1. Diode As Circuit Elements Section 3.1-3.3

2. Key Concepts Diode models –Exponential model Derivation of n –Ideal model –Constant-voltage model

3. Choosing a Diode Model Use the ideal model to develop a quick, rough understanding of a circuit. If the ideal model is not adequate, uses the constant voltage model, which is sufficient for most cases. Occasionally, we will use the exponential model

4. I S =Reverse Saturation=leakage current

5. Obtain an Exponential Diode Model Empirically

6. Ideal Model of a Diode (exponential model) (ideal model) An ideal diode will turn on even for the slightest forward bias voltage. (VD≥0) An ideal diode will turn off even for the slightest reverse bias voltage. (VD<0)

7. Behavior of Ideal Diode Ideal diode: V anode >V cathode : Diode is on V anode <V cathode : Diode is off An ideal current experieincing V anode =V cathode, carries no current

8. I/V Characteristics An Open—can’t get a current to flow. A short--can’t get a V to develop across a diode. A diode V anode >V cathode : Diode is on V anode <V cathode : Diode is off An ideal current experieincing V anode =V cathode, carries no current In practice, consider a slightly positive or negative voltage to determine the response of a diode.

9. Example 1: An OR Gate Realized By Diodes

10. Analysis of an OR Gate Observations: 1.If D1 is on, VA=VOUT and VOUT=“1” 2.If D2 is on, VB=VOUT and VOUT=“1”. 3.VOUT is 0 if and only if D1 and D2 are “0” This is an OR gate. Logic 1=3 V Logic 0=0V

11. Example of an OR Gate VA=3 V VB=0 V VOUT=2.424 V≈3V

12. In Class Exercise VA VB

13. In Class Exercise VA VB If VA=“0”, node 1 is “0”. If VB=“0”, node 1 is “0”. If both VA and VB are “1” or 3V, no current can flow through R1, node 1 is 3 V. So we have an AND gate. Assume node 2 is 3V.

14. Simulation Example VA=3V VB=0V VOUT=0.575 V

15. Simulation Example VA=3V VB=3V VOUT=3 V

16. In-Class Exercise What is logic implemented by A, B and LED?

17. Solution 0 V

18. Basic Gates

19. Decimal to Binary Converter Big idea: If you deprive an LED with current,it will turn off. If you provide an LED with current, it will turn on.

20. If VD is less than VD, On, the diode behaves like an open circuit. The diode will behave like an open circuit for VD=V D,on Constant Voltage Model

21. Analysis of an OR Gate VA=3V, VB=0V Observations: D1 is ON. VD1=0.7V So VOUT=2.3 V D2 is OFF Logic 1=3 V Logic 0=0V

22. Example of an OR Gate VA=3 V VB=0 V VOUT=2.424 V≈3V (ideal diode model) ≈2.3 V (constant voltage model)

23. Cascade Two Stages of AND gates

24. Cascade an AND with an OR

25. Limitations of Diode Logic Circuit Vout=1.23 V

26. Level Shift Vout of 606 mV Vout can depend on input voltage combination!