Chapter 7: FET Biasing

Chapter 7: FET Biasing

Common FET Biasing Circuits JFET Biasing Circuits Fixed Bias Self-Bias oltage-ivider Bias -Type MOSFET Biasing Circuits Self-Bias oltage-ivider Bias E-Type MOSFET Biasing Circuits Feedback Configuration oltage-ivider Bias 2

Basic Current elationships For all FETs: I G 0A I I S For JFETS and -Type MOSFETs: I I SS 1 GS P 2 For E-Type MOSFETs: I 2 k(gs T ) 3

Fixed-Bias Configuration S S C GS 0 GS S GG I 4

Self-Bias Configuration 5

Self-Bias Calculations For the indicated loop, GS IS To solve this equation: Select an I < I SS and use the component value of S to calculate GS Plot the point identified by I and GS. raw a line from the origin of the axis to this point. Plot the transfer curve using I SS and P ( P = GSoff in specification sheets) and a few points such as I = I SS / 4 and I = I SS / 2 etc. The Q-point is located where the first line intersects the transfer curve. Use the value of I at the Q-point (I Q ) to solve for the other voltages: S S I S S I S ( S ) 6

oltage-ivider Bias I G = 0 A I responds to changes in GS. 7

oltage-ivider Bias Calculations G is equal to the voltage across divider resistor 2 : G 1 2 2 Using Kirchhoff s Law: GS G I S The Q point is established by plotting a line that intersects the transfer curve. 8

oltage-ivider Q-point Step 1 Plot the line by plotting two points: GS = G, I = 0 A GS = 0, I = G / S Step 2 Plot the transfer curve by plotting I SS, P and the calculated values of I Step 3 The Q-point is located where the line intersects the transfer curve 9

oltage-ivider Bias Calculations Using the value of I at the Q-point, solve for the other variables in the voltagedivider bias circuit: S I S 1 I I 2 I I S 1 ( 2 S ) 10

-Type MOSFET Bias Circuits epletion-type MOSFET bias circuits are similar to those used to bias JFETs. The only difference is that depletion-type MOSFETs can operate with positive values of GS and with I values that exceed I SS. 11

Self-Bias Step 1 Plot line for GS = G, I = 0 A I = G / S, GS = 0 Step 2 Plot the transfer curve using I SS, P and calculated values of I Step 3 The Q-point is located where the line intersects the transfer curve. Use the I at the Q-point to solve for the other variables in the voltage-divider bias circuit. These are the same steps used to analyze JFET self-bias circuits. 12

Step 1 Plot the line for GS = G, I = 0 A I = G / S, GS = 0 Step 2 Plot the transfer curve using I SS, P and calculated values of I. oltage-ivider Bias Step 3 The Q-point is located where the line intersects the transfer curve is. Use the I at the Q-point to solve for the other variables in the voltage-divider bias circuit. These are the same steps used to analyze JFET voltage-divider bias circuits. 13

E-Type MOSFET Bias Circuits The transfer characteristic for the e-type MOSFET is very different from that of a simple JFET or the d-type MOSFET. 14

Feedback Bias Circuit I G = 0 A G = 0 S = GS GS = I 15

Step 1 Plot the line using GS =, I = 0 A I = /, GS = 0 Step 2 Using values from the specification sheet, plot the transfer curve with GSTh, I = 0 A GS(on), I (on) Step 3 The Q-point is located where the line and the transfer curve intersect Step 4 Using the value of I at the Q-point, solve for the other variables in the bias circuit Feedback Bias Q-Point 16

oltage-ivider Biasing Plot the line and the transfer curve to find the Q-point. Use these equations: G GS S 2 1 2 G IS I(S ) 17

oltage-ivider Bias Q-Point Step 1 Plot the line using GS = G = ( 2 ) / ( 1 + 2 ), I = 0 A I = G / S, GS = 0 Step 2 Using values from the specification sheet, plot the transfer curve with GSTh, I = 0 A GS(on), I (on) Step 3 The point where the line and the transfer curve intersect is the Q- point. Step 4 Using the value of I at the Q-point, solve for the other circuit values. 18

p-channel FETs For p-channel FETs the same calculations and graphs are used, except that the voltage polarities and current directions are reversed. The graphs are mirror images of the n-channel graphs. 19

Applications oltage-controlled resistor JFET voltmeter Timer network Fiber optic circuitry MOSFET relay driver 20