I recently saw a class in YouTube about the emitter follower where the professor said that AC sees ( for lack of other word ) DC pathways as ground , is this true ??. Also , does AC in a BJT moves back and forth as it does everywhere else ?.

 

AC sees ( for lack of other word ) DC pathways as ground , is this true ?

Yes.
Ground is ground, whether for AC or DC.
Why would you think there would be a difference?

does AC in a BJT moves back and forth as it does everywhere else ?

Yes.
But the AC typically has a DC bias with it.
If you block the DC with a series coupling capacitor, you will see just the AC.

 

Yes.
Ground is ground, whether for AC or DC.
Why would you think there would be a difference?
Yes.
But the AC typically has a DC bias with it.
If you block the DC with a series coupling capacitor, you will see just the AC.

Well the first part of my question was about AC seeing a live conductor of DC as pathway for AC too , I mean ( for what I understood in that class ) , AC can go in opposite direction to DC in the same conductor. Is that true or I missunderstood that part of the YouTube class ?.

 

AC can go in opposite direction to DC in the same conductor. Is that true or I missunderstood that part of the YouTube class ?.

Depends upon the relative DC and AC currents in the conductor.
If the AC peak current is less than the DC current (as is typical in an amplifier), then the AC will just cause the total current to increase and decrease at the AC frequency.

Below is the simulation of a simple example.;
There is a 2Vpp AC signal which is riding on 2Vdc.
Note how the voltage (green trace) and current (yellow trace) vary with the AC, but they never go to zero because of the added DC.
After passing through the DC block capacitor C1, you then see the AC signal centered around zero (red trace and blue trace).

That all make sense?

 

The initial concept of DC meaning Direct Current and AC meaning Alternating Current is simplistic and limiting.
You need to view DC and AC from a frequency perspective.

All electrical signals contain DC and AC components.
DC strictly refers to the frequency at 0Hz. Strictly speaking, there is almost no electrical source that can supply a 0Hz signal.
Therefore, if you want to stretch this definition, there is no such thing as DC voltage.
Just a thought.

 

there is almost no electrical source that can supply a 0Hz signal.

Not even a battery (excluding thermal noise)?

 

Not even a battery (excluding thermal noise)?

Not even a battery.

 

hi Chips,
Agreed.
E

 

Not even a battery.

I excluded thermal noise.
So what other AC battery voltage is there?

 

Ve

Depends upon the relative DC and AC currents in the conductor.
If the AC peak current is less than the DC current (as is typical in an amplifier), then the AC will just cause the total current to increase and decrease at the AC frequency.

Below is the simulation of a simple example.;
There is a 2Vpp AC signal which is riding on 2Vdc.
Note how the voltage (green trace) and current (yellow trace) vary with the AC, but they never go to zero because of the added DC.
After passing through the DC block capacitor C1, you then see the AC signal centered around zero (red trace and blue trace).

That all make sense?

View attachment 287693

Very nice graphs .

 

Ve
Very nice graphs .

Nice is good, but did they help you understand your question?

 

Nice is good, but did they help you understand your question?

Yes , completely, in the first graph the AC is not centered at zero since there is always a DC companent , once the Capacitor blocks the DC the graph is back to be centered at zero as it normally does since the DC component is not present anymore. It gave me a pretty good insight. O learned something new . Thanks a lot sir. I will the experiment with my oscilloscope and an amplifier.

 

Yes , completely, in the first graph the AC is not centered at zero since there is always a DC companent , once the Capacitor blocks the DC the graph is back to be centered at zero as it normally does since the DC component is not present anymore. It gave me a pretty good insight. O learned something new . Thanks a lot sir. I will the experiment with my oscilloscope and an amplifier.

Also I see that the center of the sinusoidal signal is the DC . How interesting. Thank you so much.

 

Also I see that the center of the sinusoidal signal is the DC . How interesting. Thank you so much.

And please , excuse my typo mistakes , I have seen some now.