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History of decoupling capacitors

C

Charlie Gibbs

Jan 1, 1970
0
The way I read it years ago, the bypass capacitors form a circuit path
for the signal (or signal return) around the power supply.

That's the way I've always thought of it. And we always called them
bypass capacitors - that term "decoupling" is a relatively recent
invention.
 
C

Charlie Gibbs

Jan 1, 1970
0
Many was the motorboating valve amp when a decoupling capacitor
went dry.

Or that horrible tearing screech when an i.f. stage went into
oscillation. And it wasn't limited to tube circuits - I heard
someone's transistorized receiver doing it, and immediately
diagnosed and fixed it.
 
C

CBFalconer

Jan 1, 1970
0
John said:
.... snip ...

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.

Tetrodes, in the shape of the 224 (later known as the 24) became
common in the late 20's. One of the highest gm bottles, the 2A3,
was available by the early 30s. At least in leftpondia.
 
R

Roger Hamlett

Jan 1, 1970
0
Steve Kavanagh said:
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.
Terman 'Fundamentals of Radio', in 1938, also refers to such bypassing, and
a couple of the 'example' circuits do have bypasses shown.
I think bypassing of some form, was 'known' within only a very few years of
the first amplifier stages, since the problems would soon appear, especially
once higher gains became possible.

Best Wishes
 
B

Bernd Felsche

Jan 1, 1970
0
Charlie Gibbs said:
That's the way I've always thought of it. And we always called them
bypass capacitors - that term "decoupling" is a relatively recent
invention.

I thought a decoupling capacitor was a bucket of cold water.
 
T

Té Rowan

Jan 1, 1970
0
Saith Bernd Felsche:
Charlie Gibbs said:
I thought a decoupling capacitor was a bucket of cold water.

I'm sure One-Man-Bucket's older brother would've liked that one. He'd
have given his right arm to be named Two-Dogs-Fighting.
 
P

Paul Burridge

Jan 1, 1970
0
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.

Wow! Amateurs must have enjoyed great satisfaction 'rolling their own'
passives in those days, then.
 
P

Paul Burridge

Jan 1, 1970
0
Hello,

Plus we'd get to drink the stuff that was in there before :)

Not a great idea. Heinz Katsup is very sour by itself.
 
P

Paul Burridge

Jan 1, 1970
0
I like the NE25339; it's a tetrode GaAs mesfet. Needs bypassing.

Do you have a data sheet for that device, John? Googling only gave two
unhelpful hits.
 
W

Winfield Hill

Jan 1, 1970
0
John Larkin wrote...
I like the NE25339; it's a tetrode GaAs mesfet. Needs bypassing.

Why do these FETs have such high DC gate currents?

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
J

John Larkin

Jan 1, 1970
0
John Larkin wrote...

Why do these FETs have such high DC gate currents?

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)


I think gaas, and compound semiconductors in general, have very high
defect densities; plus, these parts have schottky gates, and schottky
diodes are always leaky for some reason.

Another interesting property of gaas is "trapping states." The high
crystal dislocation density creates places where charge can get
trapped under gates, so the transconductance and pinchoff voltages of
mesfets is a nasty function of temperature and bias history. Under
constant-bias conditions, Id can change 10% overnight. I managed to
make money off this effect, by replacing a piece of very expensive
gear that depended on mesfets to behave, and they wouldn't.

Given what crummy parts gaasfets *appear* to be, they can hit noise
figures below 1 dB as RF amps.

I haven't tried any of the phemts as regards bias stability, but I'd
expect similar behavior. They do seem to be a lot more predictable
than mesfets, though. NE25339 (see the NEC.COM site, or buy from
Mouser) is a dual-gate mesfet.

John
 
J

Joerg

Jan 1, 1970
0
Hi Paul,
Not a great idea. Heinz Katsup is very sour by itself.
Just change brands then. Yesterday we had a pre-July 4th party at
neighbors and after several Mai-Tai drinks the Bacardi bottle would have
been a good candidate here :)

Regards, Joerg
 
T

Theo Markettos

Jan 1, 1970
0
Roger Hamlett said:
Terman 'Fundamentals of Radio', in 1938, also refers to such bypassing, and
a couple of the 'example' circuits do have bypasses shown.

Thanks everyone - both of those are in the library, so I'll dig them out.

Theo
 
S

Stan Barr

Jan 1, 1970
0
Was watching a show on TV about Benjamin Franklin the other day, and his
electrical experiments with lightning. There was a scene recreating an 18th
century lab, and in the background on the table were big glass and foil
Leyden Jars. The capacitance of jars wasn't much, around 500pf, but the
400KV voltage made up for it. When did the name switch to condenser?

It had switched by 1892 - as far back as I could locate quickly, (in
American Telegraphy, William Maver Jr.). The Jar as a unit of capacitance
was in use up until WW2.

--
Cheers,
Stan Barr stanb .at. dial .dot. pipex .dot. com
(Remove any digits from the addresses when mailing me.)

The future was never like this!
 
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