AM modulator circuit help ??

[Walid]

**broken link removed**

can anyone explain to me how the modulation occurs here ? how does the source VG1 cause the current gain to change ? is this something related to changing in the dynamic resistance or something else ? I don't get that plz help.

thnx

 

[trash]

VG1 is a voltage source.
If you consider the voltage drop across R1 + T1 + R2 + VG2.
T1 is changing with respect to the carrier
VG2 is changing with respect to the modulation.
If we remove the carrier and look at the DC level (fix T1's current)

Lest say the voltage is 10V. R1=10 ohms R2=1 ohm VG2=1Vpp and T1=4 ohms
10V-1V=9V 9/(10+1+4)=0.6A thus VR1=6V, VR2=0.6V Vce=2.4V (output voltage = 0.6+2.4+1V = 4V with respect to ground)

now the 180 degrees out of phase for the modulation.

10V+1V=11V 10/(10+1+4)=0.67A thus VR1=6.7V, VR2=0.67V Vce=3.63V (output voltage = 0.67+3.63-1V= 3.3V with respect to ground)


All that is happening is that you are changing the emitter current by changing the emitter voltage.
This effects the amplified carrier signal gain. That's modulation !

 

[Walid]

"All that is happening is that you are changing the emitter current by changing the emitter voltage.
This effects the amplified carrier signal gain. That's modulation ! "

The AC gain for a CE transistor circuit is Av = - Rc/re (approx) , so how would changing Ie affect the AC gain ?

ANy help plz ?!

 

[Walid]

"All that is happening is that you are changing the emitter current by changing the emitter voltage.
This effects the amplified carrier signal gain. That's modulation ! "

The AC gain for a CE transistor circuit is Av = - Rc/re (approx) , so how would changing Ie affect the AC gain ?

ANy help plz ?!

 

[trash]

The emitter current (Ie) is dependant on "re"

Ie = Ve / re or more correctly Ie= (Ve-VG2)/re

 

[RCinFLA]

With C3 as a 100 nF it won't work very well. It will attenuate most of the higher modulation frequencies and clip the lower side of modulation waveform.

 

[Walid]

The emitter current (Ie) is dependant on "re"

Ie = Ve / re or more correctly Ie= (Ve-VG2)/re

- I think u mean Ie= (Ve-VG1)/re , VG1 is the audio source , VG2 is the carrier .

- mean by re the internal small signal emitter resistance which is given by (hie / hfe) , hie = rπ .not the external RE wich is 4.7k in the schematic .

Is it that changing the value of Ie due to the audio source causes a change in (hie) which in turn changes the Ac gain Av = Rc/re ? is that wy the gain is being affected by changing Ie ?

thnx

 

[trash]

Sorry, you're correct ... I'm getting old and silly ... Yes your correct, (VG2) I meant VG1 as the source of modulation.