calculating emitter stablizied biasing sat current

[Telus News]

Hi all:
I have a simple emitter stablized circuit. with Vcc = 12V, Rc = 2.2k, Re =
1.2k
I did Vcc / (Rc + Re) to find the sat current = 3.53mA ( assuming Vsat =
0V )
but the answer on my textbook gave me 5.13 mA!!

Is it my problem or the textbook's problem?
thanks for any help...

--
{ Kelvin@!!! }

 

[Jonathan Kirwan]

On Wed, 25 May 2005 01:46:03 GMT, "Telus News"
<kelvinchan604@donot.spam> wrote:

Hi all:
I have a simple emitter stablized circuit. with Vcc = 12V, Rc = 2.2k, Re =
1.2k
I did Vcc / (Rc + Re) to find the sat current = 3.53mA ( assuming Vsat =
0V )
but the answer on my textbook gave me 5.13 mA!!

Is it my problem or the textbook's problem?
thanks for any help...

Your logic seems okay to me. But I'm not trained in electronics, so
I'll listen to others on this, too.

I tend to think about BJTs as having Vce(sat) of about < 0.4V, very
often in the near .05V-.15V range. But that's barely anything
different to what you were saying. I can imagine how to increase the
emitter current, of course, by simply jacking up the base voltage and
dumping base current like it was going out of style. But the
collector current would only then decline a little as the (Ic+Ib)*Re =
Ie*Re voltage jacked up the collector voltage, which rides at Vce(sat)
above it, and thus dropping the collector current somewhat. I assume
they were talking about "sat current" as collector current, yes?

At 5.13mA, just through the collector resistor Rc alone, you've about
used up all your 12V supply -- with only about .7V left over. This
sounds a lot like someone imagining that the saturation current is the
entire 12V supply going through the collector resistor and a diode to
ground. If the wrong polarity BJT were wired up, it might act like
that, I suppose.... But it really sounds more like you are assuming
they were talking about a collector resistor when they were really
talking about a base resistor. Or they assumed that and wrote
something else.

Oh, well. I'm stumped.

Jon

 

[Rich Grise]

On Wed, 25 May 2005 01:46:03 +0000, Telus News wrote:

Hi all:
I have a simple emitter stablized circuit. with Vcc = 12V, Rc = 2.2k, Re =
1.2k
I did Vcc / (Rc + Re) to find the sat current = 3.53mA ( assuming Vsat =
0V )
but the answer on my textbook gave me 5.13 mA!!

Is it my problem or the textbook's problem?
thanks for any help...

Your arithmetic is correct, assuming Vcesat = 0. Isat will be even less
with a non-zero Vcesat. It wouldn't be the first time that a textbook
has a typographical error. (Or maybe the purpose is to find out if
you're confident enough in your coursework to actually _find_ the
error(s) in the textbook! ;-) )

Cheers!
Rich

 

[Jerry R]

What's the Vee? -5.2V maybe?

....jerry

"Telus News" <kelvinchan604@donot.spam> wrote in message
news:vNQke.9080$HI.2333@edtnps84...

Hi all:
I have a simple emitter stablized circuit. with Vcc = 12V, Rc = 2.2k, Re =
1.2k
I did Vcc / (Rc + Re) to find the sat current = 3.53mA ( assuming Vsat =
0V )
but the answer on my textbook gave me 5.13 mA!!

Is it my problem or the textbook's problem?
thanks for any help...

--
{ Kelvin@!!! }