Chapter 5FET2

Assuming device operates in saturation thus id s

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Assuming device operates in saturation thus iD s atisfies with iD ~ vGS equation. 2. According to biasing method, write voltage loop e quation. 3. Combining above two equations and solve these equations. 4. Usually we can get two value of vGS , only the on e of two has physical meaning. MOSFET amplifier: DC analysis

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3/2/12 SJTU J. Chen 11 5. Checking the value of vDS i. if vDS vGS-Vt , the assuming is correct. ii. if vDS vGS-Vt , the assuming is not correct. We s hall use triode region equation to solve the proble m again. DC analysis
3/2/12 SJTU J. Chen 12 The NMOS transistor is operating in the saturation region due to Examples of DC analysis t GD V V < t V 2V =

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3/2/12 SJTU J. Chen 13 Ø Assuming the MOSFET operate in the saturation region Ø Checking the validity of the assumption Ø If not to be valid, solve the problem again for triode region Examples of DC analysis
3/2/12 SJTU J. Chen 14 The MOSFET as an amplifier Graph determining the transfer characteristic of the amplifier Basic structure of the common-source amplifier

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3/2/12 SJTU J. Chen 15 The MOSFET as an amplifier and as a swit ch vi Time vI Time vo Ø Transfer characteristic showing operation as an amplifier biased at point Q. Ø Three segments: § XA---the cutoff region segment § AQB---the saturation region segment § BC---the triode region segment
3/2/12 SJTU J. Chen 16 Homework April 2, 2008: 5.2 ; 5.4 ; 5.9 ; 5.10 ;

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3/2/12 SJTU J. Chen 17 Voltage biasing schem e Biasing by fixing voltage (constant VGS ) Biasing with feedback resi stor Current-source biasing scheme Biasing in MOS amplifier circuits Ø Disadvantage of fixing biasing L Fixing biasing may result in large ID variability due to deviation in device performance L Current becomes temperature dependent - Unsuitable biasing method
3/2/12 SJTU J. Chen 18 Ø Biasing using a resistance in the source lead can reduce the variability in ID Ø Coupling of a signal source to the gate using a capacitor CC1 Biasing in MOS with feedback resistor

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• Fall '10
• ChengJianping
• J. Chen

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