Depletion-type MOSFET bias circuits are similar to JFETs. The only difference is that the depletion-Type MOSFETs can operate with positive values of V GS and with I D values that exceed I DSS. Depletion-Type MOSFETs 1

The DC Analysis Same as the FET calculations Same as the FET calculations Plotting the transfer characteristics of the device Plotting the transfer characteristics of the device Plotting the at a point that V GS exceeds the 0V or more positive values Plotting the at a point that V GS exceeds the 0V or more positive values Plotting point when V GS =0V and I D =0A Plotting point when V GS =0V and I D =0A The intersection between Shockley characteristics and linear characteristics defined the Q-point of the MOSFET The intersection between Shockley characteristics and linear characteristics defined the Q-point of the MOSFET The problem is that how long does the transfer characteristics have to be draw? The problem is that how long does the transfer characteristics have to be draw? We have to analyze the input loop parameter relationship. We have to analyze the input loop parameter relationship. As R S become smaller, the linear characteristics will be in narrow slope therefore needs to consider the extend of transfer characteristics for example of voltage divider MOSFET, As R S become smaller, the linear characteristics will be in narrow slope therefore needs to consider the extend of transfer characteristics for example of voltage divider MOSFET, The bigger values of V P the more positive values we should draw for the transfer characteristics The bigger values of V P the more positive values we should draw for the transfer characteristics Depletion-Type MOSFETs 2

Analyzing the MOSFET circuit for DC analysis How to analyze dc analysis for the shown network? How to analyze dc analysis for the shown network? It is a …. Type network It is a …. Type network Find V G or V GS Find V G or V GS Draw the linear characteristics Draw the linear characteristics Draw the transfer characteristics Draw the transfer characteristics Obtain V GSQ and I DQ from the graph intersection Obtain V GSQ and I DQ from the graph intersection 3

1. Plot line for V GS = V G, I D = 0 and I D = V G /R S, V GS = 0 2. Plot the transfer curve by plotting I DSS, V P and calculated values of I D. 3. Where the line intersects the transfer curve is the Q-point. Use the I D at the Q-point to solve for the other variables in the voltage-divider bias circuit. These are the same calculations as used by a JFET circuit. 4

When R S change…the linear characteristics will change.. 1. Plot line for V GS = V G, I D = 0 and I D = V G /R S, V GS = 0 2. Plot the transfer curve by plotting I DSS, V P and calculated values of I D. 3. Where the line intersects the transfer curve is the Q-point. Use the I D at the Q-point to solve for the other variables in the voltage-divider bias circuit. These are the same calculations as used by a JFET circuit. 5

The transfer characteristic for the enhancement-type MOSFET is very different from that of a simple JFET or the depletion-typeMOSFET. Enhancement-Type MOSFET 6

Transfer characteristic for E-MOSFET Transfer characteristic for E-MOSFETand 7

Feedback Biasing Arrangement I G =0A, therefore V RG = 0V Therefore: V DS = V GS Which makes 8

1. Plot the line using V GS = V DD, I D = 0 and I D = V DD / R D and V GS = 0 2.Plot the transfer curve using V GSTh, I D = 0 and V GS(on), I D(on) ; all given in the specification sheet. 3.Where the line and the transfer curve intersect is the Q-Point. 4.Using the value of I D at the Q-point, solve for the other variables in the bias circuit. Feedback Biasing Q-Point 9

DC analysis step for Feedback Biasing Enhancement type MOSFET Find k using the datasheet or specification given; Find k using the datasheet or specification given; ex: V GS(ON),V GS(TH) ex: V GS(ON),V GS(TH) Plot transfer characteristics using the formula Plot transfer characteristics using the formula I D =k(V GS – V T ) 2. Three point already defined that is I D(ON), V GS(ON) and V GS(TH) Plot a point that is slightly greater than V GS Plot a point that is slightly greater than V GS Plot the linear characteristics (network bias line) Plot the linear characteristics (network bias line) The intersection defines the Q-point The intersection defines the Q-point 10

Example Determine I D Q and V DS Q for network below Determine I D Q and V DS Q for network below 11

Again plot the line and the transfer curve to find the Q-point. Using the following equations: Input loop: Output loop: Voltage-Divider Biasing 12

1.Plot the line using V GS = V G = (R 2 V DD )/(R 1 + R 2 ), I D = 0 and I D = V G /R S and V GS = 0 2. Find k 3. Plot the transfer curve using V GSTh, I D = 0 and V GS(on), I D (on); all given in the specification sheet. 4. Where the line and the transfer curve intersect is the Q-Point. 5. Using the value of I D at the Q-point, solve for the other variables in the bias circuit. Voltage-Divider Bias Q-Point 13

Example Determine I D Q and V GS Q and V DS for network below Determine I D Q and V GS Q and V DS for network below 14

= =- - = - = -+)( = = - + =- )( + = = - )( = = 15

= - = = =- = - = - = + -= = - + ( ) = = - = =

Troubleshooting N-channel V GSQ will be 0V or negative if properly checked N-channel V GSQ will be 0V or negative if properly checked Level of V DS is ranging from 25%~75% of V DD. If 0V indicated, there’s problem Level of V DS is ranging from 25%~75% of V DD. If 0V indicated, there’s problem Check with the calculation between each terminal and ground. There must be a reading, R G will be excluded Check with the calculation between each terminal and ground. There must be a reading, R G will be excluded 17

For p-channel FETs the same calculations and graphs are used, except that the voltage polarities and current directions are the opposite. The graphs will be mirrors of the n-channel graphs. P-Channel FETs 18

Voltage-Controlled Resistor JFET Voltmeter Timer Network Fiber Optic Circuitry MOSFET Relay Driver Practical Applications 19

JFET Voltmeter 20

Advantages High Input impedance for isolation. High Input impedance for isolation. Amount of power drawn from circuit under test is very small, so no loading effect. Amount of power drawn from circuit under test is very small, so no loading effect. Very high sensitivity. Very high sensitivity. Amplifier gain allows measurement in the mV range. Amplifier gain allows measurement in the mV range. No damage due to overload because of amplifier saturation. No damage due to overload because of amplifier saturation. 21

Single MOSFET Relay Toggle Circuit 22

For drawing an a c equivalent circuit of Amp. Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal Short circuit the d c supply Replace the FET by its small signal model

Analysis of CS Amplifier A C Equivalent Circuit Simplified A C Equivalent Circuit