Kevin Aylward wrote:



Provide a *mathematical* *explicit*
disproof of my mathematics or present your retraction.

You are correct about the specific device (BJTs) biased class-A for
multipliers such as the gilbert cell. They need to have the gm "slope"
change for the gain to change, and changing the gain is what the
modulation is all about. By definition that means those "slopes" are
tangent to something that can't be straight (the Vbe - Ic function).
I was wrong about the specific device used in this type of application.
The system in this application (DSB-AM) is most certainly linear (all
phase and amplitude information is completely preserved -- the definition
of linearity -- albeit at a translated frequency), but the Vbe - Ic
function for the transistor isn't linear and it can't be for it to work
in the given configuration.






· Gain is a direct function of gm.
· Gain must change for amplitude modulation to occur, by definition.
..·. not only must gm move along the curve, the curve has to be a
curve (not straight), otherwise the gain would not change.


Ic Ic = f(Vbe)
| .
| . .slope of Vbe - Ic = gm
| . .
| . .
| . .
| . .
Ic_q+--------------..
| .. |
| . . |
| . . |
| . . |
|. . |
| . |
+---------------+------------- Vbe
Vbeq

Pretty simple really. No wonder I was so bored and fell asleep in EDSN101.

Kevin Aylward wrote:



Provide a *mathematical* *explicit*
disproof of my mathematics or present your retraction.

You are correct about the specific device (BJTs) biased class-A for
multipliers such as the gilbert cell. They need to have the gm "slope"
change for the gain to change, and changing the gain is what the
modulation is all about. By definition that means those "slopes" are
tangent to something that can't be straight (the Vbe - Ic function).
I was wrong about the specific device used in this type of application.
The system in this application (DSB-AM) is most certainly linear (all
phase and amplitude information is completely preserved -- the definition
of linearity -- albeit at a translated frequency), but the Vbe - Ic
function for the transistor isn't linear and it can't be for it to work
in the given configuration.






· Gain is a direct function of gm.
· Gain must change for amplitude modulation to occur, by definition.
..·. not only must gm move along the curve, the curve has to be a
curve (not straight), otherwise the gain would not change.


Ic Ic = f(Vbe)
| .
| . .slope of Vbe - Ic = gm
| . .
| . .
| . .
| . .
Ic_q+--------------..
| .. |
| . . |
| . . |
| . . |
|. . |
| . |
+---------------+------------- Vbe
Vbeq

Pretty simple really. No wonder I was so bored and fell asleep in EDSN101.