E84 Home Work

1. Assume in the circuit in figure B above, $\beta=100$, $V_{cc}=20V$, $R_C=R_E=1\;K\Omega$. Find the voltages $V_E(t)$ and $V_C(t)$ when the base current is $I_B(t)=(50+30 \cos \omega t)\;\mu A$. Sketch the waveformes (over time) of the two voltages.

2. Find values of $R_C$ and $R_B$ in the circuit with $\beta=100$ and $V_{CC}=15V$ so that the Q-point is $I_C=25mA$ and $V_{CE}=7.5V$. What is the Q point if $\beta=200$?

   

3. Design a stable self-biasing transistor circuit as shown below so that the DC operating point (Q point) of $I_C=2.5mA$ and $V_{CE}=7.5V$ is in the middle of the load line for maximal dynamic range. Assume the smallest $\beta$ value allowed is $\beta_{min}=50$. To reduce the number of free parameters, we assume $R_C=2K\Omega$.

   Hint:

   • For the DC operating point to be approximately independent of the specific transistor used, we want $(\beta_{min}+1)R_E\approx \beta_{min} R_E \ge 10 R_B$ (see class notes here).
   • Start the design process from the desired Q-point, determine $R_E$, then find desired $V_{BB}$ and finally $R_1$ and $R_2$.

   

4. Verify your design in previous problem by checking the DC operating point of the resulting circuit is approximately the same as the requirement.

5. Read and understand Examples 2 and 3 in the notes here, and rederive the three circuit parameters of (1) AC input resistance, (2) AC output resistance, and (3) AC voltage gain in Example 3.