cascode question

[woody]

I am asking a general question I just picked this circuite for an example.
My question is if I replaced the source resistors r10,r11,r12 and r13 with
much larger resistors say 1k ohm each would the effective input capacitence change? I am not asking if it would work well or not just
what if any effect it would have on the input capacitence.

[gurevise]

Hi,
I do not see why input capacitance would change. It will not.
What will change is the total cascode circuit gain.

Thanks
Gurevise

[gurevise]

Woody,
I was wrong in previous post. It appears that JFET source resistance does affect input capacitance.
Look up Erno Borbely article about JFET's. It's available on internet.

Thanks

[magnoman]

Woody,

If I'm interpreting the schematic correctly then the effective input capacitance will decrease as you increase the degeneration resistors. It might help to think of it as; without the resistors all the input voltage is applied across the g-s junction whereas with the degeneration resistors some of the resulting increase in current from d-s generates a small voltage across the degeneration resistor which subtracts from the input voltage leaving a smaller voltage across the g-s terminals. This effectively means that this smaller voltage across g-s took less current to charge than would have been necessary to charge it to the full Vin level. Lots of other "stuff" happens as well.

Hope this helps
-Antonio

[magnoman]

Woody,

A little follow up:
I didn't say how much the capacitance would decrease that depends on a bit more, but basically the gain of the transistor will determine how much voltage is developed across the resistor and hence how much less the input voltage had to charge up the g-s junction. Also the d-g junction capacitance I think will remain not effected and the effective input capacitance is the parallel combination of the two (at least).

-Antonio

[woody]

Thanks for the replys. I will look up that Borbely article.

[AndrewT]

Hi,
Q5 cascode loads the 4 jFETs.
This massively reduces the apparent input capacitance.

4jFETs have a lot of input capacitance, but the cascode reduces this to manageable levels.

Now feed your perfect voltage source into a capacitor.
What is the phase angle presented to the Source?
Now add in a small resistor, effectively in series with that capacitor.
What is the new phase angle presented to the Source?
While you have those numbers to hand, what is the effective filter of that resistor+C?

Change the resistor to a bigger value.
What is the new phase angle? What is the new filter value?

The resistor changes the phase angle, and makes it easier to drive the capacitive load.
The capacitance has not actually changed.
The impedance of the load has changed.

[magnoman]

Andrew, well put.
In my reply I didn't mean to imply that the actual Fet capacitance changed but rather that the effective input impedance when viewed alone could be thought of as having a capacitive component which is reduced by both increasing the degeneration resistor and or increasing the transistor gain.

Thanks
-Antonio

[WuYit]

Andrew,
that`s a really roundabout explanation.

[AndrewT]

Hi,
it's about the best I can manage.

Better knowledge of AC would allow a simpler, more succinct answer.