Question 7.4: For the astable multivibrator shown in Fig. 7.1: (a) Find th......
For the astable multivibrator shown in Fig. 7.1:
(a) Find the value of C to provide symmetrical oscillations if R = 10 kΩ and\ f = 10 kHz.
(b) Determine the values of capacitors to provide a train of pulses 0.1 ms wide and at a frequency of 1 kHz, if\ R_{1} =R_{2} = 1 kΩ
(c) Find the minimum value of\ R_{C} in a symmetric astable if R = 10 kΩ and\ h_{FE} = 50.
(a) Given R = 10 kΩ and\ f = 10 kHz, for a symmetrical astable multivibrator.
\ f = \frac{0.7}{RC} \ C = \frac{0.7}{Rf} = \frac{0.7}{10 × 10^{3} × 10 × 10^{3}} = 0.007 μF
(b) Given\ T_{1} = duration of the pulse = 0.1 ms,\ f = 1 kHz, for the un-symmetric astable multivibrator:
\ T = \frac{1}{f} = 1 ms \ T_{2} = T − T_{1} = 1 − 0.1 = 0.9 ms \ R_{1} = R_{2} = R = 1 kΩ 0.69\ RC_{2} = 0.1ms
\ C_{2} = \frac{0.1 × 10^{−3}}{1 × 10^{3} × 0.69} = 0.145 μF 0.69\ RC_{1} = 0.9 ms \ C_{1} = \frac{0.9 × 10^{−3}}{1 × 10^{3} × 0.69} = 1.30 μF
(c) We have\ R = h_{FE}R_{C}
\ R_{C(min)} = \frac{R}{h_{FE}} = \frac{10 × 10^{3}}{50} = 200 Ω