Designing Monostable Multivibrator with BJT

Aug 4, 2024

Monostable Multivibrator Using BJT

Introduction

  • Welcome to the channel "ALL ABOUT ELECTRONICS".
  • Topic: Designing a monostable multivibrator using BJT.

Monostable Multivibrator Overview

  • Definition: Circuit where output is stable but switches to unstable state upon external trigger.
  • Duration in Unstable State: Depends on resistor (R) and capacitor (C) values.

Circuit Design

  • Components: Two transistors (Q1 and Q2), resistors (R1, R2, Rc2), and capacitor (C).
  • Configuration:
    • Q1 (First Transistor)
    • Q2 (Second Transistor)
    • R2: Base resistor for Q2
    • Rc2: Collector resistor for Q2
    • Both transistors are cross-coupled:
      • Collector of Q1 connects to base of Q2 via C
      • Collector of Q2 connects to base of Q1 via R1

Circuit Operation

Initial Power-Up

  • Q1: OFF
  • Q2: ON
  • States:
    • Q1: Cut-off (open circuit)
    • Q2: Saturation (closed switch)
  • Output:
    • Voltage at Q2 collector ≈ 0V (low output)
    • Voltage at Q1 collector ≈ Vcc (high state)

Trigger Signal Application

  • Stable State: Q2 ON, Q1 OFF
  • Capacitor C charges to Vcc - Vbe (approx. 0.7V).
  • Trigger Pulse:
    • Can be a narrow pulse or finite duration pulse.
    • Circuit acts as a differentiator; detects pulse edges.
    • Positive edge passed through diode for triggering Q1.

Transition to Unstable State

  • Applying trigger pulse:
    • Q1 goes into saturation (short circuit)
    • Q2 goes into cut-off (open circuit)
  • Voltage Behavior:
    • Left side of C grounded: Voltage = -(Vcc - Vbe)
    • Q2 cutoff leads to high voltage at its collector.

Returning to Stable State

  • Capacitor C reverse charges towards Vcc.
  • Once Vc reaches 0.7V, Q2 conducts, causing:
    • Q2 output low
    • Q1 turns OFF
  • Returns to stable state.

Timing Analysis

  • Time in Unstable State: T1
  • Expression for Capacitor Charging:
    • Vc(t) = Vc(∞) + [Vc(0) - Vc(∞)] * e^(-t/RC)
  • Initial Conditions:
    • Vc(0) = -(Vcc - Vbe)
    • Vc(∞) = Vcc
  • Transition Voltage (Vbe) at Time T1:
    • Vbe = Vc(∞) + [Vc(0) - Vc(∞)] * e^(-T1/R2*C)
    • Rearranged to solve for T1:

Final Expression for T1

  • T1 = 0.693 * R2 * C

Output Voltage Considerations

  • To achieve output close to Vcc:
    • R1 should be at least 10 times the value of Rc2.

Conclusion

  • Recap of how to design a monostable multivibrator using BJT.
  • Open to questions and suggestions in the comments.
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