PSPICE MULTISIM The circuit shown in Fig. P7.99 is known as an astable multivibrator and

Chapter 7, Problem 7.99

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PSPICE MULTISIM The circuit shown in Fig. P7.99 is known as an astable multivibrator and finds wide application in pulse circuits. The purpose of this problem is to relate the charging and discharging of the capacitors to the operation of the circuit. The key to analyzing the circuit is to understand the behavior of the ideal transistor switches Ti and T2. The circuit is designed so that the switches automatically alternate between ON and OFF. When T{ is OFF, T2 is ON and vice versa. Thus in the analysis of this circuit, we assume a switch is either ON or OFF. We also assume that the ideal transistor switch can change its state instantaneously. In other words, it can snap from OFF to ON and vice versa. When a transistor switch is ON, (1) the base current ib is greater than zero, (2) the terminal voltage vbc is zero, and (3) the terminal voltage vce is zero. Thus, when a transistor switch is ON, it presents a short circuit between the terminals b,e and c,e. When a transistor switch is OFF, (1) the terminal voltage vhe is negative, (2) the base current is zero, and (3) there is an open circuit between the terminals c,e. Thus when a transistor switch is OFF, it presents an open circuit between the terminals b,e and c,e. Assume that T2 has been ON and has just snapped OFF, while Tj has been OFF and has just snapped ON. You may assume that at this instance, C2 is charged to the supply voltage Vcc, an d tn e charge on C\ is zero. Also assume Cx = C2 and Rx = R2 = 10RL. a) Derive the expression for vbc2 during the interval that T2 is OFF. b) Derive the expression for vcc2 during the interval that T2 is OFF. c) Find the length of time T2 is OFF. d) Find the value of vce2 at the end of the interval that T2 is OFF. e) Derive the expression for /bl during the interval that T2 is OFF. f) Find the value of ibx at the end of the interval that T2 is OFF. g) Sketch vcc2 versus t during the interval that T2 is OFF. h) Sketch /M versus t during the interval that T2 is OFF.