# Ac ground in amplifiers

#### Antsy electron

Joined Dec 27, 2022
67
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 ?.

#### crutschow

Joined Mar 14, 2008
32,026
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.

#### Antsy electron

Joined Dec 27, 2022
67
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 ?.

#### crutschow

Joined Mar 14, 2008
32,026
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?

Last edited:

#### MrChips

Joined Oct 2, 2009
28,473
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.

#### crutschow

Joined Mar 14, 2008
32,026
there is almost no electrical source that can supply a 0Hz signal.
Not even a battery (excluding thermal noise)?

#### MrChips

Joined Oct 2, 2009
28,473

#### ericgibbs

Joined Jan 29, 2010
17,410
hi Chips,
Agreed.
E

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#### crutschow

Joined Mar 14, 2008
32,026
Not even a battery.
I excluded thermal noise.
So what other AC battery voltage is there?

#### Antsy electron

Joined Dec 27, 2022
67
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 .

#### crutschow

Joined Mar 14, 2008
32,026
Ve
Very nice graphs .

#### Antsy electron

Joined Dec 27, 2022
67
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.

#### Antsy electron

Joined Dec 27, 2022
67
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.