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I was studying a cascoding circuit that was built as a constant current source. I am having a few confusions in this circuit:

  1. I read that we need to increase the output resistance of the current source in order to make it work effectively. I am not able to grasp this at first. How will the load be connected and why do we need a higher resistance?
  2. Secondly in order to increase the resistance an Rs or resistance at the source was added. This was done to increase the output resistance. I am not able to understand how. Like the ro=Vd/Id where Id is the drain current. With the increase of the resistance at the source how can we increase the ro as ro is inversely proportional to the current which decreases with the increase of resistance? However, due to a higher gate voltage to be applied with the addition of a source resistance, a clever way was to add a MOSFET instead of the source resistance. But this mosfet is then used as a switch(triode region) as far as my understanding.
  3. I am not able to understand how the source mosfet increases the output resistance and why we want to increase output resistance, referring to part 1.

Kindly help me with these.

kam1212
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1 Answers1

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Q1 - Take a look here at the circuit diagram

enter image description here

Taken from here: Common drain JFET output resistance "problem"

And we are talking here about the current source output resistance, not the load resistance.

For the ideal current source, the output current does not depend on the voltage across it. But for the real current source, this is not true. The voltage across the current source will have an effect on the "output" current. And we model this effect by using a \$ro\$ resistor connected in parallel with an ideal current source. Because for an ideal current source source \$ro\$ = \$\infty\$.

So, this is the reason we want to have a current source with the highest output resistance. To get a "stiff" output current.

Q2 and 3 Rs or MOSFET adds negative current feedback to the circuit.

I/O Resistance of common source MOSFET with source degeneration

G36
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  • I understood what you said regarding the Q2, and Q3 but over there there is nothing stated regarding the MOSFET being used as a constant current source. – kam1212 Sep 15 '23 at 20:43
  • For a constant Vgs voltage and for Vds > (Vgs - Vth) - saturation region. The MOSFET will behave just like a constant current source. – G36 Sep 15 '23 at 20:46
  • And this is for the small signal anylysis only right? because we dont have ro in the DC. – kam1212 Sep 15 '23 at 23:46
  • For a DC we also see the influence of VDS voltage on drain current. – G36 Sep 16 '23 at 07:47