# designing a base bias grounded emitter amplifier.

Discussion in 'Electronic Basics' started by [email protected], Apr 1, 2007.

1. ### Guest

I'm a EET student and I have to find values for a base bias grounded
emitter amplifier given the following properties.
a. Beta DC,and AC are both 160
b.Vbe= 0.7V
c.Tj=72 degrees ferenhiet, or Vt =25.5mV
d. Q point Vce=Vc=0.5Vcc
e. Unloaded Voltage gain Av = -100
f. Output resistance = 2Kohms.

I have to find theses values for the amplifier.
a.Rc
b. r'e
c.ie
d. VCC
e. Rb
f.Rin (input resistance).

2. ### Andrew HolmeGuest

Tell us where you're stuck and maybe we can give you a hint.

3. ### JamieGuest

lets see, it's the weekend, your in school, you have been doing
every thing except your home work.
I get the notion that many young people these days think the
internet users are going to do their home work for them?
You go to school to learn, I suggest you at least give it a
try.

4. ### BobGuest

Kooky,

The answers are (not in any particular order):

5, 13.5, 78, 4, 377, 69.0

You just have to put them in the proper order.

Bob

5. ### Guest

for the fixed bias circuit I know what re' is Rc and Ie but I need to
find Vcc, but don't know what Ic is or Ib

6. ### Charles SchulerGuest

Use a 2 k ohm collector resistor (Rc). If Vce = Vc, then there is no
emitter resistor and already it is a stupid homework problem.

Use the gain and Rc to determine r'e and that will lead you to the emitter
current and then you can divide by Beta dc + 1 to find the base current and
then you can use Kirchhoff's voltage law and Ohm's Law to find Rb. Again,
it is a stupid homework problem. You might want to find a better school.

The input resistance is roughly Beta x r'e in parallel with Rb. Vcc is up
for grabs, by the way and can be selected before you start.

APRIL FOOL!

7. ### Guest

I worked out the problem and these are the numbers i got Ic =.
001267mA, Ie= .001275mA, Vcc = 3.034V, Rb =291750Kohms,and Rin =
3200ohms

8. ### John LarkinGuest

I think Kevin pointed out the sort-of-remarkable fact (actually, it's
right there in the math) that the voltage gain of a common-emitter
transistor amp is equal to the DC voltage drop in the collector load
resistor times 40. That should break the log jam.

You could, of course derive that. Or you could pick some arbitrary
operating current, solve the whole thing, compute the voltage gain,
and iterate until it's -100. But Kev's thing is easier.

Is this thing beta biased? That's ok for an exercize, but very bad
form in real life.

John

9. ### John LarkinGuest

Can't be right. At 1 ua collector current, base input resistance is
more like 3M and the gain will be far too low... less than 1, in fact.

Old farts like us know, on inspection, that 300 megohm base bias
resistors don't make sense.

John

10. ### Charles SchulerGuest

He has reported A as mA.

11. ### John LarkinGuest

That's still a 300M bias resistor. The numbers are tangled. I wonder
when the assignment was due?

And the gain is still too low.

John

12. ### Jonathan KirwanGuest

I decided to reread the numbers as Charles did. And in doing so, I
was forced to see his Kohms as ohms. I think the op is all screwed up
on these terms. They just stick "mA" onto amps, because that's the
way they have seen it in writing and they haven't really gotten the
details about scientific terms and notation, just yet. Since they
also see things usually as "Kohms", they paste that on like a label to
what they read on their calculator. I think the op has to learn the
notation rules, still, both in reading as well as writing with
comprehension.

Jon

13. ### Charles SchulerGuest

A 5-volt supply and a 500 k-ohm base resistor and a 2 k-ohm collector
resistor sorts it all out. As if anyone cares.

14. ### Charles SchulerGuest

EET is a fuzzy area.

Engineering places strict and prescribed demands on students. Quality
control is what it is all about.

A.B.E.T. accreditation is worth looking into, if you are a student following

I am sure that many EET programs are very worthwhile but shudder to think
that some programs might produce graduates who design my critical systems.
Yes, I know, some EE programs are also shaky.

The "homework problem" disturbs me as it is trivial and has little
relationship to what designers now need to know.

Original posted April 1, OH NOW I SEE IT ....