Differential Amplifier: Neglecting ro, why?

[hisotaso]

Homework Statement

Pretty straightforward. Attached is a small signal picture of a differential amplifier. Summing currents at the emitter junction, the text says: "Because the transistors have resistive loads, the output resistance will be neglected in the calculations."

Why is this?

Homework Equations

The Attempt at a Solution

 

[The Electrician]

The true load on one side of the diff amp is Rc in parallel with ro. Often instead of resistor loads (Rc), current sources are used. The equivalent impedance of a current source will be comparable to ro and then the parallel combination will be an impedance much different from the impedance of the current source.

In this case, the value of Rc is probably much less than ro, and the parallel combination of the two will have an impedance not much different than Rc. In other words, ro is so much greater than Rc that including it doesn't change the effective load; the effective load is just Rc.

Check the value of Rc and ro and see which is greater, and how much greater.