Dr. Ke Huang

COMPE 572

VLSI Circuit Design

Fall 2015

Lecture 2: MOSFET Bias Design VLSI Circuit Design

• In my file, the following students are enrolled but have not provided the proof of prerequisites yet:

8/27/2015 COMPE 572 VLSI Circuit Design 2

Announcement

• Introduction of VLSI

• Review of bipolar junction transistor (BJT)

• Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

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Review

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Review of bipolar junction transistor (BJT)

• Simplified structure of the npn and pnp transistors

npn transistor

pnp transistor

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Review of bipolar junction transistor (BJT)

• Characteristic for an npn transistor

𝑖𝐶 − 𝑣𝐵𝐸 characteristic 𝑖𝐶 − 𝑖𝐵 − 𝑣𝐶𝐸 characteristic

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Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

• Physical structure of NMOS transistors

Physical structure of NMOS Cross-section of NMOS

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Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

• Complementary MOS (CMOS)

Cross-section of a CMOS integrated circuit

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Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

• Characteristics of nMOS

G

S

D

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• Characteristics of nMOS – a 3-dimensional view

𝑉𝐷𝑆

𝑉𝐺𝑆

𝐼𝐷

MOSFET Bias Design

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• Characteristics of nMOS – a 3-dimensional view

𝑉𝐷𝑆

𝑉𝐺𝑆

𝐼𝐷

MOSFET Bias Design

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Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

• Characteristics of nMOS

𝑖𝐷 − 𝑉𝐺𝑆 − 𝑉𝐷𝑆 characteristic

G

S

D

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Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

• Characteristics of nMOS

𝑖𝐷 − 𝑉𝐺𝑆 characteristic in saturation region

or G

S

D

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Review of metal–oxide–semiconductor field-effect transistor (MOSFET)

• Characteristics of pMOS

G

D

S

• Biasing by fixing 𝑉𝐺𝑆

• Biasing using a drain-to-gate feedback resistor

• Biasing using a constant-current source

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Learning objectives

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MOSFET Bias Design

• MOSFET characteristics

G

S

D

We need to consider all the 3 regions (cutoff, triode, saturation) for designing digital circuits, today we will focus on the saturation region

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• Characteristics of nMOS

𝑖𝐷 − 𝑉𝐺𝑆 characteristic in saturation region

or G

S

D

MOSFET Bias Design

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• Why we need to bias transistors

MOSFET Bias Design

Bias: State of the transistor when there is no signal (current and voltages in all elements) Bias is constant in time (may vary extremely slowly

compared to signal) Purpose of the bias is to ensure that MOS is in saturation

at all times.

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MOSFET Bias Design

• Biasing by fixing 𝑉𝐺𝑆

G

S

D

Fixed voltage value

GND

Subject to change

Fixed voltage value

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MOSFET Bias Design • Biasing by fixing 𝑉𝐺𝑆 and connecting a resistance

in the source

The same 𝑉𝐺 applied to device 1 and device 2 will result in two 𝑉𝐺𝑆 values 𝑉𝐺𝑆1 and 𝑉𝐺𝑆2, which will in turn result in very close 𝐼𝐷 values

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MOSFET Bias Design • Biasing by fixing 𝑉𝐺𝑆 and connecting a resistance

in the source: practical implementations

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MOSFET Bias Design • Biasing by fixing 𝑉𝐺𝑆 and connecting a resistance

in the source: an example

Design the circuit on the left to establish a DC drain current 𝐼𝐷 = 0.5mA. The MOSFET is specified to have 𝑉𝑡𝑛 = 1V and 𝑘𝑛

′ 𝑊/𝐿 =1mA/V2. Use a power-supply 𝑉𝐷𝐷 = 15V and

we assume that 𝑉𝐷𝑆, 𝑅𝐷 and 𝑅𝑆 take one-third of supply voltage, respectively.

