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Transcript
MODULE- 7 FET & MOSFET Biasing 1 FET BIASING:• The purpose of biasing FET is to select the proper dc gate-tosource voltage to establish a desired value of drain current. • Hence, dc biasing is a must. • Here, we need reverse-biasing of the input junction. Hence, biasing is much simpler. • Bias stabilization is also much simpler. • Voltage – divider bias circuit is normally used. • The dc current and voltage decide the operating point (or quiescent point, or simply Q-point). JFET biasing circuits :1. Self bias circuit. 2. Current source bias circuit. 3. Voltage divider circuit. 2 SELF BIAS: • Self-bias is the most common type of JFET bias. • The condition requires a negative VGS for an nchannel JFET and a positive VGS for a p-channel JFET. • The gate resistor, RG, does not affect the bias because it has essentially no voltage drop across it. 3 JFET self bias circuit :- 4 5 JFET Self Bias dc equivalent circuit :- IG = 0 VRG = 0 Similarly, For p-channel JFET, Since IS = ID and VG = 0, then VS=ID RS NOTE: The analysis of the p-channel JFET is the same except for opposite-polarity voltages. 6 Example:1 Find VDS and VGS in Fig. below For the particular JFET in this circuit, the parameter values such as gm, VGS(off), and IDSS are such that a drain current (ID) of approximately 5mA is produced. Another JFET, even of the same type, may not produce the same results when connected in this circuit due to the variations in parameter values. 7 Solution: 8 Setting the Q-Point of a Self-Biased: • The basic approach to establishing a JFET bias point is to determine ID for a desired value of VGS or vice versa. •For a desired value of VGS, ID can be determined in either of two ways: • 1st from transfer characteristic curve for the particular JFET. • 2nd using IDSS and VGS(off) from the JFET datasheet. 9 Example:2 Determine the value of RS required to self-bias an nchannel JFET that has the transfer characteristic curve shown in Figure below at VGS = -5V. 10 Solution: 11 Example:3 Determine the value of RS required to self-bias a pchannel JFET with datasheet values of IDSS = 25 mA and VGS(off)= 15 V. VGS is to be 5 V. Solution:- 12 Midpoint Bias: • The midpoint bias allows the maximum amount of drain current swing between IDSS and 0. • The current ID is approximately one-half of IDSS when VGS = (0.3)(VGS(off)). •To set the drain voltage at midpoint (VD=VDD/2), select a value of RD to produce the desired voltage drop. 13 Voltage –Divider bias circuit : 14 Input Output 15 Example:5 Determine ID and VGS for the JFET with voltage-divider bias in Figure below, given that for this particular JFET the parameter values are such that VD =7V.). Solution. 16 JFET Voltage Divider Bias Load Line 17 Q-Point Stability:- In voltage-divider bias, the dependency of ID on the range of Qpoints is reduced because the slope of the load line is less than for self-bias for a given JFET. VGS varies quite a bit for both self-bias and voltage-divider bias. ID is much more stable with voltage-divider bias. So Voltage divider bias is more stable than self bias. 18 18 19 Current Source Bias: Current-source bias is a method for increasing the Q-point stability of a self-biased JFET by making the drain current essentially independent of VGS. This is accomplished by using a constant-current source in series with the JFET source. A BJT acts as the constant-current source because its emitter current is essentially constant if VEE>>VBE FET is also used as a constant current source. 20 20 21 22 Example:7 A current-source bias circuit like Fig. below has the following values: VDD= 9 V, VEE = -6 V, and RG = 10MΩ To produce a 10 mA drain current and a 5 V drain voltage, determine the values of RE and RD. Solution:- 23 MOSFET Biasing:•Three ways to bias a MOSFET are zero-bias, voltage-divider bias, and drain-feedback bias. • Because E-MOSFETs must have a VGS greater than the threshold value, VGS(th), zero bias cannot be used. • So two ways to bias an E-MOSFET using potential divider and drain feedback bias. • D-MOSFETs can also be biased using these methods. 24 E- MOSFET Biasing:- (Voltage- divider bias) • In either the voltage-divider or drain-feedback bias arrangement, the purpose is to make the gate voltage more positive than the source by an amount exceeding VGS(th). 25 Drain Feedback Bias :•In the drain-feedback bias circuit in Fig. (b), there is negligible gate current and, therefore, no voltage drop across RG. • This makes VGS = VDS. 26 Example: 8 Determine VGS and VDS for the E-MOSFET circuit in Fig. Assume this particular MOSFET has minimum values of ID(on)=200mA at VGS=4V and VGS(th)=2V. Sol.: 27 28 Example: 9 Determine the amount of drain current in Fig. below. The MOSFET has a VGS(th) = 3 V. Sol.: 29 D- MOSFET Biasing:• D-MOSFETs can be operated with either positive or negative values of VGS. •A simple bias method is to set VGS=0 so that an ac signal at the gate varies the gate-to source voltage above and below this 0 V bias point. 30 Example: 10 Determine the drain-to-source voltage in the circuit of Figure below. The MOSFET datasheet gives VGS(off) = -8 V and IDSS = 12 mA. Sol.: 31