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'[EE]: Cascoded?'
2002\05\28@123009 by Brandon Fosdick

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I'm reading through some apps notes and what not about amplifiers and ADC's etc.
Since my EE background is a little weak I'm encountering a lot of unfamiliar
terms, but one in particular is bugging me. I keep reading about things that are
cascoded together (transistors, stages, etc.). At first I thought it was a typo
(or bad English) and was supposed to read "cascaded", but its occurring too
frequently to be a typo. I tried dictionary.com and its clueless too. Spell
check doesn't like it either. Does anybody know what "cascoded" is and how it
relates to "cascaded"?

<rant>
On a related note, I really wish companies would hire English speakers/writers
if they're going to publish docs using English. Its hard to separate the
technical details from the bad grammar.
</rant>

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2002\05\28@124249 by Brian Aase

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The term dates from vacuum tube days: Cascade + Cathode.
It refers to a discrete two-stage amplifier circuit wherein
the first common-cathode stage is direct-coupled to the following
common-grid stage.  By extension, a two-transistor amplifier
consisting of a common-emitter stage direct-coupled to a common-base
stage has ended up with the "cascode" name as well.
Brian Aase


{Quote hidden}

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2002\05\28@124916 by Jim

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IIRC 'cascode' was a special configuration in
an amplifier (to my recollection, specifically
a low-level RF amplifier) and cascade simply
means 'A' follows 'B' in succession in a chain
of amplifiers.

From Skolnick, "Intro to RADAR Systems" dated 1962
pg 388 I find:

Cascode:

 "Grounded cathode triode followed by a grounded
  grid triode."

I suppose that the use of 'cascode' could apply
to solid state devices in similar configuration
where a higher-output Z stage (e.g. common or
grounded cathode) is loaded (followed by) a
lower-Z input stage (e.g. common or grounded grid)
could also be described as 'cascode'.

Jim

{Original Message removed}

2002\05\28@130212 by Olin Lathrop

face picon face
> I'm reading through some apps notes and what not about amplifiers and
ADC's etc.
> Since my EE background is a little weak I'm encountering a lot of
unfamiliar
> terms, but one in particular is bugging me. I keep reading about things
that are
> cascoded together (transistors, stages, etc.). At first I thought it was a
typo
> (or bad English) and was supposed to read "cascaded", but its occurring
too
> frequently to be a typo. I tried dictionary.com and its clueless too.
Spell
> check doesn't like it either. Does anybody know what "cascoded" is and how
it
> relates to "cascaded"?

I've seen that term used when two complementary transistors (NPN to PNP or
vice versa) are cascaded.  The collector of the first transistor is
connected to the base of the second.  If you tie the emitter of the first to
the collector of the second, you are left with three external connections
which can act like a single transistor with very high current gain.  The
concept is similar to a darlington, which uses like transistors to emulate
one high gain transistor.

> <rant>
> On a related note, I really wish companies would hire English
speakers/writers
> if they're going to publish docs using English. Its hard to separate the
> technical details from the bad grammar.
> </rant>

Please not to offend of honorable translator from Japanese for English.  He
job only to make manual look like good.  Americans not so smart for reading
but buy much anything we make.  Sell same for manual good or not.


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2002\05\28@131232 by Jim

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  "The concept is similar to a darlington, which
   uses like transistors to emulate one high gain
   transistor."

The 'cascode' configuration has other goals like
improved bandwidth and lower noise figure as contrasted
to simply increasing the gain, verily, voltage (and even
current) gain in a cascode configuration is _lower_
than other configurations ...

Jim





{Original Message removed}

2002\05\28@132932 by Dave Tweed

face
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Brandon Fosdick <spam_OUTbfozTakeThisOuTspamTERRANDEV.COM> wrote:
> I keep reading about things that are cascoded together (transistors,
> stages, etc.). At first I thought it was a typo (or bad English) and was
> supposed to read "cascaded", but its occurring too frequently to be a
> typo. I tried dictionary.com and its clueless too. Spell check doesn't
> like it either. Does anybody know what "cascoded" is and how it relates
> to "cascaded"?

Cascoded is a special case of cascaded; for details, see
http://www.chipcenter.com/circuitcellar/may00/ancil-0500/c0500eqa7.htm

(Let me know if you want help on Miller Effect; that one isn't online
any more.)

