Today was the first day back for the spring semester of college. I only had one class at 4pm so i managed to fit in a morning run beforehand. Starting next week, however, i'll have a lab at 9am to 11am and then will have to hang around college until the class at 4pm. It's not so bad though, in fact i'm wondering if they schedule the classes like this so that we can get work done on our FYPs.

The class was ASICs 2 (Application Specific Integrated Circuits). This is an elective i chose out of a really crap list! They used to have Active Circuit Design 5 (ACD 5) as an elective but then they removed this module completely from the course which isn't too great for me since it's my favourite subject. I had to choose 2 electives and so i went with ASICS 2 and VLSI Digital Systems Processing.

When i got back from college, my friend Rob rang me. He's also an electronic engineering student but a year behind me. He said that he was having some trouble with a design question for Active Circuit Design 3. Unfortunately for Rob, they changed the content of the ACD 1 and 2 modules (2nd year modules) to when i did them but they left ACD 3 untouched. When our year did ACD 1, it was based on the basics of transistor theory. We went through MOSFETS, JFETS, BJTS and the first chapter or two covered diodes. When Rob's year did ACD 1, the main focus was diodes. I remember looking at Robs course notes and thinking "this is crazy, you guys are barely touching on any transistor information". I have no idea what ACD 2 consisted of for Robs year but it will be interesting to find out.

Anyway, skipping forward to today, ACD 3 is all based on oscillators, Phase Locked Loops, ADCs, DACs etc. In the oscillators section they cover phase shift oscillators, wien bridge, colpitts, hartley and so on. You kind of need to know how opamps work and so need to know how transistors work. So the lecture gave Robs class the task of designing a common emitter amplifier. The only information he gave is that it has a gain of 60. I'm meeting Rob tomorrow to go through this with him but i will also post the solution here.




Let VB = 1.7v

VE = VB-VBE = 1V

IE roughly equals IC and IE = VE/RE

We want a gain (AV) of 60.

AV = RC/RE so 60 = RC/RE

Let RE = 100 so RC = 6000

Therefore IE = 1/100 = 1mA
IC = IE = 1mA

VC = VCC - (RC)(IC)

Let VCC = 12V

So VC = 6V

VB = (VCC)(R2/R1+R2)

1.7 = 12(R2/R1+R2)

R1 roughly equals (6)R2

so if we let R1 = 6K then R2 = 1K

NOTE - I'm not sure if 6k resistors are comercially available so change the values accordingly. Or put resistors in series


C'est tout!