Transistor Circuit Construction

1. Design and build the voltage amplifier based on fixed biasing shown below. Determine the value of $R_B$, $R_C$, and $V_{CC}$ to achieve maximum voltage amplification and minimum distortion by setting the DC operating point to be in the middle of the linear range of the output characteristic plot. The values of capacitors $C$ should be large enough so the impedance $Z_C=1/j\omega C$ can be assumed to be zero approximately for the signal frequency $\omega$.

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   • Calculate the DC operating point on the output characteristic plot ($I_{C}$ and $V_{CE}$) based on your design, and compare it to the actual one measured from the actual circuit.

   • Test the circuit by a sinusoidal signal of 50 mV peak-to-peak amplitude from signal generator (with output resistance $R_S=50 \Omega$) and an oscilloscope (with input resistance $R_{in}=1 M\Omega$) to monitor the input and output signals both before and after the amplification and to observe the voltage gain and waveform distortion. Compare the predicted gain based on the small signal model with the actual one.

   • Observe the polarity inversion and the signal distortion. Increase the amplitude of the input sinusoid from 50 mV to 1 V and observe the output in terms of the amount of distortion and clipping at both the positive and negative peaks of the sinusoid.

2. Repeat the above for the amplifier based on a self-biasing circuit shown in the figure below. Determine the values of $R_1$, $R_2$, $R_C$, and $R_E$ to achieve maximum voltage amplification and minimum distortion by setting the DC operating point to be in the middle of the linear range of the output characteristic plot.

   Hint 1: To minimize distortion, the DC operating point should be around the center of the linear region of the output characteristic plot, i.e., $V_{CE}\approx V_{CC}/2$.

   Hint 2: write a piece of Matlab code based on the equations for the self-biasing circuit given here to calculates the DC operating point ($I_B$, $I_C$, $V_B$, $V_C$, $V_E$, and $V_{CE}=V_C-V_E$), based on the circuit parameters $V_{CC}$, $R_1$, $R_2$, $R_C$ and $R_E$ that you choose for your design.

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   • Calculate the DC operating point on the output characteristic plot ($I_{C}$ and $V_{CE}$) based on your design, and compare it to the actual one measured from the circuit.

   • Test the circuit by a sinusoidal signal of 50 mV peak-to-peak amplitude from signal generator and an oscilloscope monitor the input and output signals both before and after the amplification and to observe the voltage gain and waveform distortion. Compare the predicted gain based on the small signal model with the actual one.

   • Observe the voltage gain as a function of the by-pass capacitor in parallel with $R_E$, and the capacitors at both the input and output ports, by trying different $C$ values.

   • Observe the voltage gain as a function of the signal frequency. Generate a Bode plot of the magnitude of the voltage gain for the frequency range of 10 Hz to 1 MHz. Find the maximum voltage gain $A_{max}$ and the freqnecy range in which this maximum gain is achieved.

   • Observe the polarity inversion and the signal distortion. Increase the amplitude of the input sinusoid from 50 mV to 1 V and observe the output in terms of the amount of distortion and clipping at both the positive and negative peaks of the sinusoid.

3. Connect a load resistor $R_L$ to the amplificaiton circuit above, measure the output voltage across $R_L$ as its value varies from $1 M\Omega$ to $10 \Omega$ in decade scale. Then insert an emitter follower in between the amplification cirucit and the load $R_L$. Re-measure the output voltage across $R_L$ when its values varies as before. Explain what you observe.

   What to submit:

   A text document that discusses concisely each of the items specified above, including

   • the design process by which you determin the circuit parameters (e.g., resistance values).
   • comparison of the predicted and measured DC operating points.
   • comparison of the predicted and measured AC voltage gains.
   • pictures of the screenshots of the input/output voltages of the circuits required in the lab.