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Midterm Exam 3 -- E84, Fall 2008

  1. Problem 1. (33 points)

    An inductive load is connected to a AC voltage source of 220 Volts (RMS) and 50 Hz. It is known that the average power is 1.1 kW and the power factor is $\lambda=0.5$. If the power factor is to be improved to 0.866 by including a capacitor in paralle with the load, what is the its capacitance?

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  2. Problem 2. (33 points)

    Find the DC operating point $(I_C, V_{CE})$ of the transistor circuit given below, where $R_1=100K\Omega$, $R_2=300K\Omega$, $R_C=R_E=2K\Omega$, $V_{CC}=12V$, and $\beta=100$. If you find the DC operating point is not in the middle of the linear region of the output characteristic plot, modify $R_2$ so that the DC operating point is in the middle of the linear region (to maximize the dynamic range of the AC output).

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  3. Problem 3. (34 points)

    The circuit shown below is called Darlington transistor amplifier which is composed of two transistors $T_1$ and $T_2$ with their collectors connected and the emitter of $T_1$ connected to the base of $T_2$. Assume $V_{CC}=20V$ and both transistors have $\beta=50$.

    1. Give the expressions of $I_{C1}$, $I_{E1}=I_{B2}$, $I_{C2}$, $I_{E2}$ all in terms of $\beta$ and $I_{B1}$.
    2. Given $R_C=1 k\Omega$, find $R_B$ so that the DC operating point $Q$ is in the middle of the linear region of the output V-I characteristic plot.
    3. Given $R_B= 4.6 M\Omega$ and the same $R_C$ as before, find the proper DC voltage $V_{cc}$ so that the Q point is in the middle of the linear region.
    For simplicity, assume for both transistors $v_{be}=0.7V$.

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Ruye Wang 2008-12-12