ECE523/421 - Lecture 3 Slide: 1University of New Mexico

Office: ECE Bldg. 230BOffice hours: Tuesday 2:00-3:00PM or by appointment

E-mail: pzarkesh@unm.edu

Payman Zarkesh-Ha

ECE 523/421 – Analog Electronics

Lecture 3: Basic MOSFET Amplifiers II

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Review of Last Lecture MOSFET Biasing for Amplification

MOSFET Small Signal (Π and T Models)

Basic MOSFET Amplifier Configurations (CS, CG, CD)

Examples

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Today’s Lecture Biasing in MOS Amplifier Circuits

• Fix VGS Biasing

• Fix VGS Biasing with Source Resistance

• Drain-to-Gate Feedback Resistor Biasing

• Constant Current Source Biasing

Role of Body Effect in MOS Amplifiers

Temperature Effect

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Problem of Fix VGS Biasing

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Advantage of RS in Fix VGS Biasing

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Example: Bias Circuit Design I

1) Design the following circuit to operate at a dc drain current of 0.5 mA and VD = +2 V. Let Vt= 1V , K’nW/ L = 1 mA/V2, and λ=0, VDD=VSS=5 V. Use standard 5% resistor values and give the resulting values of ID, VD, and VS.

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Drain-to-Gate Feedback Biasing

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Constant Current Source Biasing

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Using two transistors Q1 and Q2 having equal lengths but widths related by W2/W1=5, design the current mirror circuit to obtain I=0.5mA. Let VDD=-VSS=5V, K’n(W/L)1=0.8mA/V2, Vt=1V, and λ=0. Find the required value for R. What is the voltage at eth gate of Q1 and Q2? What is the lowest voltage allowed at the drain of Q2 while Q2 remains in the saturation region?

Example: Bias Circuit Design II

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Review of Common-Source Amplifier

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Review of Common-Gate Amplifier

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Review of Source Follower Circuit

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Quick Review of Frequency Response

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Modeling of Body Effect

mmb gg

SBf V22

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Temperature Effect Vt increases by about 2mV/ºC by temperature increase

K’n decreases more by temperature increase

Overall ID decreases as temperature increase

This is an advantage for MOSFET devices (No thermal runaway)