Class C Amplifier : Expert Help needed !

Thread Starter

haruwn

Joined Jan 30, 2009
11
Hi.... m emloying a Class C Amplifier Design for the amplification of an fsk signal
I simulated the whole thing on EWB 5 , worked pretty well, input 30dB, where voltage rms ws 20mV @ 10MHz
and i obtained about 250mV in the simulation.
Now doing this all practically, gave input of 2V rms and results were quite disappointing, my signal rather got attenuated !
I tested the results on 2N3904 BJT !
any suggestions !
I have attached the simulation print screen with results.
 

Attachments

hgmjr

Joined Jan 28, 2005
9,029
Have you a reason for using such a small value for output coupling capacitor?

hgmjr
 

KL7AJ

Joined Nov 4, 2008
2,208
Hi.... m emloying a Class C Amplifier Design for the amplification of an fsk signal
I simulated the whole thing on EWB 5 , worked pretty well, input 30dB, where voltage rms ws 20mV @ 10MHz
and i obtained about 250mV in the simulation.
Now doing this all practically, gave input of 2V rms and results were quite disappointing, my signal rather got attenuated !
I tested the results on 2N3904 BJT !
any suggestions !
I have attached the simulation print screen with results.
Remember a Class C amplifier is always a TUNED amplfier. Is the final tank circuit tuned to your input frequency, (or vice versa)?

ERic
 

Thread Starter

haruwn

Joined Jan 30, 2009
11
Infact this is a Class A amplifier associated with the tank circuit as in Class C, i calculated the values of all the components using the AC and DC analysis separately and ws perfect in theory as wel.....
input, bypass, output reactance ws setted to te critical frqncy of 1 MHz while the tank ckt is resonated on 10 MHz.....
all this is good, whts wrng with the real circuit....... ?
 

Thread Starter

haruwn

Joined Jan 30, 2009
11
Ofcourse i tuned it by watchful calculations ....... any othr help ???
should i change the transistor ........ my group fella was like, 2N3904 has lesser bandwidth...... ? may be this is the issue ?
any recommendations from ppl who had bn wrkin on BJTS ?
 

bertus

Joined Apr 5, 2008
20,050
Hello,

I think the tank circuit will not do much with the 4k7 resistor in series.
The gain is also reduced by the emitter resistor without a decoupling capacitor.

Greetings,
Bertus

PS Do not use SMS like text, this will make your text unreadable.
Use proper english.
 

Thread Starter

haruwn

Joined Jan 30, 2009
11
oK, Can anybody please ammend, or modify the circuit so that i could attain a gain = 10 ?
 

hgmjr

Joined Jan 28, 2005
9,029
Try increasing the tank capacitor to 2.56 pFd. I am assuming that your input signal is 10MHz.

hgmjr
 

KL7AJ

Joined Nov 4, 2008
2,208
using your figures for the tank circuit, I'm showing the resonant frequency being around 17 MHz
 

hgmjr

Joined Jan 28, 2005
9,029
What is the frequency of your input signal. The scope makes it looks like 10 MHz.

hgmjr
 
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