(8.1.6)--Chapter 7模拟电子技术基础.ppt

Chapter 7 FET BiasingIntroductionFixed-Bias ConfigurationDepletion-Type MOSFETsEnhancement-Type MOSFETsOutline7.1 Introduction a.Assuming device operates in saturation thus iD satisfies with iDvGS equation.b.According to biasing method,write voltage loop equation.c.Combining above two equations and solve these equations.d.Usually we can get two value of vGS,only the one of two has physical meaning.e.Checking the value of vDSi.if vDSvGS-Vt,the assuming is correct.ii.if vDSvGS-Vt,the assuming is not correct.We shall use triode region equation to solve the problem again.MOSFET Circuit at DCThe NMOS transistor is operating in the saturation region due to MOSFET Circuit at DCAssuming the MOSFET operate in the saturation regionChecking the validity of the assumptionIf not to be valid,solve the problem again for triode regionThe MOSFET As an AmplifierBasic structure of the common-source amplifier.Graphical construction to determine the transfer characteristic of the amplifier in(a).The MOSFET As an Amplifier and as a SwitchTransfer characteristic showing operation as an amplifier biased at point Q.Three segments:XA-the cutoff region segmentAQB-the saturation region segmentBC-the triode region segment7.2 Fixed-Bias Configuration Voltage biasing schemeBiasing by fixing voltageBiasing with feedback resistorCurrent-source biasing schemeBiasing in MOS Amplifier CircuitsThe use of fixed bias(constant VGS)can result in a large variability in the value of ID.Devices 1 and 2 represent extremes among units of the same type.Current becomes temperature dependentUnsuitable biasing methodBiasing in MOS Amplifier CircuitsBiasing using a fixed voltage at the gate,and a resistance in the source lead(a)basic arrangement;(b)reduced variability in ID;(c)practical implementation using a single supply;Biasing in MOS Amplifier Circuits(d)coupling of a signal source to the gate using a capacitor CC1;(e)practical implementation using two supplies.Biasing in MOS Amplifier CircuitsBiasing the MOSFET using a large drain-to-gate feedback resistance,RG.Biasing in MOS Amplifier Circuits(a)Biasing the MOSFET using a constant-current source I.(b)Implementation of the constant-current source I using a current mirror.Biasing in MOS Amplifier Circuits(d)coupling of a signal source to the gate using a capacitor CC1;(e)practical implementation using two supplies.Biasing in MOS Amplifier CircuitsBiasing the MOSFET using a large drain-to-gate feedback resistance,RG.Biasing in MOS Amplifier Circuits(a)Biasing the MOSFET using a constant-current source I.(b)Implementation of the constant-current source I using a current mirror.7.3 Depletion-Type MOSFETs UGS=-ISRS7.4 Enhancement-Type MOSFETs