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Multivibrators?

Discussion in 'Electronic Basics' started by [email protected], Dec 6, 2004.

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  1. Guest

    Do we analyse multivibrators and oscillators in the same way,
    i.e. beta*A = -1 . For example, if we look at the multivibrator
    at http://www.uoguelph.ca/~antoon/circ/2qflash.htm
    do we need to calculate the gain with a small signal equivalent
    circuit or not. Is it a voltage-shunt feedback, what approximations
    do we do to ease the analysis, and how do we get the equations
    for the frequency of oscillation?
     
  2. Guest

    No. That technique is only applicable to linear circuits e.g. sine
    wave oscillators.
    The base-emitter voltage ramps as the capacitor charges/discharges. As
    it passes 0.6V, aided by positive feedback, the transistor suddenly
    switches hard on/off. The frequency can be calculated by working out
    how long it takes to charge/discharge the capacitor.

    Download LTSpice (free) and load the following as an ASC file:

    Version 4
    SHEET 1 880 680
    WIRE 48 64 176 64
    WIRE 240 -16 240 16
    WIRE 240 112 240 320
    WIRE 160 320 240 320
    WIRE 240 320 240 384
    WIRE 32 320 96 320
    WIRE -48 320 -96 320
    WIRE -96 320 -96 112
    WIRE -16 112 -96 112
    WIRE -96 112 -96 0
    WIRE -96 -80 -96 -144
    WIRE -96 -144 240 -144
    WIRE 528 -144 528 -128
    WIRE 240 -80 240 -144
    WIRE 240 -144 528 -144
    FLAG 48 160 0
    FLAG 240 464 0
    FLAG 528 -48 0
    SYMBOL LED 224 -80 R0
    SYMATTR InstName D1
    SYMBOL res 224 368 R0
    SYMATTR InstName R1
    SYMATTR Value 51
    SYMBOL cap 160 304 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C1
    SYMATTR Value .47µ
    SYMBOL pnp 176 112 M180
    SYMATTR InstName Q1
    SYMATTR Value 2N3906
    SYMBOL res 48 304 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R2
    SYMATTR Value 47k
    SYMBOL npn -16 64 R0
    SYMATTR InstName Q2
    SYMATTR Value 2N3904
    SYMBOL res -112 -96 R0
    SYMATTR InstName R3
    SYMATTR Value 3.3e6
    SYMBOL voltage 528 -144 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value PULSE(0 3 0)
    TEXT -114 506 Left 0 !.tran 10s
     
  3. With this sort of oscillator, you have to analyze several completely
    separate phases.

    One phase starts the moment the output transistor begins to turn off,
    and collector voltage heads toward ground by the pull down action of
    the 470 ohm resistor. during this phase, D1 first forward biases and
    clamps the base voltage for Q1 to no more than a diode drop and C1
    charges through R2.

    Eventually (this is where a bit of RC analysis comes in) the current
    through D1 goes through zero and C2 changes charge rate because the
    current through R1 in series with R2 sets the charge rate for C2.
    During this phase, the base voltage at Q1 swings between negative one
    diode drop toward positive one diode drop, where Q1 begins to conduct
    as the current through R1 detours from C2 to the base emitter
    junction.

    Once Q1 conducts enough to start to turn Q2 on and the voltage drop
    across R3 starts to increase, a positive feedback loop forms that very
    quickly drives Q1 and Q2 into saturation because the positive voltage
    change across R3 gets coupled back to the base of Q1 through C1 and
    R2, making the current from R1 insignificant. This phase lasts as
    long as C1 R2 can supply enough current to Q1 to keep both transistors
    well saturated.

    Once C1 charges enough that this current is no longer available, Q1
    and Q2, while still conducting somewhat, fall out of saturation enough
    that the voltage drop across R3 starts to sag. At that moment, the
    current through C1 R2 reverses direction and Q1 shuts down and you are
    at the starting point of this description.

    Each of these phases has to be analyzed to predict both the on and off
    time.
     
  4. Guest

    I have noticed that if I increase R3 a little the oscillation stops,
    why is that, and what alternative I have to obtain a longer period /
    lower frequency?
     
  5. Guest

    I have noticed that if I increase R3 a little the oscillation stops,
    why is that, and what alternative I have to obtain a longer period /
    lower frequency?
     
  6. Guest

    I have noticed that if I increase R3 a little the oscillation stops,
    why is that, and what alternative I have to obtain a longer period /
    lower frequency?
     
  7. The current through R1 multiplied by the current gain of the two
    transistors must bias the voltage drop across R3 somewhere between cut
    off and saturated full on so that the positive feedback through C1 R2
    can swing the transistors either full on or full off from that bias
    point.

    R2 is the primary timing resistor that can be easily adjusted. Bigger
    changes require that you change C1.
     
  8. Guest

    With this sort of oscillator, you have to analyze

    Good explanation. The original of that circuit with identical parts is
    located here:

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page5.htm#flash2.gif

    And I copied it from an old GE transistor manual around 1968. It was
    originally a metronome using germanium transistors with a loudspeaker
    in place of R3.

    -Bill
     
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