Chapter 7 - SJTU 1 Chapter 7 First-Order Circuit SJTU 2 1...

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Unformatted text preview: SJTU 1 Chapter 7 First-Order Circuit SJTU 2 1. RC and RL Circuits 2. First-order Circuit Complete Response 3. Initial and Final Conditions 4. First-order Circuit Sinusoidal Response Items: SJTU 3 1. RC and RL Circuits 1. use device and connection equations to formulate a differential equation. 2. solve the differential equation to find the circuit response. Two major steps in the analysis of a dynamic circuit SJTU 4 FORMULATING RC AND RL CIRCUIT EQUATIONS SJTU 5 Eq.(7-1) Eq.(7-2) Eq.(7-3) Eq.(7-4) Eq.(7- 5) Eq.(7-6) RC RL SJTU 6 makes V T =0 in Eq.(7-3) we find the zero-input response Eq.(7-7) Eq.(7-7) is a homogeneous equation because the right side is zero. Eq.(7-8) where K and s are constants to be determined A solution in the form of an exponential ZERO-INPUT RESPONSE OF FIRST-ORDER CIRCUITS SJTU 7 Substituting the trial solution into Eq.(7-7) yields OR Eq.(7-9) characteristic equation a single root of the characteristic equation zero -input response of the RC circuit: SJTU 8 Eq.(7-10) Fig. 7-3: First-order RC circuit zero-input response time constant TC=R T C SJTU 9 Graphical determination of the time constant T from the response curve SJTU 10 Eq.(7-11) Eq.(7-12) The root of this equation The final form of the zero-input response of the RL circuit is Eq.(7-13) SJTU 11 EXAMPLE 7-1 The switch in Figure 7- 4 is closed at t=0, connecting a capacitor with an initial voltage of 30V to the resistances shown. Find the responses v C (t), i(t), i 1 (t) and i 2 (t) for t 0. Fig. 7-4 SJTU 12 SOLUTION: SJTU 13 EXAMPLE 7-2 Find the response of the state variable of the RL circuit in Figure 7-5 using L 1 =10mH, L 2 =30mH, R 1 =2k ohm, R 2 =6k ohm, and i L (0)=100mA Fig. 7-5 SJTU 14 SOLUTION: SJTU 15 2. First-order Circuit Complete Response When the input to the RC circuit is a step function** Eq.(7-15) The response is a function v(t) that satisfies this differential equation for t ≥ 0 and meets the initial condition v(0). If v(0)=0, it is Zero-State Response . Since u(t)=1 for t ≥ 0 we can write Eq.(7-15) as Eq.(7-16) SJTU 16 divide solution v(t) into two components: natural response forced response The natural response is the general solution of Eq.(7-16) when the input is set to zero. SJTU 17 The forced response is a particular solution of Eq....
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Chapter 7 - SJTU 1 Chapter 7 First-Order Circuit SJTU 2 1...

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