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/showthread.php?postid=1135809#post1135809
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/showthread.php?postid=1135809#post1135809
roender said:
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!
Hi
Piercarlo
roender said:
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.
Hi
Piercarlo
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.
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.
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.
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.
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.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=63187&highlight=
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.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=63187&highlight=
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.
--- 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.
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.
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.
forr said:
Well... but your circuit cease to be a cascode.
Hi
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.
Hi
Piercarlo
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