Cascade simply means to hook in series. Two transistors can be hooked
in series for a number of reasons, including producing a wider output
swing or simply enabling/disabling a particular function.

Most often, the term simply refers to a multistage amplifier in which the
output of one stage feeds the input of the next.

-- Dave Tweed
  Circuit Cellar EQ editor

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2002\05\28@133657 by Bob Blick

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> check doesn't like it either. Does anybody know what "cascoded" is and how it
> relates to "cascaded"?

If you want to get the most accuracy out of a transistor, keeping the
collector-emitter voltage constant is best. To cascode a transistor is to
attach another transistor in the collector, wired common-base. In other
words, transistor 2 needs a voltage reference for its base, has its
emitter connected to the collector of transistor 1.

Then transistor 2 does all the dirty work, but transistor 1 feels a
constant collector voltage so is very fast and accurate gain-wise.

Both transistors are the same polarity.

Cheerful regards,

Bob Blick

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2002\05\28@134123 by Scott Stephens

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From: Olin Lathrop <.....olin_piclistKILLspamspam@spam@EMBEDINC.COM>
Subject: Re: [EE]: Cascoded?


>I've seen that term used when two complementary transistors (NPN to PNP or
>vice versa) are cascaded.  The collector of the first transistor is
>connected to the base of the second.

Isn't that the Sziklai or complementary Darlington?

>If you tie the emitter of the first to
>the collector of the second, you are left with three external connections
>which can act like a single transistor with very high current gain.  The
>concept is similar to a darlington, which uses like transistors to emulate
>one high gain transistor.


Very poor for RF because of the Miller Effect.


>> On a related note, I really wish companies would hire English
>speakers/writers
>> if they're going to publish docs using English. Its hard to separate the
>> technical details from the bad grammar.


Really! Some people can't spell catalog or color et. Wasn't it Webster (the
guy that wrote the dictionary) that tried to modify American grammer to
simply all those goofy old complexifications? I can just see a bunch of old
drunk Lords and Priests figuring out new and clever ways to confound the
ignorant peasants, and changing rules keep the younger generations scholars
humble and dependant. Kind of like Micro$oft with Windows. Or the government
with the tax code.

Scott

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2002\05\28@134135 by Scott Stephens

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From: Brandon Fosdick <bfozspamKILLspamTERRANDEV.COM>
Subject: [EE]: Cascoded?


>Does anybody know what "cascoded" is and how it
>relates to "cascaded"?

It (AFAIK) usualy refers to a common emitter cascaded with a common base,
the idea being that the direct connection of the common base stage prevents
'Miller' capacitance (base-collector) capacitance of the first stage from
loading the input and ruining gain. Good input impedance, good gain, output
impedance and isolation. So its a common amplifier for RF.

Scott

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2002\05\28@134943 by Jim

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... continuing from Skolnick dated 1962 on the
subject of 'cascode' ...

* * * * * * * * * * * * * * * * * * * * * * * * * *

"There are three basic methods of using a triode in a
circuit, depending upon whether the cathode, grid or
anode is grounded. There are, in all, nine combinations
possiblewhen using two triodes in each of the above
three ground configurations.

It was found experimentally that the best of then nine
configurations was obtained with a grounded-cathode
triode followed by a grounded-grid triode, a combination
known as 'cascode amplifier'.

The grounded-grid stage loads the first-stage grounded-
cathode triode so that the first stage gain is essentially
unity. Thus neautralization of the grounded-cathode
stage is not needed. The second triode is necessary in
order to achieve gain.

The cascode amplifier provides the stability, gain, and
non-criticalness of a pentode and the low noiseof the
triode.

Noise figures of .25 dB are possible at a frequency
of 6 MC (Ed. note: MHz) 1.25 dB at 30 MC and 5.5 dB
at 180 MC."

