# Resistive bridge question

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

1. ### bumclouds Thread Starter Active Member

May 18, 2008
82
0
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?

Thanks!
Andrew

2. ### JDT Well-Known Member

Feb 12, 2009
658
85
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
4,846
63
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.