppose every DC
PSU has a smoothing capacitor doing roughly the same job but I was more
thinking about adding capacitors around the circuit to provide localised
decoupling. Were they necessary in valve (vacuum tube) circuits? If not,
in what technology did they first appear (RTL/DTL/TTL etc)? Was there any
prior use in another field (say RF) and they were later used in logic
circuit?

The earliest boards I can remember (late 70s/early 80s) seem to have them
on, but I haven't seen many earlier boards and can't say I've been looking
for them specifically...

Thanks,
Theo
 
H

Howard Long

Jan 1, 1970
0
Theo Markettos said:
I was just wondering...

Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design?

I'm certain that DTL PCBs used them. The problem is to do with current
spikes that occur when the gates change state. The power supply lines have
inductance and so a fast spike has the potential to reduce the voltage on
the power supply lines sufficiently to take the device out of spec on the
voltage, with the potential for flip flops to unexpectedly change state. By
having localised caps, these spikes are smoothed away.

The caps have to be smallish values as larger caps have too much inductance:
electrolytics, for example, are made by tightly rolling a conductor and
dielectric together, perfect for a bit of inductance.

Valves tend to have less dependency on the stabilised power supplies used by
solid state logic - building stabilised power supplies for the HT valves
require was difficult in the old days, so circuit design took that into
account. Inately therefore, valve designs were generally less susceptible to
current spikes.

Because analogue circuits tend not to have state, and because they don't
usually generate large current spikes, decoupling caps aren't used so much.

In RF and analogue terms, coupling (rather than decoupling) caps are used
extensively, allowing various stages to have differing DC biases. Decoupling
caps are also often used in RF and analogue designs primarily to prevent
gain stages going into self-oscillation.

Kind Regards, Howard
 
R

Reg Edwards

Jan 1, 1970
0
Decoupling capacitors and other components have been used in audio and RF
amplifier circuits for 100 years, ever since the earliest days of valve
amplifiers. They were used and are still used in telephone circuits
although not so obvious. A microhone is a mechanical amplifier and currents
must be controlled and diverted into the right channels.

Decoupling of individual components and pins of integrated circuits are
precautions. Mainly because of the very high frequencies involved and chip
and track physical dimensions being an appreciable fraction of a wavelength.
Or an appreciable fraction of 90 degrees phase shift along tracks and across
ground planes. Or unwanted radiation and crosstalk. You digital
chip-catalogue merchants can escape from analogue only at your own peril.
 
J

John Larkin

Jan 1, 1970
0
I was just wondering...

Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design? I'm thinking mostly logic circuits here, though I can
see that audio/RF circuits might have needed them too. I suppose every DC
PSU has a smoothing capacitor doing roughly the same job but I was more
thinking about adding capacitors around the circuit to provide localised
decoupling. Were they necessary in valve (vacuum tube) circuits? If not,
in what technology did they first appear (RTL/DTL/TTL etc)? Was there any
prior use in another field (say RF) and they were later used in logic
circuit?

The earliest boards I can remember (late 70s/early 80s) seem to have them
on, but I haven't seen many earlier boards and can't say I've been looking
for them specifically...

Thanks,
Theo

The earliest audio/radio stuff used batteries, so had no hum problems,
and used small numbers of tubes with such low gain that circuits were
stable without supply bypasses. I could poke through my old books, but
I'd guess that filter and bypass caps became common sometimes in the
1920's maybe.

John
 
T

Tam/WB2TT

Jan 1, 1970
0
Theo Markettos said:
I was just wondering...

Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design? I'm thinking mostly logic circuits here, though I can
see that audio/RF circuits might have needed them too. I suppose every DC
PSU has a smoothing capacitor doing roughly the same job but I was more
thinking about adding capacitors around the circuit to provide localised
decoupling. Were they necessary in valve (vacuum tube) circuits? If not,
in what technology did they first appear (RTL/DTL/TTL etc)? Was there any
prior use in another field (say RF) and they were later used in logic
circuit?

The earliest boards I can remember (late 70s/early 80s) seem to have them
on, but I haven't seen many earlier boards and can't say I've been looking
for them specifically...

