Download Source Degeneration, Cascodes

E3 238 Analog VLSI Circuits
Lecture 11: Source Degeneration, Cascodes
Gaurab Banerjee
Department of Electrical Communication Engineering,
Indian Institute of Science, Bangalore
[email protected]
CS Amplifier with Source Degeneration
CS Amplifier with Source Degeneration
KCL =>
Small
•
Source Degeneration reduces effective transconductance -> Set by Rs
•
Linearizes the output
Source Degeneration
Intuitive Approach:
This is a source follower vin ≈ vs
Set by passives, gain more accurate -> this is an
aspect of feedback -> more later!
Output Resistance
Rs gets multiplied by
the self-gain of M1
Cascode Common Source Amplifier
By inspection, and comparing the Source Degenerated CS amplifier:
No degeneration
on M1
As RL -> ∞, i.e., the load becomes an ideal current source,
•
High gain with current reuse
•
Good isolation and stability
•
Good frequency response -> later!
Active Cascode
Can we make this
voltage constant
when vout is varied?
VX = VB
•
Put node X in feedback and hold it at a constant voltage.
•
If Vx= VB, (op-amp has infinite gain), VDS, M1 is held constant -> the current
in M1 is determined only by its Vgs
•
Change in the drain (output) current in response to changes in output
voltage is zero -> infinite output impedance.
•
In reality, op-amp has finite gain (a) which rolls off at high frequencies.
•
The gain “a” regulates Vx -> regulated or “active” cascode.
Analysis
>> 1
Active Cascode
Output impedance
of M1
Self gain of M2
“active”
multiplying factor
The gain is given by:
Very high gain circuit -> This technique is sometimes called “gain boosting”