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Question 7.8: A symmetric astable multivibrator with vertical edges (see i......

A symmetric astable multivibrator with vertical edges (see in Fig. 7.20) has the following parameters,\ V_{CC} = 15 V,\ R_{C} = 3 kΩ,\ R_{1} = R_{2} = R = 30 kΩ,\ R_{3} = 2 kΩ and silicon transistors with\ h_{FE} = 50 are used.\ C_{1} = C_{2} = C = 0.01 μF. Verify whether the ON device is in saturation or not. Find its frequency.

fig 7.20
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Assume\ Q_{1} is OFF and\ Q_{2} is ON and in saturation. If\ Q_{2} is ON and in saturation\ V_{C2} = V_{CE(sat)} = 0.2V,\ V_{B2} = V_{σ} = 0.7 V then\ D_{2} is ON. The collector load is\ R_{3}||R_{C}.

\ R^{′}_{C} = R_{3}||RC = \frac{3 × 2}{5} = 1.2  kΩ        I_{C2} = \frac{V_{CC} − V_{CE(sat)}}{R^{′}_{C}}= \frac{15 − 0.2}{1.2  kΩ} = \frac{14.8  V}{1.2  kΩ} = 12.3 mA

\ I_{B2} = \frac{V_{CC} − V_{σ}}{R} = \frac{15 − 0.7}{30  kΩ} = \frac{14.3  V}{30  kΩ} = 0.477 mA    \ I_{B2}(min) = \frac{I_{C2}}{h_{FE(min)}}= \frac{12.33  mA}{50} = 0.247 mA

\ I_{B2} \gg I_{B2}(min)

Hence,\ Q_{2} is in saturation.
To find,\ f :

For a symmetric astable multivibrator:\ f = \frac{0.7}{RC} = \frac{0.7}{30 × 10^{3} × 0.01 × 10^{−6}} = 2.33 kHz

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