BasicBJTSmall-SignalModel.pptVIP

Basic BJT Small-Signal Model Computing Small-Signal Models Basic BJT Small-Signal Model Basic Frequency Effects BJT Small-Signal Model Frequency Dependence of a BJT Current Gain versus Frequency Frequency Dependence of a BJT Complete Small-Signal Model Additional Effects Complete Circuit Element Model * gmV ro rp + V - VC VE VB VC VE VB gm = IC / UT rp= b UT / IC ro = VA / IC Key Terms: gm rp= b ; gm ro = VA / UT Current Gain Voltage Gain VE VC VB IC = IC0 e e VBE/UT Forward-Active Model: (Exponential Form) VCE/VA gm = = IC0 e e (1 / UT) = IC / UT dIC dVBE VBE/UT VCE/UT 1/ ro = = IC0 e e (1 / VA) = IC / VA dIC dVCE VBE/UT VCE/VA gmV ro rp + V - Vout Vin AI = gm rp= b Current Gain Voltage Gain AV = gm ro = VA / UT GND gmV rp + V - Iin Iout GND GND Iin GND GND Iout GND GND Vin GND Vout Iref Vdd Cb = Base-Charging Capacitance of the Small Signal BJT Model Modifying Charge from the Base when changing Base voltage: Cb = tF gm Cje = base-emitter depletion capacitance (forward biased) Cjbc = base-collector depletion capacitance (reversed biased) Cp = Cje + Cb; Cm = Cjbc gmV ro rp + V - VC VE VB Cp Cm Basic measures of Frequency Characteristics: fT 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 0 10 1 10 2 Frequency (Hz) Collector current / Base current b0 fT Collector-Base Resistance of the Small Signal BJT Model (rμ) In general, rμ 3 10 boro for the npn BJT and about 2-5 boro for the lateral pnp BJT Series Resistance (rb, rc, re) ---- Finite conductivity of doped regions