Common Gate, Source Follower, Cascode

Single Stage Amplifiers (2)
• Source Followers (Common Drain)
• Common Gate
• Cascode
Source Follower
• You can’t drive a low impedance load
with a common source amplifier
• Can be used as a buffer to drive a low
impedance load
• Can provide voltage level shift
Input/Output Characteristic
As Vin increases, ID1 increases leading to an increase of Vout.
Small Signal Gain
Can you derive a Av without explicitly using the
small signal model?
Source Follower Example
gmbs=0.328 mS
Res=6K Ohm
Vin, pp=2
Av=0.796 (from simulation)
Av=0.801 (from small signal calculation)
Vin=330 mV
More Distortion
Linearity as a function of Vin
Small Signal Gain as a function
of Vin
gm depends on ID. When Vin=0, gm=0.
gm will increase as Vin increases. Thus Av will approaches 1.
Notice that as you move the red vertical line horizontally,
Av changes quite about. So there is quite a bit of nonlinearity.
Source Follower using an NMOS
transistor as a current source
If we can stabilize the current through M1, then gm1 will be
stabilize against Vin.
Av is less sensitive to Vin.
The linearity of the circuit will improve.
Output Resistance of a Source
Have you seen this
Alternative Approach to Rout
Av when RS is large
If RS is sufficiently large, then the small signal gain of the amplifier
can be obtained using thevenin’s equivalent circuit (see hand out)
SF with a NMOS CS Load
Design Example
• SF with an NMOS Current Source
Vout, pp=1.705 mV
Av=0.857 (Analytical)
Vin=330 mV
Vin=330 mV
Vin=330 mV
SF with a Diode Connected
Impedance into S of M2
Reduce sensitivity of Av due to
gm1 is constant.
gmb1 is noticeable.
The NWELL of M1 is tied to its source.
Thus, body-source effect is negligible.
Extra Voltage Headroom
Required by SF
Without the SF, the minimum voltage of VX is VDSAT1.
If SF is used as a buffer, Vout must be greater than VDSAT3.
The minimum voltage at X is increased because we have to
maintain sufficient voltage for VGS2.
As a result the minimum voltage at X is raised.
Compare SF to CS
DC Level Shifter
By using a SF, Vin can go to a higher voltage without driving
M1 into the linear region.
Common Gate Amplifier
Direct Vs. Capacitive Coupling
(Direct Coupling)
(Capacitive Coupling)
Gain of CG
If RS=0 and channel length modulation is ignored, Av is
CS followed by a CG
Using formula
from the
previous slide
Input Resistance of CG
Special cases:
1. RD=infinity (current
2. RD=0
Input resistance is approximately the drain resistance
divided by (gm+gmb)ro
Output Resistance of CG
• Similar to CS
• Cascode=CS+CG
M1 generates a small signal
drain current proportional to Vin.
M2 simply routes the current to RD.
DC Bias of a cascode
M1 and M2 must be kept in saturation.
Output Resistance of a
M2 boosts the output resistance of M1 by a factor
of (gm2+gmb2)ro2ro1
Triple Cascode
Advantage: higher output resistance
Disadvantage: the minimum output voltage is equal to
the sum of three overdrive voltages.
Voltage Gain
Exact Voltage Gain of Cascode
with a Current Source Load
Implementation of Current
Source Using Cascode
Shielding Property of a Cascode
Current Mismatch
Current Mismatch Using
Folded Cascode
Output Resistance
Choice of Device Model
• Break the circuit down into a number
of familiar topologies
• Use the simplest model
• If the drain of a device is connected to
high impedance (i.e. drain of another),
then add ro to its model
• Introduce a more sophisticated circuit
model if necessary

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