# 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.

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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

Question: 7.1

## Design an astable multivibrator, assuming that silicon devices with hFE(min) = 40 are used. Also assume that VCC = 10 V, IC(sat) = 5 mA. Let the desired frequency of oscillations be 5 kHz. For transistor used, VCE(sat) = 0.2 V, VBE(sat) = Vσ = 0.7 V. ...

\ R_{c} = \frac{ V_{CC} − V_{CE(sat)}}{I_{C...
Question: 7.7

## A symmetrical collector-coupled astable multivibrator has the following parameters: VCC = 10 V, RC = 1 kΩ, R = 10 kΩ, and C = 0.01μF. Silicon transistors with hFE = 50 and rbb′ = 0.2 kΩ are used. Plot the waveforms and calculate the overshoot. Also plot the waveforms if the circuit uses p-n-p ...

Given$\ V_{CC}$ = 10V,\ R_{C}[...
Question: 7.6

## In an astable multivibrator if R2 = 60 kΩ, R1 = 40 kΩ, C1 = C2 = 2.9 nF, find its frequency and duty cycle. ...

\ T_{2} = 0.69 R_{1}C_{1} = 0.69 × 40 × 10^...
Question: 7.5

## For the multivibrator shown in Fig. 7.13(a), VCC = 20 V, VBB = 10 V, R1 = R2 = R = 10 kΩ, C1 = C2 = C = 0.01μF. Find the time period and the frequency. ...

\ T = 2RC  ln \left(1 + \frac{V_{CC}}{V_{BB...
Question: 7.4

## 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 R1 =R2 = 1 kΩ (c) Find the minimum value of RC ...

(a)   Given R = 10 kΩ and$\ f$ = 10 k...
Question: 7.3

## For the astable multivibrator shown in Fig. 7.1: (a) Determine the time period and the frequency of oscillations if R1 = R2 = R = 10 kΩ , C1 = C2 = 0.01 μF. (b) Determine the time period and the frequency of oscillations if R1 = 1 kΩ, R2 = 10 kΩ, C1 = 0.01μF,C2 = 1μF. ...

(a)   Given$\ R_{1} = R_{2} = R$ = 10...
Given$\ V_{CC2}$ = 18 V,\ R_{3...