Single Stage Amplifier

• Common Source Amplifier• Source Follower• Common Drain

Common Source Amplifier

• Resistive Load• Diode Connected• Current Source Load• Triode Load• Source Degeneration

CS with Resistive Load

CS with Resistive Load

W/L=35.6 um/0.6um

Small Input Signal

Amplitude of input: 2 mV (peak to peak)Amplitude of output: 19.5 (peak to peak)Av=9.75

Small Signal Operation

Calculation of Small Signal Gain

(Valid only for device in saturation region)

Small Signal Model

1. small signal analysis2. the intuitive approach

Numerical Calculation

• Small Signal Gain:– gm=2 mS

– ro=1/gds=22.288 Kohms

– RD=6 Kohms

– AV=-gm(roRD/(ro+RD))=-9.454

What if the magnitude of the input signal is increased?

Distortion is observed when Vin swings high.

Triode Region (1)Triode Region

Reduction of transconductancein the triode region

Triode Region (2)

Disadvantage of resistively loaded CS:The DC level of Vout is tied to the AV.An increase of RD will increase AV, but at the price of pushingthe transistor closer into the triode region.

Diode Connected Load

• Common Source– Diode Connected Load– NFET Implementation– PFET Implementation

Diode Connected Load

Impedance:

(Body Effect ignored.)

Useful when tightly controlled resistors are not available, or physically not realizable.

Diode Connected Load With Body Effect

Example

M1: W/L=4.62 um/0.6umBias Current: 100 uAgm2=8 mSgds2=35.43 uSgmbs2=100.6 uSRX=1,068 Ohms

CS Stage with Diode Connected Load

Not sensitive of bias current, e.g. gm

Better linearity.

Diode Connected Load (NFET)

W/L_To=35.6 um/0.6um; W/L_T1=4.62 um/0.6um

Amplitude of input: 2 mV (peak to peak)Amplitude of output: 4.447 mV (peak to peak)Av=2.22

Design Issue

• If the bias current is fixed and the dc Vout (VGS) is fixed, then (W/L) of the load is fixed.

• It maybe difficult to increase Av.

Diode Connected PFET with CS

Plot

Amplitude of input: 2 mV (peak to peak)Amplitude of output: 15.24 mV (peak to peak)Av=7.62

Gain Calculation

M1: W/L=6.00 um/0.6umBias Current: 100 uACurrent source: 75 uAgm=2 mSgm2=200 uSgds2=4.69 uSgds=44.8 uS

Av=-8.013

CS Stage with Current Source Load

ro2 depends on L and ID of a transistor.|VDS2,min=VGS-Vth2| can be reduced by increasing the width of M2.

Downside: the DC output voltage is not well-defined.

Spice Example

Amplitude of input: 2 mV (peak to peak)Amplitude of output: 62.92 mV (peak to peak)Av=-31.44

Sensitivty

Even a slight deviation in the gate voltage of PFET is enoughto change the output voltage signficantly.E.g. Sensitivity of output voltage around 0.6 V.

Calculation:

W/L for T2: 24.06um/0.6umro1=22.288 Kohm

ro2=53.20 Kohm

gm=2 mS

Av=-31.41 Ideal Vout=0.6 VVth2=-0.269Vod=-0.213Vb=717.64 mV

CS Stage with Triode Load

Disadvantage: Sensitivity to a precise Vb.Advantage: Vout, max=VDD

CS Stage with Source Degeneration

• Intuition• Small signal gain• Output resistance

Intuition

Vin↑, ID ↑, VS ↑Δvin is dropped across RS, thus leading to a smoother variation of ID.

Small Signal Gain

If gmRS>>1, AV is approximately RD/RS

ДID=ДVin/RS

Effective Gm with Non-negligible body effect

Gain By Inspection

Interpretation: The resistance at the drainDivided by the resistance in the source path

Determination of Gain by Inspection Example

Output Resistance

Homework: Derive the output resistance using the smallequivalent circuit

A More Intuitive Approach

Analysis

Norton Equivalent Circuit

Gain of a Degenerated Current Source

Conductance with body effectOutput Resistance