* * * * * * * * * * * * * * * * * * * * * * * *

Jim

{Original Message removed}

2002\05\28@135944 by Harold M Hallikainen

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       The original cascode circuits were vacuum tube where a common cathode
amplifier directly drove a common grid amplifier. The tubes were wired
"in series", cathode of the bottom tube to ground, plate of the bottom
tube to the cathode of the top tube, and plate of the top tube through a
load resistor to B+.
       The same thing can be done with transistors (whether BJT or FET). For
BJT, it's emitter to ground (or a small bias resistor to ground),
collector to the emitter of the top transistor, and collector of the top
transistor to a load resistor to supply. The bases are biased from a
voltage divider. The top transistor base is held at AC ground with a
bypass capacitor. Again, this is a common emitter amplifier driving a
common base amplifier.
       The collector of a transistor can be thought of as a controlled current
source (Hfe * Ib) with a parallel resistor (Hoe). This parallel resistor
causes the collector current to not be constant as the collector voltage
varies. However, having the collector drive the very low input resistance
of the "top" common base amplifier keeps the collector voltage relatively
constant while maintaining the current gain (since the current gain of
the common base transistor is very close to 1).
       Cascode seems to be used a lot in VHF or higher preamplifier designs.
I've never designed one myself, but I've taught the basics of it in the
circuit analysis classes I teach.

Harold


On Tue, 28 May 2002 12:31:15 -0400 Brandon Fosdick <.....bfozKILLspamspam.....TERRANDEV.COM>
writes:
{Quote hidden}

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com


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2002\05\28@142049 by Peter L. Peres

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On Tue, 28 May 2002, Jim wrote:

>   "The concept is similar to a darlington, which
>    uses like transistors to emulate one high gain
>    transistor."
>
>The 'cascode' configuration has other goals like
>improved bandwidth and lower noise figure as contrasted
>to simply increasing the gain, verily, voltage (and even
>current) gain in a cascode configuration is _lower_
>than other configurations ...

Cascode gain is only slighlty lower than with emitter coupled stages. The
main advantage of the cascode is the very low output-input coupling. The
other is the possibility to match the output and input very well (better
than emitter coupled stages). Source: Sedra-Smith Microelectronic Circuits
and other books.

Peter

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2002\05\28@142708 by Peter L. Peres

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On Tue, 28 May 2002, Scott Stephens wrote:

>From: Olin Lathrop <EraseMEolin_piclistspam_OUTspamTakeThisOuTEMBEDINC.COM>
>Subject: Re: [EE]: Cascoded?
>
>
>>I've seen that term used when two complementary transistors (NPN to PNP or
>>vice versa) are cascaded.  The collector of the first transistor is
>>connected to the base of the second.
>
>Isn't that the Sziklai or complementary Darlington?

No the Sziklai is the comlpementary darlington, but there are cascode
variants that use pnp/npn and/or fet's for one or the other of the stages.
The Y final amplifiers in most scopes and many TV video amplifiers (tube
cathode modulation) are cascodes (in this case the second element of the
cascode is the picture tube's triode section).

Peter

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2002\05\28@143319 by Peter L. Peres

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On Tue, 28 May 2002, Peter L. Peres wrote:

Errata: The line below should read 'only slightly higher' ...

>Cascode gain is only slighlty lower than with emitter coupled stages. The
>main advantage of the cascode is the very low output-input coupling. The
>other is the possibility to match the output and input very well (better
>than emitter coupled stages). Source: Sedra-Smith Microelectronic Circuits
>and other books.

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2002\05\28@145320 by Jim

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   "The main advantage of the cascode is the
    very low output-input coupling"

Has the concept of "neutralization" (a means of
countering a finite figure for input-output
isolation or "reverse" gain) been lost on the
more recent, solid-state and IC generation?

Jim

{Original Message removed}

2002\05\28@162256 by Olin Lathrop

face picon face
>    "The concept is similar to a darlington, which
>     uses like transistors to emulate one high gain
>     transistor."
>
> The 'cascode' configuration has other goals like
> improved bandwidth and lower noise figure as contrasted
> to simply increasing the gain, verily, voltage (and even
> current) gain in a cascode configuration is _lower_
> than other configurations ...

Yes, I was apparently confusing the terms "pseudo darlington" with
"cascode".


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2002\05\28@162911 by Olin Lathrop

face picon face
> >I've seen that term used when two complementary transistors (NPN to PNP
or
> >vice versa) are cascaded.  The collector of the first transistor is
> >connected to the base of the second.
>
> Isn't that the Sziklai or complementary Darlington?

