Chapter 3 Diodes and Applications - t U

1 52 | P a g e Chapter 3 Diodes and Applications Introduction [5], [6] diode is the most basic of semiconductor device. It should be noted that the term of diode refers to the basic p-n junction diode . All other diode types have other identifying names, such as zener diode , light-emitting diode and so on. A diode is a two-electrode (two-terminal) device that acts as a one-way conductor. When forward biased, the diode will conduct. When reverse biased, diode conduction will drop to nearly zero. Here, a diode circuit is shown in Figure Figure diode circuit symbol [6] 53 | P a g e Bias Connections [5] Forward-Bias connection: A diode is forward-biased when a voltage source is connected as shown in Figure (a). The positive terminal of the source is connected to the anode through a resistor. The negative terminal of the source is connected to the cathode.

2 The forward current (IF) is from anode to cathode as indicated. The forward voltage drop (VF) due to the barrier potential is from positive at the anode to negative at the cathode. Reverse-Bias Connection: A diode is reverse-biased when a voltage source is connected as shown in Figure (b). The negative terminal of the source is connected to the anode side of the circuit, and the positive terminal is connected to the cathode side. The reverse current is extremely small and can be considered to be zero. Notice that the entire bias voltage (VBIAS) appears across the diode . Figure Forward-bias and reverse-bias connections showing the diode symbol. [5] 54 | P a g e diode Circuit Analysis [5], [6] Figure shows a diode circuit containing a voltage source and a resistor. Here, in order to analyze the current passing the diode (ID) and the voltage across the diode (VD), we can use 4 methods as mentioned below.

3 Mathematical Model Ideal diode Model Constant Voltage Drop (CVD) Model Complete diode Model Figure diode circuit containing a voltage source and a resistor. Steps to analyze ID and VD in the diode circuit: 1. Select a model for the diode . 2. Make a guess concerning the region of operation for the diode based on the circuit configuration. 3. Analyze the circuit using the model appropriate for the assumption in step 2. 4. Check the results to see if they are consistent with the assumptions. 55 | P a g e Analysis Using the Mathematical Model Example 1: Use Mathematical Model to analyze ID and VD from Figure Apply KVL to the loop. 10 + 104ID + VD = 0 104ID + VD = 10 (1) From the equation Substitute (2) in (1) with IS = 10-13 A,VT = V. (for Si diode @ room temperature) Solve (3) for VD, then VD = V and ID = 10-4 A = mA Analysis using Ideal diode Model The ideal model of a diode is the least accurate approximation and can be represented by a simple switch.

4 When the diode is forward-biased, it ideally acts like a closed (on) switch, as shown in Figure (a). When the diode is reverse-biased, it ideally acts like an open (off) switch, as shown in part (b). In Figure , the ideal V-I characteristic curve graphically depicts the ideal diode operation. The diode is assumed to have a zero voltage across it when forward-biased, as indicated by the portion of the curve on the positive vertical axis. Since the reverse current is neglected, its value is assumed to be zero, as indicated in Figure by the portion of the curve on the negative horizontal axis. (2) ]1[eI I)VV(SDTD (3) 10 V ]1[e10D40V-9D 56 | P a g e Figure The ideal model of a diode . [5] (a) Equivalent circuit under forward bias (on or short circuit). (b) Equivalent circuit under reverse bias (off or open circuit).

5 57 | P a g e Figure Ideal V-I characteristic curve (blue) [5] Therefore, using ideal diode model under this diode circuit condition, we can conclude that; For forward bias: =0 V, and = For reverse bias: =0 A , and = 58 | P a g e Example 2: Use ideal diode model to analyze ID and VD from Figure Solution : Consider from 4 steps to analyze ID and VD in the diode circuit: 1. Use the ideal diode model for this circuit condition (from Figure ) (Figure ) 2. Because the voltage source appears to be trying to forward bias the diode , we assume that the diode is on or acts like a short circuit, see Figure Figure For Example 2 3. Find ID: = =10 V10 k =1 mA 4. The current ID > 0, which is consistent with the assumption that the diode is on. 59 | P a g e Example 3: Use ideal diode model to analyze ID and VD of both Diodes from Figure Figure For Example 3 Solution : Use the ideal diode model Make assumptions.

6 Because we have 2 Diodes in this circuit. Therefore we can assume 4 conditions, as mentioned in Table 1. Table 1: 4 possible diode conditions for 2 Diodes 60 | P a g e First try : Both Diodes are on. Under this assumption, the diode circuit from Figure becomes Figure diode circuit with both Diodes assumed to be on. Find ID1 and ID2. The result ID1 < 0 is inconsistent with our assumption that D1 is on. Our assumption must be incorrect. 61 | P a g e Second try : D1 is off and D2 is on. Under this assumption, the diode circuit from Figure becomes Figure diode circuit with D1 Off and D2 On Find ID2 and VD1. Figure diode circuit with D1 Off and D2 On 62 | P a g e Since ID2 > 0 (D2 is on) and VD1 < 0 (D1 is off or reverse biased), they are consistent with our assumptions. We can conclude that ID1=0 A , VD1= V,ID2= mA, VD2=0 V Example 4: Determine ID1, ID2 , VD1 and VD2, using ideal diode model.

