ahndy05,
Thanks for your reply. We still need some clairification.
I will try to find your transistor later, but the "H1061C" P/N is not immediately familiar. Do you have a manufacturer for it, so I'll know where to look? A link to the transistor's data sheet is always a useful thing to include in your post, so we can all be looking at the same information.
What is Vab? I don't see any such label on the schematic.
Pending confirmation, I'm assuming that the "output" is across RL in the schematic.
The temperature is important, because the output will vary with the temperature. The temperature dependance is a vital characteristic of this circuit in its usual application. Over the -55C to +150C range, the output may vary over a range of around 2V. Do we really need to consider this entire temperature range?
I was not sufficiently clear about what I meant about the adjustment range. I'll try to do better:
The range adjustment you will actually get, with a circuit made from real parts, will depend on those particular parts, and as I said, the temperature. So I was trying to find out if the endpoint values you gave for the adjustment, 2.5V and 4.5V, are values that should be inside the adjustment range in every case and over the temperature range. Such a design will result in the endpoints of the adjustment range usually being well outside the specified 2.5V and 4.5V levels.
Have you figured-out how the circuit works? Can you tell us the practical application for this circuit?
Again, can you give us some idea why you are asking this particular question?
I hope this is of some help to you.
Ted