Yes, I got my terms mixed up.  I used a cascode configuration in a
transmitter once, not knowing it had a special name.  The advantage was that
the transistors could be run at higher frequency because the miller effect
is nullified in both transistors.  It worked like a charm dumping 1MHz AM
into the power line.


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2002\05\28@164407 by Russell McMahon

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>If you tie the emitter of the first to
>the collector of the second, you are left with three external connections
>which can act like a single transistor with very high current gain.  The
>concept is similar to a darlington, which uses like transistors to emulate
>one high gain transistor.

Emitter? Collector?
Are they perchance anything like Anode & Cathode ? :-)

> Very poor for RF because of the Miller Effect.

And yet capable of improving performance of valve front ends substantially.
Long long ago I used an ARC-5 receiver (US WW2 war surplus still available
on amateur market decades after their birth) as a ham band receiver. I
changed the RF front end to Cascode configuration using a dual-triode with
all the (simple) circuitry being implemented in a space construction on a
valve base so the whole affair plugged into the original valve socket (not
my design). I now can't remember what the intended advantages were in this
context but the practical result was a vast improvement in the sensitivity
of the receiver. This was on the "80 metre" (3.5 - 3.9 MHz ham band) which
nowadays would almost not count as RF :-)


       Russell McMahon


           .

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2002\05\28@170314 by Russell McMahon

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>     "The main advantage of the cascode is the
>      very low output-input coupling"
>
> Has the concept of "neutralization" (a means of
> countering a finite figure for input-output
> isolation or "reverse" gain) been lost on the
> more recent, solid-state and IC generation?


Apparently yes. An unseen gain of modern technology. When old enemies are
(largely) beaten terms used to describe ways to combat them or suffer from
them tend to fade away :-).

Defenestration anybody?



       RM

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2002\05\28@171047 by Dave Dilatush

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Jim wrote...

>Has the concept of "neutralization" (a means of
>countering a finite figure for input-output
>isolation or "reverse" gain) been lost on the
>more recent, solid-state and IC generation?

Nope.

The term itself has fallen into disuse but the concept is very
much alive, having broadened into a set of methods for
stabilizing feedback loops.  For example, putting a small shunt
capacitance across the feedback resistor in an opamp circuit
(often necessary when the resistance is very large) to damp out
high-frequency ringing in its step response is, in a sense,
"neutralizing" the opamp's input capacitance since that
capacitance is often a major contributor to the ringing.

DD

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2002\05\28@172416 by Jim

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  "putting a small shunt capacitance across the
   feedback resistor in an opamp circuit"

I don't think this quite qualifies ... there is
no intrinsic 'feedback' or 'reverse gain' (input
to output isolation problem) within the device,
rather the phase-gain margin over the frequency
range of device is such that in some circuit
configurations the "feedback" must compensate for
this phase shift with frequency ...

Neutralization does just that - 'neutralizes' that
feedback component which would otherwise exist.

I have an old tube RF amp in which the neutralization
can be adjusted with the power off - by measurement
of the S12 or reverse direction gain A/K/A isolation
figure from output to input - adjustment calls
for adjusting the tuning for a *minimum* or a null
in the signal from output to input using a signal
generator and spectrum analyzer/receiver.

Jim


{Original Message removed}

2002\05\28@180318 by Dave Dilatush

picon face
Jim wrote...

>I have an old tube RF amp in which the neutralization
>can be adjusted with the power off - by measurement
>of the S12 or reverse direction gain A/K/A isolation
>figure from output to input - adjustment calls
>for adjusting the tuning for a *minimum* or a null
>in the signal from output to input using a signal
>generator and spectrum analyzer/receiver.

My very first paying job, back when I was in high school, was on
a production line doing just that.  We were making these little
2-meter Nuvistor preamps for somebody, and my job was sitting
there tweaking the trimmers to get that null.  Over and over, all
day long...

DD

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2002\05\28@182527 by Russell McMahon

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> My very first paying job, back when I was in high school, was on
> a production line doing just that.  We were making these little
> 2-meter Nuvistor preamps for somebody, and my job was sitting
> there tweaking the trimmers to get that null.  Over and over, all
> day long...


