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Old 17th February 2007, 07:40 PM   #11
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Cascodding will reduce change in collector-emitter voltage.
That reduces change in collector-base capacitance and thus 'linearises' the transistor.
It also reduces the effect of the capacitance by providing a lower impedence for the transistor being cascoded.
A third advantage of a cascode is to increase voltage and power capability by dividing power/voltage between the two transistors.

Because the two transistors are connected in 'series', the same current is going through each transistor. With the same current, phase will also be the ssame.
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Old 17th February 2007, 08:20 PM   #12
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Quote:
Originally posted by roender
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?
I a cascode circuit lower stage has no voltage gain at all. Just current and power gain but *not* voltage gain.

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Old 24th February 2007, 04:49 AM   #13
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I agree with Mr.Piercarlo
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Old 25th February 2007, 04:00 AM   #14
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i tried modifying a particularly troublesome amp design using a cascode in the VAS, and it made a difference, so i added a cascode to the predriver stage and got some improvement in distortion, but went back to square one as far as stability...... hmmm.....

voltage source voltages for cascodes seem to be somewhat critical, and i need to "create" some new zeners for LTSpice, so i can "tweak" the cascode voltages.
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Old 25th February 2007, 09:23 AM   #15
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Hi, Unclejed613,

There's a good thread here by Darkfenriz about cascodes that said to be carefull with implementation of cascodes. It can easily turned to oscilator. Base stoppers helps with cascodes.

Cascodes- the truth is out there...
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Old 25th February 2007, 09:49 AM   #16
forr is offline forr  France
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Hi Myhrrhleine
--- the two transistors are connected in 'series', the same current is going through each transistor. ---

Self mentions the curious fact that you can have different currents through the two devices in series, using a bleeding load in parallel of one of them, and that this can provide better linearity.
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Old 25th February 2007, 10:51 AM   #17
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Hi,
a major advantage gained by adding a cascode if much improved/extended high frequency response. This implies that removal of the cascode reduces the frequency response.

It follows that a stage, without cascode, has a low frequency response that will demand a different overall stability strategy than a stage (with added cascode) with more higher frequency gain.

As I see it, adding a cascode will require the whole amplifier to be stabilised for the new open loop gain. It is not a case of becoming more susceptible to oscillation, it is effectively a new amplifier and back to basics.
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Old 25th February 2007, 11:37 AM   #18
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Quote:
Originally posted by forr
Hi Myhrrhleine
--- the two transistors are connected in 'series', the same current is going through each transistor. ---

Self mentions the curious fact that you can have different currents through the two devices in series, using a bleeding load in parallel of one of them, and that this can provide better linearity.
Well... but your circuit cease to be a cascode.

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Piercarlo

PS - for Unclejed. The good working of a cascode circuit rely heavily on the quality of grounding the base of upper devices for all the bandiwidth of interest. If somewhere in the bandiwidth the impedance from base to ground increase or change nature (i.e. from resistive to reactive), cascode turn itself in something of "hybrid" which must be carefully anallized on the field before judging if it is again a good fitting for the bandwidth of concern or must be substituted by others, more suitable circuits.
in any case, never let working a cascode circuit outside the bandwidth for wich it is designed.

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