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Old 16th February 2007, 01:09 PM   #1
roender is offline roender  Romania
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Default Cascode and phaseshift relation

There is any relation between cascoding amp stages and phaseshift?
Cascoding as many as possible stages will reduce or increase phaseshift?
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Old 16th February 2007, 01:54 PM   #2
ilimzn is offline ilimzn  Croatia
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Do you mean cascOding or cascAding?
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Old 16th February 2007, 01:59 PM   #3
roender is offline roender  Romania
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cascOding ...
Like in this design. In LTspice i don't see any difference in gain/phaseshift with or without cascode BJT's
http://www.diyaudio.com/forums/showt...09#post1135809
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Old 16th February 2007, 09:43 PM   #4
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Quote:
Originally posted by roender
cascOding ...
Like in this design. In LTspice i don't see any difference in gain/phaseshift with or without cascode BJT's
Cascoding is just a mean for improving separation between input and output port of an "equivalent" CE (or CS if a FET is used). It's benefits are evident at high frequency and thus cascodes are more useful as video amplifier than audio amplifier.

In low frequency amplification there is only a situation where cascoding is really useful: the combine of good low level inputs specs of small signal (and weak) transistor as may be BC 550 or BC 560 with the somewhat rugged output specs of medium power / high voltage devices (as may be TIP 31/32C for example). For other question casconding (at low frequency) is just a matter of taste... or fashion!

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Old 17th February 2007, 09:27 AM   #5
roender is offline roender  Romania
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It's a well known fact that cascoding reduce Miller capacitance and increase freq responce.
May I presume that it reduce phaseshift also?
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Old 17th February 2007, 09:32 AM   #6
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Quote:
Originally posted by roender
It's a well known fact that cascoding reduce Miller capacitance and increase freq responce.
May I presume that it reduce phaseshift also?
Are the two side of the same medal.

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Old 17th February 2007, 10:13 AM   #7
roender is offline roender  Romania
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Are you shure?
It's a very good thing if the total phaseshift of the composed transistor is decreased ...
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Old 17th February 2007, 11:19 AM   #8
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Quote:
Originally posted by roender
Are you shure?
It's a very good thing if the total phaseshift of the composed transistor is decreased ...
I'm relatively sure (Shure, I can't... ).

If you reduce Miller effect, increase bandwidth and so on, then phase-shift (lag) between input-output is reduced. bandlimiting and phase shifting are due to the action of the same reactive components (in this case, capacitive coupling between input and output). If you couple the input and output of your cascode with a capacitor equivalent to that exist on base-collector junction of a single transistor you obtain again the same bandwidth and phase shift you have with a single transistor. Only a thing really change: the external capacitor of the cascode is a *real* capacitor, whence the junction capacitance on B-C terminal appear as a capacitor (with small signal at input) but isn't a capacitor at all (and is very non-linear and distorting).

With cascoding that you really improve with BJT is the stableness of the bandwidth which is less dependent from the quiescent current of device.

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Old 17th February 2007, 11:29 AM   #9
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http://passdiy.com/pdf/ZV9.pdf


http://www.passlabs.com/downloads/articles/cascode.pdf
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Note to myself: Never again use fingers to check if there still is voltage left in a capacitor...
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Old 17th February 2007, 04:07 PM   #10
roender is offline roender  Romania
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Very interesting!
What happened if upper device is selected upon pinch off voltage to force lower one to work in triode region? It will impruve THD for input diff stage without to much reduction in gain?
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