For the uninitiated, a Nuvistor is a FET with a heating element inside :-)

Well, maybe not quite. They were (are?) very small vacuum tubes with a metal
casing - size was about the tip of a typical small finger AFAIR. Excellent
VHF characteristics due to small size and low interelectrode capacitance
etc. Much loved by radio amateurs for use on 2 metres (144 MHz) and probably
higher.


       RM

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2002\05\28@191026 by Andrew Warren

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Russell McMahon <PICLISTspamspam_OUTmitvma.mit.edu> wrote:

> For the uninitiated, a Nuvistor is a FET with a heating element inside :-)
>
> Well, maybe not quite. They were (are?) very small vacuum tubes with a metal
> casing

   "Were", not "are"; as far as I know, no one is making them
   anymore.

   However, they ARE being designed into new products; Musical
   Fidelity's "Nu-Vista" amplifiers and CD player use nuvistors,
   and MF are apparently stocking a spare set of nuvistors for each
   unit they ship.

   -Andy

=== Andrew Warren -- @spam@aiwKILLspamspamcypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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2002\05\29@042411 by Alan B. Pearce

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>The 'cascode' configuration has other goals like
>improved bandwidth and lower noise figure as contrasted
>to simply increasing the gain, verily, voltage (and even
>current) gain in a cascode configuration is _lower_
>than other configurations ...

Not sure why you perceive it as having low voltage gain, one of my memories
of the use of this configuration is to have a low voltage swing input drive
a high voltage transistor in the output stages of oscilloscopes for
deflection plate drive.

The original configuration was designed for RF use when the devices
available had high miller feedback effects. The use of two devices in the
cascode configuration resulted in much better output to input isolation,
allowing much higher gain. Originally used quite widely in the RF stages of
TV tuners, back in the days when these were built with vacuum tubes, but I
imagine the configuration was used long before that.

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2002\05\29@110535 by Jim

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  "Not sure why you perceive it as having low
   voltage gain"

Trade-offs are often made in the pursuit of other
of goals at the cost of sheer overall "gain" - such
as achieving a usually elusive  _low noise figure_
and circuit stability - factors closely tied to the
physics and structure of the devices (tubes) that
were used in the time frame when the 'cascode'
configuration was 'discovered' and the term
coined.

In today's world those goals are much easier to
meet but were a bonafide _challenge_ fifty years
ago given the devices and technology available ...

I think the quotes I excerpted from Skolnick said
it best in this regard.

   "Originally used quite widely in the RF
    stages of TV tuners,"

We might find out the origins of the 'cascode' circuit
configuration if we were to research this subject in
the MIT "Rad Lab Series" (meaning it was probably first
used in RADAR apps beginning in the forties and prior
to use in TV tuners) ...

Jim


{Original Message removed}

2002\05\29@140522 by Peter L. Peres

picon face
On Tue, 28 May 2002, Jim wrote:

>   "putting a small shunt capacitance across the
>    feedback resistor in an opamp circuit"
>
>I don't think this quite qualifies ... there is
>no intrinsic 'feedback' or 'reverse gain' (input
>to output isolation problem) within the device,

I think that he meant feed-forward reaction (small cap from output to +
input) to increase the apparent slew rate at the output. This used to be
popular to make 'high speed' opamps out of 741s and the like.

Peter

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2002\05\29@140526 by Peter L. Peres

picon face
On Tue, 28 May 2002, Jim wrote:

>    "The main advantage of the cascode is the
>     very low output-input coupling"
>
>Has the concept of "neutralization" (a means of
>countering a finite figure for input-output
>isolation or "reverse" gain) been lost on the
>more recent, solid-state and IC generation?

<g> no, but everyone remembers tetrodes (and up) and shielded-cb-junction
transistors like the BF173 (which is great by the way). Even if they are
younger than you think ...

Peter

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2002\05\29@171729 by Dave Dilatush

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Peter wrote...

>On Tue, 28 May 2002, Jim wrote:
>
>>   "putting a small shunt capacitance across the
>>    feedback resistor in an opamp circuit"
>>
>>I don't think this quite qualifies ... there is
>>no intrinsic 'feedback' or 'reverse gain' (input
>>to output isolation problem) within the device,
>
>I think that he meant feed-forward reaction (small cap from output to +
>input) to increase the apparent slew rate at the output. This used to be
>popular to make 'high speed' opamps out of 741s and the like.

