Diode Circuits

Outline Large-Signal and Small-Signal Operation Application of Diodes (Up to 3.5.1)

c03f19 How does a small increase in Vad affect Vout?

c03f20 (Slope)

Small Signal Resistance Small Signal Resistance: ID/VT

Measured Diode Voltage

Small Signal Resistance VF (V) IF (Computed) 0.70 5.74 mA 0.748 13.8 mA Dynamic Resistance from the measurement: (0.748-0.70)/(13.8 mA-5.74 mA)= 48 mV/8.06 mA =5.95 Ohms From the manufacture’s specification=8.33 Ohms, using data from 0.7V and 0.725 V in Figure 4. (0.748-0.70)/(13.8 mA-5.74 mA)= 48 mV/8.06 mA =5.95 Ohms

c03f22 Using Small Signal Model in a Small Signal Calculation

c03f23

Small Signal Model of Adaptor c03f25 Change in Vad

c03f26 Change in Vout due to a drop in c03f26

c03f27 c03f27

c03f28 Half-Wave Rectifier Assume constant voltage model for diodes Voltage offset Diode is off for Vin < VD,on Assume constant voltage model for diodes

c03f29 On for Vin less than –VD,on

c03f31 An AC-DC Converter Circuit Ideal Diode Model

c03f30 An AC-DC Converter Circuit Constant Voltage Diode Model Issue: It remains to be seen whether it can provide a current to a load

How should a load be conceptualized? A laptop consumes an average power of 25 W with a supply voltage of 3.3 V. Determine the average current drawn from the batteries or the adaptor. P=VI I= 7.56 A If the laptop is modeled by a resistor, then RL is 0.436 Ohms.

c03f32 Rectifier Driving a Resistive Load The diode is turned on again ripples Vin begins to fall after t1 because RL provides a discharge for C1. C1 must be sufficient large so that the current drawn by RL does not reduce Vout significantly.

c03f33 C1 is called smoothing capacitor

c03f34 Magnitude of the Ripple Tin

c03f35 The Maximum Forward Bias Current Maximum Forward Current Occurs at the point when the diode turns on (t1) C1dVout/dt is maximum The current supplied to RL is (Vp-VD,on)/RL

Screen Shots

Smoothed Capacitor