A symmetric astable multivibrator with vertical edges (see in Fig. 7.20) has the following parameters, VCC = 15 V, RC = 3 kΩ, R1 = R2 = R = 30 kΩ, R3 = 2 kΩ and silicon transistors with hFE = 50 are used. C1 = C2 = C = 0.01 μF. Verify whether the ON device is in saturation or not. Find its

Question

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

Accepted Answer

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

[latex]\ 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}[/latex] = 12.3 mA

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

[latex]\ I_{B2} \gg I_{B2}(min)[/latex]

Hence,[latex]\ Q_{2}[/latex] is in saturation.
To find,[latex]\ f[/latex] :

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

Chapter 7

Q. 7.8

Q. 7.8

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