No, I meant exactly what I said, and it had absolutely nothing to
do with either feed-forward techniques or with the use of
positive feedback such as what you've described above.

FWIW, I agree with Jim's comment that the opamp compensation I
spoke of isn't all that closely related to the process of
"neutralizing" an RF amplifier; I can't think of any really good
non-RF analogies to neutralization.

DD

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2002\05\29@173012 by Barry Gershenfeld

face picon face
>>I have an old tube RF amp in which the neutralization
>>can be adjusted with the power off - by measurement
>>of the S12 or reverse direction gain A/K/A isolation
>>figure from output to input - adjustment calls
>>for adjusting the tuning for a *minimum* or a null
>>in the signal from output to input using a signal
>>generator and spectrum analyzer/receiver.
>
>My very first paying job, back when I was in high school, was on
>a production line doing just that.  We were making these little
>2-meter Nuvistor preamps for somebody, and my job was sitting
>there tweaking the trimmers to get that null.  Over and over, all
>day long...

Some the great old HeathKit radios had a piece of wire sticking
up next to the final power amplifier.  You were supposed to
push it around a bit until the capacitance between it and the
tube was just right, thus "neutralizing" the final stage.  As
I recall the instructions had you disconnect something (probably
the plate supply) and adjust it for a null, as cited above.

wa2qmi

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2002\05\29@180310 by Dave Dilatush

picon face
Barry Gershenfeld wrote...

>Some the great old HeathKit radios had a piece of wire sticking
>up next to the final power amplifier.  You were supposed to
>push it around a bit until the capacitance between it and the
>tube was just right, thus "neutralizing" the final stage.  As
>I recall the instructions had you disconnect something (probably
>the plate supply) and adjust it for a null, as cited above.

And don't forget "gimmick" capacitors- a couple of short pieces
of insulated wire twisted together.  Tighten or loosen the twists
to change the capacitance and make the circuit work right...

DD

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2002\05\29@222321 by Bill Colville

flavicon
face
Jim wrote:

> We might find out the origins of the 'cascode' circuit
> configuration if we were to research this subject in
> the MIT "Rad Lab Series" (meaning it was probably first
> used in RADAR apps beginning in the forties and prior
> to use in TV tuners) ...

Out of curiosity I looked in the MIT Radiation Lab books. The
cascode amplifier is described in volume 18, written in 1946. The
description dealt mainly with increasing the gain of a triode amp to
make it approximately equal to a pentode without having to use a
screen grid power supply. It was described in the Direct Coupled
Amplifier section. No mention was made of its use as an RF amp.

Another reference I found mentioned that the cascode amp was
developed in the MIT labs during WW2 and described it's use as a
low noise VHF RF amp.

I first used a cascode amp to replace a pentode RF amp in a
communications receiver I had in 1949. It really pepped up the
performance on 10 and 20 meters.

Cheers,
Bill

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'[EE]: Cascoded?'
2002\06\03@131629 by Brandon Fosdick
flavicon
face
Q. How do you flood your inbox with email?

A. Ask the PIClist a question about something very old and hard to remember. :)

Thanks for all of the replies. Now that I know cascoded really is a word I can
get back to translating my stack of jenglish data sheets.

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2002\06\03@131634 by Brandon Fosdick

flavicon
face
Olin Lathrop wrote:
> Please not to offend of honorable translator from Japanese for English.  He
> job only to make manual look like good.  Americans not so smart for reading
> but buy much anything we make.  Sell same for manual good or not.

So terribly true.

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2002\06\03@135839 by Jim

flavicon
face
As long as the schematic (schematic circuit diagram)
is correct - we're happy ...

Jim

----- Original Message -----
From: "Brandon Fosdick" <KILLspambfozKILLspamspamTERRANDEV.COM>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Monday, June 03, 2002 11:57 AM
Subject: Re: [EE]: Cascoded?


> Olin Lathrop wrote:
> > Please not to offend of honorable translator from Japanese for English.
He
> > job only to make manual look like good.  Americans not so smart for
reading
> > but buy much anything we make.  Sell same for manual good or not.
>
> So terribly true.
>

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