Question 15.9: Objective: Design a monostable multivibrator to produce a gi...
Objective: Design a monostable multivibrator to produce a given pulse width.
Specifications: The circuit with the configuration shown in Figure 15.37 is to be designed to produce an output pulse that is 1µs wide. Assume parameters of V_{P} = 10 V, V_{γ} = 0.7 V and R_{1} = R_{2} = 20 kΩ.
Choices: Assume an ideal comparator is available. Use standard-valued element values in the final design.
Since V_{γ} \ll V_{P} and R_{1} = R_{2}, then from Equation (15.92), we have
T = τ_{x} \ln \left[\frac{1 + (V_{γ}/V_{P})}{(1 − β) } \right ] (15.92)
T = 0.69τ_{x}or
τ_{x} = R_{X} C_{X} = \frac{T}{0.69} = \frac{1}{0.69} = 1.45 µs
If R_{X} = 10 k\Omega, then C_{X} = 145 pF.
Trade-offs: Using standard-valued elements of R_{X} = 10 k\Omega and C_{X} = 150 pF produces a pulse width of 1.035 µs. Element tolerances must also be taken into account in the final design.
Comment: In actual monostable multivibrator ICs, R_{X} and C_{X} are external elements to allow for variable times.