Thanks,
Theo

Bypass capacitors were used with vacuum tube audio and RF amplifiers. One
very important reason was to prevent feedback, and possible oscillation, by
way of the plate supply voltage.

Tam
 
J

Joerg

Jan 1, 1970
0
Hi Theo,

Nowadays the word "decoupling" is use to describe decoupling current
spikes from the power supply. IOW to prevent them from entering there or
causing a momentary sag in voltage.

In the olden days the word also meant decoupling one stage from another,
for example the plate DC level from the much lower gate DC bias of the
next amplifier stage. In that respect they are really historic. Even the
vintage telephones with cranks and separate ear pieces had a decoupling
cap. They just didn't call it capacitor but condenser.

Regards, Joerg
 
C

CBFalconer

Jan 1, 1970
0
Theo said:
Roughly when were power supply decoupling capacitors first deemed
necessary in circuit design? I'm thinking mostly logic circuits
here, though I can see that audio/RF circuits might have needed
.... snip ...

When local current spikes became significant. So called high
frequency tube circuits always needed them. In general,
transistors draw heavier currents than tubes, at lower voltages,
and so are more likely to need a local supply. It's function, not
time.
 
M

Morten Reistad

Jan 1, 1970
0
I dissassembled an old German valve.based radio from the 1930's, and
those decoupling capacitors were present there; especially around
swithces; e.g. from MW to KW.
The earliest audio/radio stuff used batteries, so had no hum problems,
and used small numbers of tubes with such low gain that circuits were
stable without supply bypasses. I could poke through my old books, but
I'd guess that filter and bypass caps became common sometimes in the
1920's maybe.

The oldest radio hardware I have inspected was from 1924, and that
was a very simplistic design for a MW radio. No excess capcitors there.

-- mrr
 
J

John Larkin

Jan 1, 1970
0
I dissassembled an old German valve.based radio from the 1930's, and
those decoupling capacitors were present there; especially around
swithces; e.g. from MW to KW.


The oldest radio hardware I have inspected was from 1924, and that
was a very simplistic design for a MW radio. No excess capcitors there.

-- mrr

Actually, capacitors - and resistors, for that matter - were expensive
in the early days of radio. In 1921, a 1M grid-leak resistor could
cost a dollar or two, and paper or mica caps, a few nf, were similar.
A lot of early radios had no fixed capacitors at all.

John
 
M

Max Hauser

Jan 1, 1970
0
I agree with those here having experience with VT (valve) schematics.
Mention of "power supply spikes" suggests either digital (rare in consumer
products until cheap digital ICs circa 1970, you could see them arrive
gradually, and the hype that came with them) or more specialized, pulsy
circuitry as in TV scan; yet I've seen many multiple-stage VT audio
amplifier schematics with successive R-C networks, sometimes inductors too,
leading from one power supply feed to the next, "decoupling" stages from
each other to discourage unwanted intercourse via power supply line [1].
That "decoupling capacitor" language in English long predated commonplace
digital hardware. When early transistor amplifier circuitry mimicked its VT
predecessors closely, the decoupling networks came along too.

I didn't see it mentioned here, but decoupling caps INside monolithic
circuits have also been common practice for many years, if the chip needs
them and can fit caps of useful size. (I'm talking pure-monolithic; much
more can be done with specialized components added to a leadframe or header
before the package is finished.) Oxidized silicon grown from crystal is
actually an outstanding capacitor dielectric in multiple ways. (And it's
amazing what a few picofarads can do, in the right spot, to the interior of
the inductance matrix of the leadframe.) And then there are the RF ICs ...

Capacitors have come far since the adventurous but fortunate Mynheer van
Musschenbroek [2]. If you missed the good stuff, it's currently at

http://tinyurl.com/33h2u

-- Max


[1] First I edited "intercourse" to "communication." Then I changed it
back.

[2] "Pure Hydrogen has been often respired by different philosophers,
particularly by Scheele, Fontana, and the adventurous and unfortunate
Rosier." -- Humphry Davy, _Researches Chemical and Philosophical,_ Division
1, 1799.
 
