Common emitter vs. LC tuned amplifier

Hi everyone,

I have a question about the difference between common emitter amplifier to LC tuned amplifier.

In a common mode amplifier , the gain is the :
= sum of the resistors in the collector/ sum of the resistors in the emitter.
The output voltage in CE is:

Vout= Vcc - Idc*Rc - ic*Rc (where Vcc-Idc*Rc is the DC voltage and ic*Rc is the AC signal).
The voltage in the common emitter output as I understood, will never succeed Vcc.

on the contrary, for LC tuned amplifier, where the collector Load is L||C||Rc (paralell resonance) the voltage can succeed Vcc.

my question is, how come? how can a signal in a LC tuned amplifier can be bigger then Vcc, and if so, why in common emitter it cant?

for example, suppose the collector voltage in common emitter amplifier is 3.2V (for 3.3V p.s) and the gain is 100.

if the Vin is 10mV sin then the AC signal needs to be 1V at the output of the CE amplifier and the voltage at the output should be : 3.2V + 1V*sin(wt) which is 4.2V max and 2.2V min.

In LC tuned amplifier (read the attachement) the voltage can succeed the Vcc.

I will apprichiate your help.

Best regards

In LC tuned amplifier ,the signal at the output is high because of the resonance.At resonance there is max and equal voltage across the L and C.At resonance the voltage across the reactance can exceed the applied voltage.
the gain can be changed by changing the values of reactive elements.

Can you explain more on this for example why the gain is depend on the LC since at the Resonance the L and C cancell each other and it's depend only on the parallel resistor Rp.

why in the analytical calculations, your remarks aren't inserted?

does someone has a site which explains it? or can light the issue?

Regards,

Hi,


basically if you loo at the small signal model of the LC tuned amplifier. We expect that the open and the capcitor as short at the frequencies of interest.

And hence the DC quiescnet point is at VCC and hence the voltage would swing w.r.t to VCC. while this doesnt happen in CE amplifier because of emitter resisitor.

Regards,
thoppay