Determine the values of 𝑅𝐺1 and 𝑅𝐺2. Calculate the percentage change in the value

of 𝐼𝐷 obtained when the MOSFET is replaced with another unit having the same but 𝑉𝑡𝑛 = 1.5V.

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MOSFET Bias Design • Biasing by fixing 𝑉𝐺𝑆 and connecting a resistance

in the source: an example

𝑉𝐷𝑆 takes one-third of supply voltage:

15V ×1

3= 5V

𝑅𝐷 =𝑉𝐷𝐷 − 𝑉𝐷

𝐼𝐷=

15 − 10

0.5= 10 kΩ

𝑅𝑆 =𝑉𝑆

𝐼𝐷=

5

0.5= 10 kΩ

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MOSFET Bias Design • Biasing by fixing 𝑉𝐺𝑆 and connecting a resistance

in the source: an example

Determine 𝑉𝐺𝑆

𝐼𝐷 =1

2𝑘𝑛′(𝑊/𝐿)(𝑣𝐺𝑆 − 𝑉𝑡𝑛)2

⇒ 𝑣𝐺𝑆 = 2 V

0.5 =1

2× 1 × (𝑣𝐺𝑆 − 1)2

⇒ 𝑣𝐺 = 𝑉𝑠 + 𝑉𝐺𝑆 = 5 + 2 = 7 V

We can set 𝑅𝐺1 = 8 MΩ and 𝑅𝐺2 = 7 MΩ

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MOSFET Bias Design • Biasing by fixing 𝑉𝐺𝑆 and connecting a resistance

in the source: an example

If 𝑉𝑡𝑛 = 1.5V this time

𝐼𝐷 =1

2× 1 × (𝑣𝐺𝑆 − 1.5)2 … (1)

𝑣𝐺 = 𝑉𝑠 + 𝑉𝐺𝑆 = 𝐼𝐷𝑅𝑆 + 𝑉𝐺𝑆

7 = 104𝐼𝐷 + 𝑉𝐺𝑆 … (2)

⇒ 𝐼𝐷 = 0.455mA

Percentage change is |0.455−0.5|

0.5= 9%

Combining (1) and (2), we have

• Biasing by fixing 𝑉𝐺𝑆

• Biasing using a drain-to-gate feedback resistor

• Biasing using a constant-current source

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Learning objectives

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MOSFET Bias Design • Biasing using a drain-to-gate feedback resistor

𝑉𝐷𝐷

Slope = −1/𝑅𝐷

• Biasing by fixing 𝑉𝐺𝑆

• Biasing using a drain-to-gate feedback resistor

• Biasing using a constant-current source

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Learning objectives

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MOSFET Bias Design • Biasing using a constant-current source

General scheme

Large input resistance value

Constant current source

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MOSFET Bias Design • Biasing using a constant-current source

Implementation of the constant-current source 𝐼 using a current mirror

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MOSFET Bias Design • Biasing using a constant-current source

Analysis of current mirror

⇒

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MOSFET Bias Design • Biasing using a constant-current source

Analysis of current mirror

The output current value 𝐼 (or 𝐼𝐷2) can be easily controlled by setting the W and L in the transistor 𝑄2

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MOSFET Bias Design • Biasing using a constant-current source

Example of current mirror application

Using two transistors 𝑄1 and 𝑄2 having

equal lengths but widths related by 𝑊2

𝑊1= 5,

design the circuit to obtain 𝐼 = 0.5 mA. Let 𝑉𝐷𝐷 = 5 V, −𝑉𝑆𝑆 = −5 V, 𝑘𝑛

′ (𝑊/𝐿)1=0.8mA/V2, 𝑉𝑡𝑛 = 1V. Find the required value for 𝑅. What is the voltage at the gates of 𝑄1 and 𝑄2 ? What is the lowest voltage allowed at the drain of 𝑄2 while 𝑄2 remains in the saturation region?

85k, -3.5V, -4.5V

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Summary

• Biasing by fixing 𝑉𝐺𝑆

• Biasing using a drain-to-gate feedback resistor

• Biasing using a constant-current source