Example Vnr1sqr=2.3288 x 10-19 Vnr3sqr=7.7625 x 10-20 Vnoutsqr=3.1050 x10-19
Noise Spectrum Shaping by a Low Pass Filter As R increases, 4kTR increases, but the bandwidth decreases. Therefore, the bandwidth is constant. Pn,out can only be decreased by increasing C.
MOSFETS (Typically 2/3, not to be confused with body effect coefficient)
Noise Voltage Generated Per Device VDS=0.6 I1=100 uA
Flicker Noise Model The flicker noise is modeled as a voltage source in series with the gate: The trap-and-release phenomenon associated with the dangling bond occurs at low frequencies more often. Device area can be increased to decrease noise due to flicker noise.
Corner Frequency • Definition: the frequency at which the thermal noise equals the flicker noise.
Representation of Noise in Circuits • Output noise • Input noise • Voltage noise source • Current noise source
Problem of Output Noise • Output noise depends on the gain of the amplifier, for example.
Input-Referred Noise Voltage Problem: only valid for when source impedance is low.
Calculation of Input-Referred Noise Low source impedance High source impedance
Input Current More significant at High frequencies!
Noise in Single Stage Amplifier • Equivalent CS Stages • CS • CG • SF • Cascode • Differential Pairs
Common Source Amplifier The transconductance of M1 must be maximized in order to minimize input-referred noise.
Input Referred Thermal Noise Voltage M2 acts as the current source. The gm of M2 should be minimized. M1 acts as the amplifier. The gm of M1 should be maximized.
Noise Simulation Thermal noise 3.758 nV/sqrt(Hz) Av=28.711
Noise Simulation 3.126 nV/sqrt(Hz) Av=33.42
Comparison (simulated input-referred thermal noise) Av=33.42
Flicker Noise Dominated by Flicker noise Dominated by Thermal noise
Common Gate Amplifier • Need to consider • Input referred voltage source • Input referred current source
Gain of CG If RS=0 and channel length modulation is ignored, Av is
Input-Referred Current Source Does not produce a current to the output
Design Example Design criteria: gm/ID=5 for M0, M2, M3 and M4. gm/ID=20 for M1. I1=10 uA I2=10 uA I(M1)=40 uA
Source Followers High Input impedance, noise current source is negligible.
Differential Pair (negligible)