Common emitter amplifier design

Thread Starter

Mohaamed Yousry

Joined Sep 23, 2017
14
Hello , i've a 16 Mhz oscillator that produces 100 mV peak sine output . i'm trying to use that as a carrier to do ASK modulation but i think the power is too low to go to the antenna , so i tried to design a CE bjt amplifier for my carrier , i've a 9v battery as my supply , so i thought a gain of 90 would give me 9v p-p ,
i've set the emitter at 1v , calculated current Ic , and put the collector at half the supply , then i use a series resistance with bypass capacitor to set the Ac gain.

i've searched on the forums trying to learn how to design it properly and now i understand that my input voltage will cause the output to be distorted , but i have no idea how to calculate the distortion or how bad will it be ? , also i may be wrong in my entire design can i get a desired output from this circuit by adjusting R5 to get the right gain ?
 

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AnalogKid

Joined Aug 1, 2013
8,157
16 MHz and 39 dB works out to a gain-bandwidth product of over 1.4 GHz. That's a lot, and way more than a 3904 can deliver. Also, at that high a frequency, parasitic effects plague the circuit's construction. Stray capacitances become significant current paths among the components and the power and ground connections. You need to treat this more as an RF amplifier than an audio stage.

ak
 

Thread Starter

Mohaamed Yousry

Joined Sep 23, 2017
14
I'm sorry this is my first time trying something with rf , i'm only trying to get a transmitter with a range of a couple of meters , which i assume needs some milliwatts of power . what i understood from your answer is that i need a transistor with a higher gain-bandwidth product or maybe get 39 db with multistage of CE amplifier with 2n3904s ? what should i take into consideration ? if there are any books or application notes you can direct me to i'd be grateful .
 

Ylli

Joined Nov 13, 2015
799
Start out by replacing that collector resistor with a parallel LC network resonant at 16 MHz.

Assuming you are just using a short piece of wire as an antenna, try this:Capture.JPG
 

Zeeus

Joined Apr 17, 2019
468
Hello , i've a 16 Mhz oscillator that produces 100 mV peak sine output . i'm trying to use that as a carrier to do ASK modulation but i think the power is too low to go to the antenna , so i tried to design a CE bjt amplifier for my carrier , i've a 9v battery as my supply , so i thought a gain of 90 would give me 9v p-p ,
i've set the emitter at 1v , calculated current Ic , and put the collector at half the supply , then i use a series resistance with bypass capacitor to set the Ac gain.

i've searched on the forums trying to learn how to design it properly and now i understand that my input voltage will cause the output to be distorted , but i have no idea how to calculate the distortion or how bad will it be ? , also i may be wrong in my entire design can i get a desired output from this circuit by adjusting R5 to get the right gain ?
hmm..please can you explain why gain should be 90
 
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