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Our teacher gave us a homework assignment to design a common-emitter amplifier.

My task is design an amplifier which has 12 voltage gain rate.

Firstly, I drew its small signal model, nd determined its voltage gain formula.

Then, I assumed some values. For instance RE, RS, RC, RL, \$\beta\$(beta) and tried to calculate R1 and R2 according to 12, voltage gain.

Unfortunately, when I calculate R1 and R2, the rate was negative. I said that is not possible because resistances are always positive. Unfortuntaley I couldn't make my homework but I want to learn what my problem is and what I did wrong.

My solution and question:

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    \$\begingroup\$ .... Hence the gain is always negative. \$\endgroup\$ Commented Nov 30, 2020 at 11:19
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    \$\begingroup\$ But did you forgot about Ohm's Law? If Vcc is 20V and R1= R2 then the base voltage is 10V and the emitter voltage is 9.3V. Thus, Ie = Ic = 9.3V/500R = 18.6mA and because Rc is 12k IC_max = 20V/12k = 1.6mA. All this means that your BJT is saturated. And it will not work as an amplifier. You cannot randomly select the component values. \$\endgroup\$ Commented Nov 30, 2020 at 14:50
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    \$\begingroup\$ What exactly your teacher tells you to do? Did he specify the voltage gain value only? What about Vcc, Rs, and RL values? \$\endgroup\$ Commented Nov 30, 2020 at 16:07
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    \$\begingroup\$ Well for Ic = 2.4mA and Rc =12k we have Ve =1.2V and V_Rc = 29V Thus, Vcc = 60V The gain will be around 8k/(1/gm + 500R) = 15V/V; Thus to get 12V/V we need to attenuation it by 0.8 or so. Thus, R1||R2 needs to be equal to around 2k. And at the same time, the voltage at the base needs to be equal to Ve+Vbe = 1.2V + 0.7V = 1.9V. So you need to choose R1 and R2 os that R1||R2 = 2k and Vcc*R2/(R1 + R2) =1.9V Can you do it? \$\endgroup\$ Commented Nov 30, 2020 at 18:15
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    \$\begingroup\$ Reduce the input signal amplitude. \$\endgroup\$ Commented Nov 30, 2020 at 19:47

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