P

#### Peter

- Jan 1, 1970

- 0

I'm now taking an Analog Electronic course and the instructor is having us

go a little more into the circuit than I was taught.

To start off... I'm trying to understand a simple 4 resistor amplifier and

then add capacitors in there so I can amplify AC (I wont go into that now).

I'll use R1 and R2 for the voltage divider on the base of the transistor (R1

being the top resistor and R2 the bottom), R3 will be Rc and R4 will be Re.

Then we can assume a beta of 100, our input voltage will be 20 and a Vce of

approx 10 (half way from 20 so the signal can swing up and down).

Now the way I was originally taught... calculate your voltage divider and

ignore base current. So if R1 and R2 are both 1k, then your base voltage is

10volts, your Ve is 9.3 (assuming 0.7volt drop Vbe), you calculate your

emitter current and voltage, then you consider Ie and Ic equal and calculate

your collector stuff.

About a year ago I wanted to learn how to actualy "design" so someone

suggested the follow which is a GREAT way of doing it. A transistor data

sheet has beta for given values of Ic and Vce, so you first calculate your

R3 value to get your Vc to the value your looking for. In this case we can

consider Ic and Ie equal for purproses of discussion. (I know Ic is beta

divided by beta +1 times Ie higher). Then you set your R4 so your Vce will

be the 10volts that I'd like to get. Now at this point last year I was given

two pieces of advice. I could ignore base current if it's small enough or

(the way I used all the time) I calculate base current based on Ie/beta=Ib

(assuming worse case beta, for a 2n2222a I believe the lowest is 50, but

let's stick with 100) then I base my R1 on being 20 times higher than my

base current for a rule of thumb. Then R2 is selected based on the total

current calculated through R1 and subtract the base current to get your

current through R2.

This was the way I used and stuck with. But now that I'm in school, the

teacher is teaching us impedance and gains which is something I wasn't

using. He'll tell us to make Vce 10volts for max swing (understandable).

Then in order to get a gain of say 10, we make R3 10k and R4 1k, calculate

our currents and voltages. Now the totally confusing part for me is.... we

need to get our R1 and R2 values. The teacher uses Thev. equivelant by

saying our load looking into the base is Re*beta +1, our Vthev is what the

load sees looking the other way (into R1 and R2), and our Rb is R1 in

parallel with R2. This is easy to understand, but then the teacher says,

let's "assume" R1 and R2 is 1k, that would be a Vthev of 10volts and and

Rthev of 500 ohms and we do a loop equation based on that. I told him about

my 20 times higher rule and he said, that can be a way of doing it 'I

guess', but doing it this way you can come up with your own equation and

it's more accurate.

My confusion is what to make R1 and R2. My book (which is impossible to

understand, even the teacher doesn't understand why the school picked it)

goes into a few 'rules of thumb' as well which states something about make

this 1/3 of that and this 1/3 of this. But the strange thing is, if I'm

designing circuit after circuit, all my circuits are going to be exactly

alike because I'm making everything 1/3 of this and 1/3 of that.

Forinstance, my company pretty much uses 15volts for most of it's circuits,

5 volts for the logic and 24 for the higher powered stuff. All my amplifier

circuits would be based on 1/3 of 15volts and 1/3 of that, blah blah. I

could simply make an excel spread sheet and never have to calculate anything

ever again. Then how could I ever call myself an enginee? I'd never be able

to figure out where a problem is in my design if I only followed rules. but

anyway... I know I want my R1 and R2 high enough so that when base current

changes due to temp or the variations in beta that we don't have to worry

about it. But I'm trying to understand everything. The teacher gives us a

loop equation and distracts us with that, then plugs "assumptions" in for

values and bases his entire circuit on throwing whatever resistor value that

pops in his head in the circuit. I think it's becasue he has done if for so

many years, he knows what to use and a person that is trying to understand

why he does it doesn't follow his steps.

If someone can help me out with the R1 and R2 stuff, that would be great. I

don't think I need to waste anyone's time with R3 and R4, I know my formulas

and I know how to use the loop equation. We are really getting into the

details of a transistor and now I just found out that the emitter changes

25mv per ma I believe, so now does the transistor end up running away

because the emitter voltage increases, then the collector increases, changes

the beta then changes the base current, then changes the emitter voltage,

then changes the collector voltage, then the beta. I NEVER thought a 4

resistor and 1 transistor circuit can be so confusing. I thought I really

understood these circuits until I started this class.

Thanks in advance!!!!!!