Sorry for asking a question about the same subject as my last question, but I am once again stuck on a BJT Amplifier design problem.
Where the beta parameter may vary from 100 to 800, the voltage between the emitter and the base equals 0.6V (active mode), Vt=25mV and the Early Effect may be ignored.
It can also be supposed that the bypass capacitors simply act as a short circuit for AC and open circuit for DC.
There are two constraints:
- Input impedance > \$2k\Omega\$
- Maximum possible output signal swing
What have I already done (\$i_C\$ is the polarization current which runs through the collector):
I found the signal swing equations:
\$ V_{o_{max}} = 19.8 - i_C(R_C + R_E)\\ V_{o_{min}} = -i_C * R_C//R_L\$
I also found out that the imput impedance will be \$r_\pi = \frac{\beta V_T}{i_C}\$ from the small signal model. One can infer that if the input impedance > \$2k\Omega\$ for \$\beta = 100\$, then it will continue > \$2k\Omega\$ for \$\beta = 800\$. So we can work with \$\beta = 100\$, which yields:
\$R_i = r_\pi = \frac{\beta V_T}{i_C} = \frac{100 * 0.025}{i_C} \rightarrow \frac{2.5}{i_C} > 2000 \rightarrow i_c < 1.25mA\$
From now, I don't know what to do. I have already tried some values for \$i_c\$, being able to calculate the resistances (only supposing symmetrical output) and I noticed that bigger \$i_c\$ gives biggers signal swing. How can prove that? Also, how can I solve the problem without supposing symmetrical output (having one less equation [\$ |V_{o_{max}}| = |V_{o_{min}}|\$])?