EE 461G Introduction to Electronics

Homework # 10 Due November 16, 1999

In Sedra and Smith read sections 4.14, 5.5 and 5.7

Problem 1

This homework demonstrates the characteristics of the common source amplifier. You can compare it to the BJT amplifier you studied last week. The circuit is the same as the common emitter amplifier except that an n-channel MOSFET takes the place of the BJT. Once biased, the circuit may be used as a voltage amplifier by connecting an input signal to the gate of the transistor, and connecting a load to the drain. Once again, these connections are capacitively coupled. A capacitor is connected in series with the signal source and load, providing an open circuit during DC operation in order to prevent the source and load from changing the circuits quiescent operating point. These same capacitors provide a short circuit during AC operation so that they do not effect the AC signal. The circuit is shown below. Beside it is the small signal model of the MOSFET.

For the small signal model, where gm is the MOSFET’s transconductance, and ro is the MOSFET’s output resistance. The output resistance is due to channel modulation. You may assume ro is infinite. Let Rin = 1 kW, RD = 1 kW, RS = 100 W, RL = 1kW, Kp = 0.25 mA/V2 , Vt = 1.6V, IR1 = IR2 = 1.475mA, and VDD = 15 V.

  1. Derive and draw the DC load line equation for the circuit.
  2. Determine the operating point IDQ and VDSQ at the intersection of the midpoint of the load line and the characteristic curves for the NMOSFET. You can draw these curves knowing Kp. for
  3. Find the values of R1 and R2 that meet the above requirements.
  4. Determine the small signal voltage () and current gain () of the circuit for CS = 0. For this calculation assume Rin is zero and RL is infinite. Repeat the calculation with the given values of Rin and RL.
  5. Determine the small signal voltage () and current gain () of the circuit for CS so large it shorts out RS. For this calculation assume Rin is zero and RL is infinite. Repeat the calculation with the given values of Rin and RL.
  6. Determine the input and output resistance of the circuit. Does it depend on CS?
  7. What value of CS must you choose in order to short out RS? Assume the signal frequency is 10 kHz.
  8. How would you find the input and output resistance of the circuit above experimentally?
  9. Construct Spice model of the above circuit above using values given or calculated above.
  10. Use Spice to compute the quiescent operating point by using its operating point analysis. Be sure to set the quiescent values of your sources. What are IDQ and VDSQ, and how do the Spice values compare with the values computed above?
  11. Apply a 10 KHz sine wave to the input with a value that does not push the MOSFET into cutoff or its ohmic (triode) region with CS connected. Plot the output voltage and record the peak value of the input voltage.
  12. Increase the peak value of the sine wave until the MOSFET goes into cut-off or the ohmic region. How can you tell when the MOSFET has left the saturated region?
  13. Use your result in part 11) to compute the small signal AC gain. Compare your Spice determined gain to the incremental gain found above.
  14. Determine the input and output resistance of the MOSFET common source amplifier using Spice. Compare the results with the calculated values.
  15. Explain how the common source MOSFET amplifier differs from the BJT common emitter amplifier.

Problem 2

If RC = 20KW, VCC = 5V, b = 10 - 100, VfBE = 0.6V and VCEsat = 0.3V

  1. Choose RB so that the minimum Vin to saturate the transistor is 5V over the specified range of b.
  2. If Vin = 0.45V, what is Vout?
  3. If Vin = 5V, what is Vout for b = 10 and b = 100?
  4. Make a Spice model of this circuit using the above circuit values and your RB. Make the input a pulse waveform with minimum value equal to 0.3V, maximum value equal to 5V, and a frequency of 100kHz. Plot the output voltage and IC for b=10 and b=100. Use the spice default value for Is.