# Electronic – Common Emitter not amplifying

amplifiergainoscillator

I am trying to make a 27 MHz carrier wave transmitter from a crystal oscillator and a secondary amplifier. Thi is the complete circuit:

simulate this circuit – Schematic created using CircuitLab

The first part, left of C6 is a colpitts crystal oscillator. And on the right side of C6 is the common emitter amplifier. Colpitts crystal oscillator that I've built can be found here.

Q1 and Q2 datasheets can be found here.

The problem is the following. If I have CE amplifier disconnected, and measure the voltage with oscilloscope at O1, I get expected 150 mV peak-to-peak. But as soon as I connect the CE amplifier and measure the voltage at O2, I get around 300 mV peak-to-peak (note that antenna at this time is not connected), which is far less than I was expecting.

Values choosen for colpitts oscillator are the same as on that website I posted a link to. For the CE amplifier I calculated the values my self, here's how I did it:

1. \$\beta=100\$
2. I chose: \$I_C=I_E=1mA\$
3. I chose: \$V_E=1V\$, so \$V_B=1.7V\$
4. \$R_6=\dfrac{1V}{1mA}=1k\Omega\$
5. \$I_B=\dfrac{Ic}{\beta}=10uA\$, \$I_{R5}=100uA\$, \$I_{R4} = 110uA\$
6. \$R_5=\dfrac{1.7V}{100uA}=18k\Omega\$, \$R_4=\dfrac{9V-1.7V}{110uA}=66k\Omega\$
7. \$R_7=\dfrac{9V-4.5V}{1mA}=4.7k\Omega\$
8. For \$C_4\$ I read somewhere: \$X_{C4}<=\dfrac{1}{10}\times R_6\$, and I get \$C_4 >= 60pF\$

C5 and C6 were chosen arbitrarily, If someone could tell me how to precisely calculate their values I'd really appreciate it.

So shouldn't the gain of the amplifier be:\$r_e=\dfrac{25mV}{I_C}\$ \$A_v=\dfrac{-R_C}{r_e}=-188\$?, while I'am getting gain of only 2.

What could be the problem? I read somewhere that impedence missmatch can affect the power of the signal transmitted, could this be the case here, since the output impedence of colpitts oscillator is relatively low, while the input impedence of the CE amplifier is relatively high?

Any help is greately appreciated!

EDIT:

I know I havent explicitly stated, but I would really appreciate it, if one could also suggest a solution to this problem.

EDIT2:

What if I were to use BS270 MOSFET in common gate mode instead of 2N3904, would the gain increse? I've read somewhere that MOSFETs are faster and seen them used in HF applications. Because I have them at hand and can't buy any components at the moment.