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PostSubject: Multivibrators   Wed Feb 18, 2009 10:00 am

Those who feel that slide presentation is insufficient can refer to this

A multivibrator is an electronic circuit used to implement a
variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two
amplifying devices (transistors, electron tubes or other devices) cross-coupled
by resistors and capacitors. The most common form is the astable or oscillating
type, which generates a square
—the high level of harmonics
in its output is what gives the multivibrator its common name. The multivibrator
originated as a vacuum tube
(valve) circuit described by William Eccles and F.W. Jordan in 1919.
There are three types of multivibrator circuit:

  • astable, in which the circuit is not stable in either state—it
    continuously oscillates from one state to the other.
  • monostable, in which one of the states is stable, but the other is
    not—the circuit will flip into the unstable state for a determined period, but
    will eventually return to the stable state. Such a circuit is useful for
    creating a timing period of fixed duration in response to some external event.
    This circuit is also known as a one shot. A common application is in
    eliminating switch
  • bistable, in which the circuit will remain in either state
    indefinitely. The circuit can be flipped from one state to the other by an
    external event or trigger. Such a circuit is important as the fundamental
    building block of a register or memory device. This
    circuit is also known as a flip-flop.

Astable multivibrator circuit

This circuit shows a typical simple astable circuit, with an output from the
collector of Q1, and an inverted output from the collector of Q2.
Suggested values which will yield a frequency of about 0.48Hz:

  • R1, R4 = 10K
  • R2, R3 = 150K
  • C1, C2 = 10μF
  • Q1, Q2 = BC547 or similar NPN switching transistor

Basic mode of operation

The circuit keeps one transistor switched on and the other switched off.
Suppose that initially, Q1 is switched on and Q2 is switched off.
State 1:

  • Q1 holds the bottom of R1 (and the left side of C1) near ground (0V).
  • The right side of C1 (and the base of Q2) is being charged by R2 from below
    ground to 0.6V.
  • R3 is pulling the base of Q1 up, but its base-emitter diode prevents the
    voltage from rising above 0.6V.
  • R4 is charging the right side of C2 up to the power supply voltage (+V).
    Because R4 is less than R2, C2 charges faster than C1.

When the base of Q2 reaches 0.6V, Q2 turns on, and the following positive feedback
loop occurs:

  • Q2 abruptly pulls the right side of C2 down to near 0V.
  • Because the voltage across a capacitor cannot suddenly change, this causes
    the left side of C2 to suddenly fall to almost -V, well below 0V.
  • Q1 switches off due to the sudden disappearance of its base voltage.
  • R1 and R2 work to pull both ends of C1 toward +V, completing Q2's turn on.
    The process is stopped by the B-E diode of Q2, which will not let the right side
    of C1 rise very far.

This now takes us to State 2, the mirror image of the initial state,
where Q1 is switched off and Q2 is switched on. Then R1 rapidly pulls C1's left
side toward +V, while R3 more slowly pulls C2's left side toward +0.6V. When
C2's left side reaches 0.6V, the cycle repeats

Monostable multivibrator circuit

When triggered by an input pulse, a monostable multivibrator will switch to
its unstable position for a period of time, and then return to its stable state.
The time period monostable multivibrator remains in unstable state is given by t
= ln(2)*R2*C1. If repeated application of the input pulse maintains the circuit
in the unstable state, it is called a retriggerable monostable. If
further trigger pulses do not affect the period, the circuit is a
non-retriggerable multivibrator

Bistable multivibrator circuit

Suggested values:

  • R1, R2 = 10K
  • R3, R4 = 10K

This circuit is similar to an astable multivibrator, except that there is no
charge or discharge time, due to the absence of capacitors. Hence, when the
circuit is switched on, if Q1 is on, its collector is at 0 V. As a result, Q2
gets switched off. This results in nearly +V volts being applied to base of Q1,
thus keeping it on. Thus, the circuit remains stable in a single state
continuously. Similarly, Q2 remains on continuously, if it happens to get
switched on first.
Switching of state can be done via Set and Reset terminals connected to the
bases. For example, if Q2 is on and Set is grounded momentarily, this switches
Q2 off, and makes Q1 on. Thus, Set is used to "set" Q1 on, and Reset is used to
"reset" it to off state.

Last edited by Sreeram on Wed Feb 18, 2009 4:27 pm; edited 1 time in total (Reason for editing : I apologize for forgetting to include diagram- Please apologize)
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