Common emitter amplifier question (rc || rl)

[cheese9988]

Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac signal?
It looks more to me like rl is in parellel with the transistor and both
being in series with rc. Can anyone explain this?

 

[JeffM]

How are these two in parallel with an ac signal?
It looks more to me like rl is in parellel with the transistor
and both being in series with rc.
cheese9988

Think *Thevenin's impedence of an ideal voltage supply*.
http://www.google.com/search?&q=Thevenin

 

[Andrew Holme]

cheese9988 wrote:

Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac signal?
It looks more to me like rl is in parellel with the transistor and
both being in series with rc. Can anyone explain this?

In the AC equivalent circuit (where there is no DC - only AC signals) :
1. The +ve supply (Vcc) is ground.
2. The DC-blocking (aka coupling) capacitor between the collector and RL is
a short circuit.
3. The transistor is an AC current source

So, the AC equivalent circuit is a current source, RL and RC - in parallel.

A current source with parallel resistance can be transformed to a voltage
source with series resistance; therefore, the AC equivalent circuit can also
be drawn as a voltage source in series with RL and RC.

 

[Fred Bloggs]

cheese9988 wrote:

Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac signal?
It looks more to me like rl is in parellel with the transistor and both
being in series with rc. Can anyone explain this?

In order to compute the output voltage, you will want to look at the
equivalent circuit seen by the transistor at the collector- this is
clearly rl||rc. Then when it comes time to compute the voltage developed
across rl, you look back in and see rc || transistor. Does that make sense?

 

[Pooh Bear]

cheese9988 wrote:

Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac signal?
It looks more to me like rl is in parellel with the transistor and both
being in series with rc. Can anyone explain this?

It's only a convenience for modelling using equivalent circuits. rc doesn't
actually exist as a resistance, it's simply a way of modelling the
transistor's sensitivity to collector voltage vs collector current when
inserted into the appropriate equation..

In comparison Rbb and Ree are real ( bulk resistance of semiconductor
material ).


Graham

 

[Paul Burridge]

On 29 Jan 2005 13:39:18 -0800, "cheese9988" <cheese9988@hotmail.com>
wrote:

Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac signal?
It looks more to me like rl is in parellel with the transistor and both
being in series with rc. Can anyone explain this?

They are effectively in parallel because at signal frequencies the all
voltage supply points are ground (there is an 'invisible short' across
the battery or power supply at signal frequencies so your Vcc and GND
are one and the same). This only applies at signal frequencies, of
course, so at DC they're still pretty much isolated from eachother.

--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[lemonjuice]

On Sun, 30 Jan 2005 09:06:35 +0000, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> attempted to answer

I read in sci.electronics.design that cheese9988
cheese9988@hotmail.com> wrote (in
1107034758.554400.279030@c13g2000cwb
.googlegroups.com&gt about 'Common emitter amplifier question (rc ||
rl)', on Sat, 29 Jan 2005:
Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac
signal?
It looks more to me like rl is in parellel with the transistor and
both
being in series with rc. Can anyone explain this?


As far as the signal is concerned, the collector DC supply is at
ground
potential (or should be).

stop wasting bandwidth by repeating his question. The question is

why is it at Ground. Either give an answer or go and mow your lawn.
Maybe you're Kevin Alyward under a different name?

 

[Active8]

On 1 Feb 2005 12:48:52 -0800, lemonjuice wrote:

On Sun, 30 Jan 2005 09:06:35 +0000, John Woodgate
jmw@jmwa.demon.contraspam.yuk> attempted to answer

I read in sci.electronics.design that cheese9988
cheese9988@hotmail.com> wrote (in
1107034758.554400.279030@c13g2000cwb
.googlegroups.com&gt about 'Common emitter amplifier question (rc ||
rl)', on Sat, 29 Jan 2005:
Hi, I have been trying to figure this out and its bugging me. On a
common emmitter amplifier, you have rl (load resistance) and rc
(collector resistor). How are these two in parallel with an ac
signal?
It looks more to me like rl is in parellel with the transistor and
both
being in series with rc. Can anyone explain this?


As far as the signal is concerned, the collector DC supply is at
ground
potential (or should be).

stop wasting bandwidth by repeating his question. The question is
why is it at Ground. Either give an answer or go and mow your lawn.
Maybe you're Kevin Alyward under a different name?

Maybe he is, but no matter what you call yourself, you're an
asshole.

--
Best Regards,
Mike

 

[Paul Burridge]

On 2 Feb 2005 03:43:26 -0800, "lemonjuice" <exskimos@anonymous.to>
wrote:

Ummm I'm not not satisfied with your answers. maybe you can't explain
yourself . Why should you bypass something that is there? Why is the
impedance of the DC voltage source low? Why is it a short at AC?
Try again.

It was a lousy answer of the kind Mike's notorious for. He and Kevin
are very similar. They only understand half a question and then
proceed to waffle on about the *other* half. Personally, I killfiled
'em both several months ago.
No regrets.... ;-)

--

"What is now proved was once only imagin'd." - William Blake, 1793.