03-25-2010 05:51 AM
You're close. I don't believe there's a pulldown resistor in a "classic" bipolar TTL output stage. It's the strength of the transistor that pulls the output low. In the modern CMOS equivalent of the output driver I don't believe there are any resistors used for the output drive.
Transistors have the ability to sink and/or source current but are still restricted to V-I curves for a certain point of operation. Look at data sheets for random transistors. Popular (old) items I'm designing into bits and pieces these days include the 2N3904 (NPN bipolar) and 2N7002 (N-channel FET). You'll see operating graphs that shine some light into how these non-resistor elements have variable behaviors depending on the currents and voltages.
If you haven't had the opportunity to investigate semiconductor physics, I'd suggest you could get a lot out of some coursework or reading through a college text. I haven't looked through any syllabus there but the courses offered at MIT are available for free online. Check out http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/ for a full list. It may be the Microelectronics Devices and Circuits courses would provide some extremely helpful basic information.
03-25-2010 06:03 AM
Thank you John, i got it now.
I studied semiconductor physics back in college days but it has been some years now without revising such matters.I didn't recall about the I/V restriction.
Your help was without a doubt precious. You should write a book you know :)
I'll keep your contact if you don't mind.
03-25-2010 06:23 AM
I thought I did just write a book ;-D
I'm happy to help where I'm helpful which is the main reason I'm here in the first place. As long as someone does enough research to know what questions to ask in the first place, the experience is pleasant.