J

Joerg

Jan 1, 1970
0
Hi John,
Actually, capacitors - and resistors, for that matter - were expensive
in the early days of radio. In 1921, a 1M grid-leak resistor could
cost a dollar or two, and paper or mica caps, a few nf, were similar.
A lot of early radios had no fixed capacitors at all.
On the other hand in those days people were still able and willing to
"roll their own". Some parchment type paper, or at least construction
paper, lots of foil from chocolate wrappers plus litz wires and you had
a capacitor. Resistor could also be "drawn" on paper.

Regards, Joerg
 
In alt.folklore.computers Joerg said:
On the other hand in those days people were still able and willing to
"roll their own". Some parchment type paper, or at least construction
paper, lots of foil from chocolate wrappers plus litz wires and you had
a capacitor. Resistor could also be "drawn" on paper.

FWIW, homemade capacitors are often the rule with homebrewed high-voltage
stuff (ie, tesla coils). Glass bottles wrapped with foil and filled with
salt water are common, albeit crude, but workable. One can dramatically
increase the voltage handling of an air-dielectric variable capacitor
(typical radio tuning cap) by submerging it in mineral oil. Try pricing a
<lots of kilovolts> variable cap sometime.
 
J

Joerg

Jan 1, 1970
0
Hello,
FWIW, homemade capacitors are often the rule with homebrewed high-voltage
stuff (ie, tesla coils). Glass bottles wrapped with foil and filled with
salt water are common, albeit crude, but workable.
Plus we'd get to drink the stuff that was in there before :)
One can dramatically increase the voltage handling of an air-dielectric variable capacitor (typical radio tuning cap) by submerging it in mineral oil.
Did that with a high power dummy load once. But after a while some oil
started to seep out and made a huge mess.
Try pricing a <lots of kilovolts> variable cap sometime.
Yes, I painfully rememember forking over lots of bills for a variable
vacuum cap.

Regards, Joerg
 
F

fcweed

Jan 1, 1970
0
Joerg said:
Hi Theo,

Nowadays the word "decoupling" is use to describe decoupling current
spikes from the power supply. IOW to prevent them from entering there or
causing a momentary sag in voltage.

In the olden days the word also meant decoupling one stage from another,
for example the plate DC level from the much lower gate DC bias of the
next amplifier stage. In that respect they are really historic. Even the
vintage telephones with cranks and separate ear pieces had a decoupling
cap. They just didn't call it capacitor but condenser.

Where I grew up, we used the terms "decoupling" and "bypass" capacitors,
where the latter provided a noise glitch bypass from power to ground.

wrt. the original question, bypass/decoupling caps for digital circuits
were already necessary with RTL. The first flip-flop circuit we built in
the first digital logic course I took was a simple 4-stage ripple
counter - Not much to screw up, but it didn't work. The instructor
didn't even look at the circuit wiring, he just clipped a small cap
between power and ground and things worked as expected - Which may have
been the real point of the whole exercise.
 
H

Hank Oredson

Jan 1, 1970
0
Theo Markettos said:
I was just wondering...

Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design? I'm thinking mostly logic circuits here, though I can
see that audio/RF circuits might have needed them too. I suppose every DC
PSU has a smoothing capacitor doing roughly the same job but I was more
thinking about adding capacitors around the circuit to provide localised
decoupling. Were they necessary in valve (vacuum tube) circuits? If not,
in what technology did they first appear (RTL/DTL/TTL etc)? Was there any
prior use in another field (say RF) and they were later used in logic
circuit?

The earliest boards I can remember (late 70s/early 80s) seem to have them
on, but I haven't seen many earlier boards and can't say I've been looking
for them specifically...


I would guess you mean "in computers".

The early mainframes that used tubes had plenty of them!
I still have some .02 250V ceramics from that era, from
Univac surplus, left over from the 1103 days ...

When transistors were first used there were decoupling
capacitors on each voltage on the cards that plugged
into the backplane, and others as needed scattered around.
Some backplanes had caps here and there as well.

