Transistor amp question ---- not homework

Discussion in 'Electrical Engineering' started by Peter, Sep 5, 2005.

  1. Peter

    Peter Guest

    I've posted a circuit on my webpage for everyone to view since I don't
    think I should upload pictures here.

    http://home.comcast.net/~bostonpeter73/


    I'm trying to understand how to find Vout. Last night I played around with
    loop equations, assumptions, and after hours of trying I had no luck.

    The biggest part that is killing me is that 68ohm resistor up top because
    you can't just say 20 volts goes to the left part and the right part of the
    circuit. I know approx. what I should get because I plugged this into
    MultiSim, but that's cheating in my book, I should know how to calculate
    this circuit.


    If anyone knows the best way to come up with the answer, please let me
    know.



    Thanks in advance!
     
    Peter, Sep 5, 2005
    #1
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  2. Peter

    ehsjr Guest

    Peter wrote:
    > I've posted a circuit on my webpage for everyone to view since I don't
    > think I should upload pictures here.
    >
    > http://home.comcast.net/~bostonpeter73/
    >
    >
    > I'm trying to understand how to find Vout. Last night I played around with
    > loop equations, assumptions, and after hours of trying I had no luck.
    >
    > The biggest part that is killing me is that 68ohm resistor up top because
    > you can't just say 20 volts goes to the left part and the right part of the
    > circuit. I know approx. what I should get because I plugged this into
    > MultiSim, but that's cheating in my book, I should know how to calculate
    > this circuit.
    >
    >
    > If anyone knows the best way to come up with the answer, please let me
    > know.
    >
    >
    >
    > Thanks in advance!


    Here's how to "sketch" it in a post, and a hint
    at the bottom:


    o +12
    |
    [R1] 68
    |
    +------------+
    | |
    [R2] 8.25K [R5] 150
    | |
    | e/
    +-----------| Q1 2N3906
    | \
    | |
    [R3] 36.4K +---- Vout
    | |
    | /
    +-----------| Q2 2N3904
    | e\
    | |
    [R4] 8.25K [R6] 150
    | |
    +------------+
    |
    Gnd

    Hint: You were concerned about the left hand side and the
    68 ohm resistor. What is the lowest possible voltage,
    with respect to ground, at the bottom of the 68 ohm
    resistor? Can you analyze the circuit, if the 68 ohm
    resistor was replaced by a jumper wire?

    Ed
     
    ehsjr, Sep 5, 2005
    #2
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  3. Peter

    Peter Guest


    >
    > Hint: You were concerned about the left hand side and the
    > 68 ohm resistor. What is the lowest possible voltage,
    > with respect to ground, at the bottom of the 68 ohm
    > resistor? Can you analyze the circuit, if the 68 ohm
    > resistor was replaced by a jumper wire?
    >
    > Ed
    >


    I believe it can be done. you pretty much ONLY have I1 on the left and I2
    on the right because the emitter current of Q2 is the collector current of
    Q1 and the emitter current of Q1. So it's almost a parallel circuit.
     
    Peter, Sep 5, 2005
    #3
  4. Peter

    ehsjr Guest

    Peter wrote:
    >>Hint: You were concerned about the left hand side and the
    >>68 ohm resistor. What is the lowest possible voltage,
    >>with respect to ground, at the bottom of the 68 ohm
    >>resistor? Can you analyze the circuit, if the 68 ohm
    >>resistor was replaced by a jumper wire?
    >>
    >>Ed
    >>

    >
    >
    > I believe it can be done. you pretty much ONLY have I1 on the left and I2
    > on the right because the emitter current of Q2 is the collector current of
    > Q1 and the emitter current of Q1. So it's almost a parallel circuit.


    Ok, so what do you get (voltages) at the base of the transistors
    that way? Will B-E current flow? What will happen in terms
    of C-E current?

    Ed
     
    ehsjr, Sep 6, 2005
    #4
  5. Peter

    Peter Guest

    ehsjr <> wrote in news:fDnTe.1204$d6.317@trndny05:

    > Peter wrote:
    >>>Hint: You were concerned about the left hand side and the
    >>>68 ohm resistor. What is the lowest possible voltage,
    >>>with respect to ground, at the bottom of the 68 ohm
    >>>resistor? Can you analyze the circuit, if the 68 ohm
    >>>resistor was replaced by a jumper wire?
    >>>
    >>>Ed
    >>>

    >>
    >>
    >> I believe it can be done. you pretty much ONLY have I1 on the left
    >> and I2 on the right because the emitter current of Q2 is the
    >> collector current of Q1 and the emitter current of Q1. So it's almost
    >> a parallel circuit.

    >
    > Ok, so what do you get (voltages) at the base of the transistors
    > that way? Will B-E current flow? What will happen in terms
    > of C-E current?
    >
    > Ed
    >


    I think I need a few loop equations and not just pulling the 68ohm resistor
    out of the circuit to make assumptions. There should be a scientific
    approach to this. In any case, if I don't hear from someone soon, I'll pull
    the circuit off the webpage.
     
    Peter, Sep 7, 2005
    #5
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