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Old 26th August 2005, 12:08 AM   #1
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Default Cascodes- the truth is out there...

I've just came up to an idea about cascodes, precisely about some threat in circuits with casodes, like cascoded voltage amplification stages or cascoded current sources.
I feel there is a lack of theory concerning cascodes, namely there is no explaination why cascodes don't always sound better, despite of technical advantages (reduced Early effect and Miller effect).
Some just state, that simpliest circuits sound best with no further technical explaination, others prefer cascodes because of theoretical advantages disregarding real listening tests.
The only technical drawback being an ultra high output impedance.?? I don't believe this matters so much.
For start I attatch cascoded voltage gain stage to visualise.

to be continued...
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Old 26th August 2005, 12:09 AM   #2
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Now, reduction of capacitive negative feedback known as miller effect is considered the greatest advantage of cascode.
And the only band-limiting phenomenon is lowering current gain of a transistor with rising frequency, finite ft.
Needless to say modern audio transistors have very high ft, in range of hundreds of Mhz.
But things become look complicated when one considers in-built capacitances of active devices, frequency limits are so high, that these capacitances start to be significant.
First glance-...yes, the common base stage has significantly capacitive load at emitter. Warning!!

to be continued...
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Old 26th August 2005, 12:10 AM   #3
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To confuse more, the next step is to consider parasitic capacitances and inductances of PCB traces. Intuition makes me draw some of them in particular places. Doesn't it resemble anything? Look carefully.

to be continued...
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Old 26th August 2005, 12:11 AM   #4
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Confusion's gone, yes this is a goal of what I have in mind. A Colpitts oscillator.

to be continued...
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Old 26th August 2005, 12:13 AM   #5
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I don't state that all the cascodes are oscilators of course. But surely they may suffer from RFI, RF noise amplification, RF ringing and so on...
They also create complex eigenvalue (linear transfer function) with poles and zeroes and mad phase shifts at RF, thus feedback systems with cascodes may be significantly hard to stabilize.
Even if the whole system is stable internal parts may be ringing, which no good in audio, because of intermodulation.
Maybe no capacitive feedback (Miller effect) believed biggest advantage is actually biggest fault?

Your opinions?

Be merciful please. If what's above is utter ******** just forgive the offence of logic. Anyway it is worth trying to think and seek solutions, even wrong

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Old 26th August 2005, 12:31 AM   #6
Mr Evil is offline Mr Evil  United Kingdom
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As with everything, cascodes are neither good nor bad inherently, it's how you use them that's important. If you just go around cascoding everything willy-nilly of course you will end up with something that performs unpredictably. Add them where they address a specific need.

I don't recall ever seeing a cascode oscillate on is own as you theorize it might, but of course they do add phase shift and so may cause an amp as a whole to become unstable. A base stopper resistor and good PCB layout would alleviate the problem if it came up.
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Old 26th August 2005, 07:14 PM   #7
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Quote:
Originally posted by darkfenriz
I don't state that all the cascodes are oscilators of course. But surely they may suffer from RFI, RF noise amplification, RF ringing and so on...
They also create complex eigenvalue (linear transfer function) with poles and zeroes and mad phase shifts at RF, thus feedback systems with cascodes may be significantly hard to stabilize.
Even if the whole system is stable internal parts may be ringing, which no good in audio, because of intermodulation.
Maybe no capacitive feedback (Miller effect) believed biggest advantage is actually biggest fault?

Your opinions?

Be merciful please. If what's above is utter ********just forgive the offence of logic. Anyway it is worth trying to think and seek solutions, even wrong

best regards
make a comparison between the two. from just throwing in parts, i think you'll find that the CE amplifier also has wiring parasitics, also has a bias current, ect... in the end you can simplify both to the same circuit...

without looking at numbers, and only looking at unvalued models, well, its impossible to say what you're going to get. is L1 signifigant, or does the oscillator have a center frequency of 25Ghz? if you really wanted to blindly simplify things to make things worse, you could add in distributed effects.
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Old 26th August 2005, 08:00 PM   #8
Cortez is offline Cortez  Hungary
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Good topic, lets explore this theme !
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Old 26th August 2005, 08:09 PM   #9
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Default Re: Cascodes- the truth is out there...

Quote:
Originally posted by darkfenriz
I feel there is a lack of theory concerning cascodes.........
Abidi, A. A., ‘On the Operation of Cascode Gain Stages’, IEEE Journal of Solid-State Circuit,
vol. SC-23, No. 6 pg. 1434, December 1988.


Jaeger, R., et al ‘On the Performance of the Differential Cascode Amplifier’, IEEE Journal of
Solid-State Circuit, vol. SC-8, No. 2 pg. 169, April 1973.
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Old 26th August 2005, 08:45 PM   #10
mikeks is offline mikeks  United Kingdom
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...anyone interested in those refs should send me mail...
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