Going back a bit further (into the 1920s) one would find
them on power supplied to audio / RF / IF stages. Usually
wound foil capacitors with values of .005 to maybe .1 uFd.
These were perhaps .5 cm diameter and 1 - 2 cm long.

I guess the "right" answer is "There never have NOT been
decoupling capacitors in any electronic device."

--

... Hank

http://horedson.home.att.net
http://w0rli.home.att.net
 
R

Rich Grise

Jan 1, 1970
0
fcweed said:
Where I grew up, we used the terms "decoupling" and "bypass" capacitors,
where the latter provided a noise glitch bypass from power to ground.

The way I read it years ago, the bypass capacitors form a circuit path
for the signal (or signal return) around the power supply.

Cheers!
Rich
 
R

Rob Storey

Jan 1, 1970
0
Theo said:
I was just wondering...

Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design? I'm thinking mostly logic circuits here, though I can
see that audio/RF circuits might have needed them too. I suppose every DC
PSU has a smoothing capacitor doing roughly the same job but I was more
thinking about adding capacitors around the circuit to provide localised
decoupling. Were they necessary in valve (vacuum tube) circuits? If not,
in what technology did they first appear (RTL/DTL/TTL etc)? Was there any
prior use in another field (say RF) and they were later used in logic
circuit?

The earliest boards I can remember (late 70s/early 80s) seem to have them
on, but I haven't seen many earlier boards and can't say I've been looking
for them specifically...

IIRC, bypass caps became particularly important with the advent of TTL
logic, since (again IIRC) the totem-pole outputs in these devices have a
very brief period during switching when both output transistors are on,
causing a short short(!!), which in turn caused supply line spikes.
DEC used to recommend a cap (0.1uF or 0.01uF) on every IC package.

Rob STorey
 
K

Ken Taylor

Jan 1, 1970
0
Rob Storey said:
IIRC, bypass caps became particularly important with the advent of TTL
logic, since (again IIRC) the totem-pole outputs in these devices have a
very brief period during switching when both output transistors are on,
causing a short short(!!), which in turn caused supply line spikes.
DEC used to recommend a cap (0.1uF or 0.01uF) on every IC package.

Rob STorey

Many was the motorboating valve amp when a decoupling capacitor went dry.

Ken
 
S

Steve Kavanagh

Jan 1, 1970
0
Theo Markettos said:
Roughly when were power supply decoupling capacitors first deemed necessary
in circuit design?

Stuart Ballantine, in "Radio Telephony for Amateurs, Second Edition"
(Philadelphia, 1923) devotes a few pages to a scheme for reducing
instability in multistage amplifiers wherein there is a bypass
capacitor from each tube's plate supply lead to ground and (if
necessary) a series inductor between this capacitor and the power
supply.

So the concept was certainly well known and available to the general
public by 1923. Though the lack of use of this circuit in the other
parts of the book would tend to indicate that it was considered a bit
extravagant.

Steve
 
J

John Larkin

Jan 1, 1970
0
Stuart Ballantine, in "Radio Telephony for Amateurs, Second Edition"
(Philadelphia, 1923) devotes a few pages to a scheme for reducing
instability in multistage amplifiers wherein there is a bypass
capacitor from each tube's plate supply lead to ground and (if
necessary) a series inductor between this capacitor and the power
supply.

So the concept was certainly well known and available to the general
public by 1923. Though the lack of use of this circuit in the other
parts of the book would tend to indicate that it was considered a bit
extravagant.

Steve

I have a few references from the 1920's. Mostly what I see is battery
powered with hardly any caps at all. Interstage coupling is mostly via
transformers... one transformer would give about as much gain as an
additional R-C coupled tube stage, replacing 7 expensive parts. Tubes
mostly ran at Idss in those days, so the transformer was ideal.
Transformer coupling also greatly reduces the need for supply
bypasses.

By the mid-30s, tubes were cheaper and had much higher Gm's, voltages
were higher, AF coupling was mostly R-C, grids were back-biased,
tetrodes were appearing, and supply bypassing was universal.

I like the NE25339; it's a tetrode GaAs mesfet. Needs bypassing.

John
 
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