Lecture 18 MOSFET (cont d) MOSFET 1-1
Outline Continue Enhancement-type MOSFET (E- MOSFET) Characteristics C Biasing Circuits and Examples MOSFET 1-
E-MOSFET (Quick Review) MOSFET is also known as nsulated-gate FET (GFET) MOSFET 1-3
E-MOSFET Symbol n-channel E-Type MOSFET p-channel E-Type MOSFET MOSFET 1-4
E-MOSFET Characteristic The transfer curve is not define by Shockley s equation anymore. t is defined by the equation: Where k is defined by: The term on means that the values where the device has reached its saturation level As for the example of the characteristic curve given in slide 3, k k V VT for V VT V V ( on) ( on) the (on) values are V (on) =+8V where (on) =10mA or V (on) =+7V where (on) =7mA or V (on) =+6V where (on) 4.5mA or any other values that have reached saturation level T MOSFET 1-5
E-MOSFET Characteristic Usually, we take the highest saturation level shown in the graph (i.e., V (on) =+8V where (on) =10mA ) So, the value of k for the characteristic example given above is: k V V 8 ( on) ( on) T 10m Notice the unit of k is in ma/v because the numerator is current and the denominator is voltage So, the equation for this example will become: 0.78 ma/v V 0.78m MOSFET 1-6
E-MOSFET Transfer Curve Plotting all the V S(sat) from the characteristic curve, the transfer curve can be obtained: V 0.78m MOSFET 1-7
E-MOSFET mportant Relationships k G 0 k S V V T (on) V V (on) T for V V T MOSFET 1-8
E-MOSFET C Biasing Fixed-bias, self-bias and many more bias configuration can be applied to enhancement-type MOSFET Two most popular E-MOSFET biasing configurations Feedback-bias configuration Voltage-divider bias configuration MOSFET 1-9
Feedback-Bias Configuration As the situation G = 0 still applied, the resistor R G will be ignored resulting in the drain and gate terminal to have the same voltage (V G = V ) MOSFET 1-10
Example (1) etermine V Q and Q for the E-MOSFET shown in the figure MOSFET 1-11
Example (1) Solution For the E-MOSFET s equation, the value of k have to be obtained first: k V V 8 3 (on) (on) the equation for the device: G V V 1 k T 0.4m V 6m 3 for V 3 Since G equals zero, then 0.4 ma/v nserting the V equation into the device equation: V V V 1 k 0 1 k G S MOSFET 1-1
Example (1) Solution Substituting the equation for the MOSFET 0.4m V 0.4m 960 3 0.4m1 k 3 0.4m9 k 81 36 4 k M 19.44m 8.64 960 9.64 19.44m Solving the equation, we get: b b a 4ac 9.64 7.5 ma and.79 ma E-MOSFET doesn t have limitation for saturation current ( SS ), the true value of is the smaller one (since V should be greater than V T ) Q 0 ( 9.64) (960) 4(960)(19.44m).79 ma and V 1 k 6.4 V MOSFET 1-13
Example (1) Solution For graphical approach, several plot points have to be obtained first: 0.4m V V 3 for V 3 3 V 0 ma 4 V 0.4 ma 5 V 0.96 ma 6 V.16 ma 7 V 3.84 ma 8 V 6 ma For the bias line, only two plot points are required: V 1 k V 1 V 0 ma 0 V 6 ma MOSFET 1-14
Example (1) Solution Plots all the device transfer curve and device representation points: V T MOSFET 1-15
Voltage-ivider Bias Configuration Basically, the configuration is the same as in depletiontype MOSFET, JFET or BJT except the change of device to the enhancementtype MOSFET All the calculation would be the same except for the transfer curve of enhancement-type MOSFET is different from those depletion-type MOSFET and JFET MOSFET 1-16
Example () etermine Q and V Q MOSFET 1-17
Example () Solution etermining V G : For V S : So, for V : k 18M V G 40* 18 V 18M M 3 V S 0.810 3 V V V 18 0.810 G V V 10 5 ( on) ( on) T S 3m 0.1 ma/v nserting the circuit representation equation into the device equation: 0.110 80.69 0.110 3 3 V 5 for V 5 3 V 5 0.110 18 0.8k 5 3.56 0.810 3 0 MOSFET 1-18
Example () Solution Solving the equation, we get: For b b 4ac 3.56 a 37.4 ma and 6.7 ma We take the smaller value: V 18 0.8k 6.7 ma, V ( 3.56) 18 0.810 3 4(80.69)(0.8m) (80.69) 6.7 ma (6.7m) 1.49 V The Q-point for MOSFET is defined by V Q Q 6.7 ma 1.49 V MOSFET 1-19
Example () Solution For graphical approach, several plot points have to be obtained first: 0.1m V 5 for V 5 For the bias line, only two plot points are required: V 18 0. 8k V 5 V 0 ma 10 V 3 ma 15 V 1 ma 0 V 7 ma 5 V 48 ma 30 V 75 ma V 18 V 0 ma 0 V 1.95 ma MOSFET 1-0
Example () Solution Plots all the device transfer curve and device representation points: MOSFET 1-1
p-channel Enhancement-Type MOSFET t is the complement to n-channel enhancement-type MOSFET All the current flow will be in the opposite direction Although the current direction is reverse, however the current equation are still the same (just like in JFET and depletion-type MOSFET) Construction Transfer Curve Characteristics MOSFET 1-
Comparison between -MOSFET and E-MOSFET MOSFET 1-3
Lecture Summary Covered material Continue Enhancement-type MOSFET Characteristics Biasing Circuits and Examples Material to be covered next lecture ntroduction to BJT-FET Combination Circuits MOSFET 1-4