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MODULE- 7
FET & MOSFET Biasing
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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.
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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.
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JFET self bias circuit :-
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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.
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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.
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Solution:
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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.
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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.
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Solution:
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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:-
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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.
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Voltage –Divider bias circuit :
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Input
Output
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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.
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JFET Voltage Divider Bias Load Line
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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.
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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.
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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:-
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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.
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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).
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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.
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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.:
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Example: 9 Determine the amount of drain current in Fig.
below. The MOSFET has a VGS(th) = 3 V.
Sol.:
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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.
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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.:
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