ASTABLE MULTIVIBRATOR PRODUCING WAVEFORMS WITH SHORT RISE TIME

EDITOR: B. SOMANATHAN NAIR

1. INTRODUCTION

The output waveforms of a conventional astable multivibrator shows curved edges due to the transient effects created by the two capacitors in the circuit. Ideally, we want square pulses with sharp rising and falling edges. To get these ideal pulses, at least approximately, the capacitors C₁ and C₂ must be de-coupled from the output, so that the rise and fall of the output waves are not affected by them. For vertical rise and fall in waves, we may use reverse-biased diodes to isolate the capacitors from the output, as shown in Fig. 1.

2.  SPECIFICATIONS

·         Free-running Frequency                :           5 kHz

·         Output-voltage Swing                     :            10 V

·         Rise time                                           :           As minimum as possible

3. DESIGN PROCEDURE

The circuit additionally incorporates two resistors R₁ and R₂ and two diodes D₁ and D₂ connected as shown in Fig. 1. The capacitors C₁ and C₂ now charge and discharge through R₁ and R₂ instead of through R_B! and R_B2. The output wave then will have almost vertical edges. The small deviation from the verticality is due to the reverse-biased junction capacitances of the diodes. It can be seen that R₁ comes in parallel with R_C1 and R₂ comes in parallel with R_C2. Then the values of R₁, R_C1, R₂, and R_C2 must be such that (for symmetrical design)

                                                        R₁= R_C1= R₂= R_C2 =2 R_C          (1)

where R_C is the value of the collector resistor in the previous design (4.7 kΩ). The circuit with designed values is shown in Fig. 2. Diodes D₁ and D₂ must be such that they should have a reverse break down voltage large enough to withstand the supply voltage V_CC. They also must have very small junction capacitance. Point contact diodes (e.g., 1N 34 or OA 79) are highly suitable for this purpose, as they have very low junction capacitance and sufficiently high reverse break down voltage. The fully designed circuit is shown in Fig. 2.