HW Set 5 Name:____________________________________________ SID:_______________________ Question 1: Derive the equation for the small signal resistance for a diode: HW Set 5 Name:____________________________________________ SID:_______________________ To derive rD, find a Q point on an IV curve and find the tangent (slope) of the diode curve at that point. The inverse of the slope will be the small signal resistance rD. Figure 1: Diode IV curve. The full diode equation is given by: 1 Assume that: 1 And On the IV curve above this means Vd is between .2 and .6Volts. Let The diode current will become: Q point Tangent at Q HW Set 5 Name:____________________________________________ SID:_______________________ Expand This can be seen to be: Assume 1 Talyor expansion 1 Expand Which clearly has a DC part and an AC part. HW Set 5 Name:____________________________________________ SID:_______________________ Question 2: Show on the plot below for the D9124 Diode from Motorola, where the equation: for small signal resistance is valid. By eye between these two points. HW Set 5 Name:____________________________________________ SID:_______________________ The full diode equation is given by: 1 The lower limit is given by: 1 Numerically this can be given by: 100 Rearrange: ln100 ln100 If VT=.0259V, and n=1.752, the lower limit on VD is .21Volts Upper limit: 1 Assume that: 1 And Numerically .01 HW Set 5 Name:____________________________________________ SID:_______________________ . lnIDIS lnIDIS ln.01VDRIS.01 We can assume that ln.01VDRIS We still Have to solve numerically. Plot the intersection of VD and nVTln(.01Vd/IS/Rs) vs. VD Vd upper is .7 Volts. HW Set 5 Name:____________________________________________ SID:_______________________ Question 3: Verify the small signal analysis works using LTspice for the following circuit: The dc op point is: Full solution: HW Set 5 Name:____________________________________________ SID:_______________________ HW Set 5 Name:____________________________________________ SID:_______________________ Using the small signal analysis technique find the Vout for the following circuit by hand and Ltspice: Note the diode is different! Find dc operating point: HW Set 5 Name:____________________________________________ SID:_______________________
View Full Document
Unlocking...