Design of Low Power Instrumentation Amplifier

Instrumentation Amplifier

Design Goal

• I want to design an amplifier with more than 40 dB in Gain

• Less than 10 uA of current

Top Level Schematic

1st stage amplifier 2nd stageBias circuit

Design Strategy

• Get a voltage gain of 10 from the first stage (By choosing R1 and RG carefully)

• Get the rest of the gain from the second stage

• Reuse amplifier cell

Active Loaded Differential Amplifier

2 uA of currentVoltage gain of 24.56

Biasing of M3

2 uA of current

Testing of the ECG Circuit

(Biasing Circuit)

Low Voltage Cascode Bias Cicuit

To keep M2 in saturation: Vx>Vb-Vth→Vx+Vth2>Vb

To keep M1 in saturation: VA>Vx-Vth1

Since VA=Vb-VGS2, Vb>Vx-Vth1+VGS2

Design criteria for M2

Vb Requirement

Vb=VOD3+VGS4 to produce a minimum outputVoltage of VOD3 and VOD4.

By design, VGS4=VGS2, VA=VB

Vb=VOD2+VTH2+VOD1.

Vb Generation

Requirement:Vb=VOD2+VTH2+VOD1

VGS5=VGS2

VOD1=VGS6-VTH7

Problem: M5 suffers from no body effectM2 suffers from body effect

Design M7 (Large W7/L7) so that VGS7 is approx. VTH7

Need to have sufficiently largeVGS6, otherwise M6=triode since M7=Sat.

Testing of the ECG Circuit

(Biasing Circuit)

Biasing Circuitry

Top Level Schematic

1st stage amplifier 2nd stageBias circuit

2 uA

2 uA

2 uA

2 uA

Gain of the Amplifier

Trasient Simulation

Vin,diff=2 mVVout,single=336 mVVout,single/Vin,diff=168