7 Figure For Example 4 63 | P a g e Solution : Keep in mind that by using ideal diode model, For on condition : VD = 0 V and ID > 0 A For off condition : VD < 0 V and ID = 0 A First try : Assume diode 1 is on, diode 2 is off. Under this assumption, the diode circuit from Figure becomes Figure For Example 4 Find ID and VD off both Diodes . 64 | P a g e Second try : Assume both Diodes are off. Under this assumption, the diode circuit from Figure becomes Figure For Example 4 Find ID and VD off both Diodes . 65 | P a g e Constant Voltage Drop (CVD) Model or Practical diode Model The Constant Voltage Drop (CVD) Model (or Practical diode Model) includes the barrier potential. When the diode is forward-biased, it is equivalent to a closed switch in series with a small equivalent voltage source (VF) equal to the barrier potential ( V for Si diode , V for Ge diode ) with the positive side toward the anode, as indicated in Figure (a).

8 This equivalent voltage source represents the barrier potential that must be exceeded by the bias voltage before the diode will conduct and is not an active source of voltage. When conducting, a voltage drop of V appears across the Si diode or V appears across the Ge Diodes . When the diode is reverse-biased, it is equivalent to an open switch just as in the ideal model, as shown in Figure (b). The barrier potential does not affect reverse bias. Figure The constant voltage drop (CVD) model (or practical diode model of a diode ). [5] The characteristic curve for the practical diode model is shown in Figure Since the barrier potential is included, the diode is assumed to have a voltage across it when forward-biased, as indicated by the portion of the curve to the right of the origin. 66 | P a g e Figure V-I characteristic curve (blue) of CVD model for Si diode . [5] Therefore, using CVD model under this diode circuit condition, we can conclude that; For forward bias: = V ( for Si diode ) and = V ( for Ge diode ) Here, applying Kirchoff s voltage to find IF: =0 And = Therefore, =( )/ For reverse bias: =0 A , and = 67 | P a g e Example 5: Use CVD model to analyze ID and VD from Figure Solution : Consider from 4 steps to analyze ID and VD in the diode circuit: 1.

9 Use the CVD model for this circuit condition (from Figure ) (Figure ) 2. Because the voltage source appears to be trying to forward bias the diode , we assume that the diode is on, and we can replace Si diode by VF = V, see Figure Figure For Example 5 68 | P a g e 3. Find ID: 4. The current ID > 0, which is consistent with the assumption that the diode is on. ** Here, comparison of diode Circuit Analysis (from Example 1, 2, 5) Results Example 6: Use CVD model to analyze ID1, ID2, ID3, VD1, VD2, and VD3 . Figure For Example 6 69 | P a g e Solution : Here, we assume D1 = off, D2 = on, D3 = on. Figure For Example 6 70 | P a g e Example 7: Using CVD model, calculate VO, ID1, ID2, and I for the following conditions: (a) V1 = V2 = 0 V (b) V1 = 20 V, V2 = 0 V Figure For Example 7 Solution : (a) When V1 = V2 = 0 V, we assume D1 = on, D2 = on, see Figure 1212010 5 5* / 2 / 2 5* ( ) DDDDIIIAIIIAVIor IV 71 | P a g e Figure For Example 7 (b) When V1 = 20 V, V2 = 0 V, we assume D1 = off, D2 = on, see Figure Figure For Example 7 122220 = 0 A_____#I = 10 = 5I + + 5I = A = _____#V = + 5 = + 5 ( ) = V_____#DDDDDIIIII72 | P a g e Homework 3 1.

10 Using ideal diode model, calculate the voltage across each diode and the current flowing through each diode in Figure (a), (b) and (c). Figure For problem 1 [5] 2. Using ideal diode model , calculate ID1, ID2 in the circuit for each case. (1) VIN = 0 V (2) VIN = 10 V (3) VIN = 10 V 73 | P a g e Figure For problem 2 3. Assume D1, D2 and D3 are Si Diodes . Find ID1, ID2, ID3, VD1, VD2 and VD3 in the circuit of Figure , using the ideal diode model. Figure For problem 3 74 | P a g e 4. Assume D1 and D2 are Si Diodes . Find ID1, ID2, VD1, VD2 and VO in the circuit of Figure , using the ideal diode model. Figure For problem 4 5. Assume D1 and D2 are Si Diodes . Find ID1, ID2, VD1, VD2 and IO in the circuit of Figure , using the ideal diode model. Figure For problem 5 ID1 0 V 2 k ID2 - 5 V 1 k + _ VD1 + _ VD2 - 5 V VO 1 k 75 | P a g e 6. Using CVD model, assume D1, D2, D3 are Si Diodes .