Resistive bridge question

Discussion in 'Homework Help' started by bumclouds, Aug 20, 2009.

  1. bumclouds

    Thread Starter Active Member

    May 18, 2008
    Hey guys,

    I got given an assignment which involves being given a resistive bridge that looks like this:


    Where the resistors are all variable. I applied +10V and V+ and -10V at V-. At points A and B we got (pretty close to) 0V.

    Then we applied points A and B to this circuit where the black dots are:


    And wham.. the collector voltages of the transistors are now very closely matched... A little while earlier, we had just grounded these terminals (effectively 0V as well), and the collector voltages weren't so closely matched.

    So why is *this* 0V matching the voltages so much better than the other 0V?

  2. JDT

    Well-Known Member

    Feb 12, 2009
    The collector voltages depend on what's connected to the collectors - which you don't show. The collector currents will be equal only if base currents I1 and I2 are equal. Which will only happen if the bridge is balanced: assuming R = R then Rx = Ra. The voltage difference between A and B will be zero.

    This all assumes that the base currents are small compared to the currents flowing through the bridge resistors.
  3. mik3

    Senior Member

    Feb 4, 2008
    What are the values of the resistors in the bridge?

    I think you have closely matched collector voltages just by luck because the resistances in the bridge have different values (and the transistors are not matched).

    Put all the resistors in the bridge the same value and check it again.