Applications of Diodes:

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Part of original deleted when doing summary.
Applications of Diodes
The goal of this lab you was to build,
characterize and analyze a variable attenuator
using a diode. Next you were to build and test
some clipper/clamping circuits and a DC
restoration circuit.

• Description of Experiment A
• Build the small signal variable attenuator shown
  to the right.
• vin(t) is a10kHz sine wave
• 1N555 Diode
• C1 3.3mF, R 2.2kW, RC 400W

Leave out C2 and RL. Measure vo(t) across diode.
The purpose of C1 is to block DC current so the
Q-point of the diode is not affected.
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The attenuator uses the fact that that small
signal resistance of the diode changes as the
diode current changes. See Equation 10.22. In
practice the purpose of the variable attenuator
is to keep the signal at a constant strength. In
the circuit below, feedback is used to keep the
output voltage of the attenuator constant. In
this experiment keep the output voltage at
20mV(peak-to-peak) by adjusting the input
voltage. You will serve as the feedback loop.

• Start with iD 20mA and then reduce it in steps
  until the diode current is about 10mA keeping vo
  at 20mV(peak-to-peak). It takes a light touch to
  get good measurements.
• Record iD, VC and vin(peak-to-peak) for each
  step.
• Plot the voltage gain (Gain vo/vin) of the
  attenuator versus iD.
• Make the plots in two ways
• As a linear plot of gain versus iD
• As 20log10(gain) dB versus log10(iD)
• You may have problems adjusting the input
  voltage for small values. There is a 20dB button
  on the function generator. Also, you can build a
  voltage divider like the following between the
  function generator and the test circuit.

Approximate values. Use what you have.
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• Calculate the gain versus iD using Equation
  10.22 to calculate rd. I measured a value of
  n1.33 last week. Assume VT 26mV. Dont forget
  to include the resistors, R, RC and RL.
• The resistors of the voltage divider should not
  be included since you are measuring vin at the
  output of the divider.
• Plot the gain versus iD on the same plots
  (linear and log) as the experimental data.
• Compare and comment on the accuracy of the
  calculation.
• Can you think of ways to improve the circuit?
• For example, how could you obtain a larger range
  of gain control.
• Set iD to about 1mA. Capture the waveform for vo
  of 20mV(peak-topeak) and save the file. By
  decreasing iD and increasing vin, adjust vo to
  100mV(peak-to-peak). Do the same measurement
  with 200mV(peak-to-peak). Capture these
  waveforms.
• Explain why the three waveforms have different
  shapes.

• Description of Experiment B
• 1) Build the clipper/clamping circuit to the
  right using
• 1N555 diodes
• R1 2.2kW
• V1 V2 5V

• Drive the circuit the following signals and
  capture the waveform of each and save the file
• 20V peak-to-peak square wave
• 20V peak-to-peak triangular wave
• 2) Calculate the output waveform for each case
  using the simple piecewise-linear diode model
  shown in Figure 10.23 of the textbook.
• Comment on the accuracy of the results compared
  to the experiment.
• 3) Plot the DC transfer characteristic, vo versus
  vin for the circuit.
• 4) Calculate the DC transfer characteristic using
  the simple piecewise-linear diode model.
• Comment on the accuracy of the results and why
  errors occur.

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• Description of Experiment C
• 1) Modify the circuit used in Experiment C by
  adding resistors as shown.
• Let R2 1kW and R3 500W

• Drive the circuit with a 20V peak-to-peak
  triangular wave and save a file of the waveform.
• 2) Calculate the waveform using the simple
  piecewise-linear diode model.
• Compare to the experimental results and comment
  on the accuracy.
• 3) Plot the DC transfer characteristic with vin
  going from 10V to 10V.
• 4) Calculate the DC transfer characteristic
• Compare to the experimental results and comment
  on the accuracy

• Description of Experiment D
• 1) Remove V1, V2, R2 and R3 from the circuit
• Let R1 10W

• Apply a 2V peak-to-peak triangular wave to the
  input
• Be careful not to get the voltage too high since
  you could burn out a resistor.
• Start with the amplitude on the function
  generator turned to minimum.
• Save a file of the output waveform.
• How does this waveform differ in shape from the
  ones made in Experiments C and D.
• 2) Calculate the output waveform using the
  simple piecewise-linear diode model using
  Vf0.6V.
• Does the calculation give a good fit to the
  experimental data? Why not?

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• Description of Experiment E
• Build the DC restoration circuit shown to the
  right.
• Let C1mF and use the 1N555 diode.
• Drive the circuit with a triangular wave with a
  peak-to-peak voltage of 10V.
• Observe the input voltage, the output voltage
  and vC with the oscilloscope.
• Reverse the diode and repeat the measurements.
• Explain how the circuit works.
• Assume the capacitor had zero charge when you
  applied the input voltage. How and when did the
  capacitor get charged?

• Lab Report
• Follow the guidelines given for the earlier labs
  and on the grading sheets given to you by Tim. F.
  
• Be sure to clearly document all of your
  measurements, observations and conclusions.

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• Description of Experiment F
• Set up the circuit shown with
• R4 15kW and VB 5VDC.
• Connect vin to the triangular output of the
  function generator.
• Set the waveform to 10kHz with a peak-to-peak
  voltage of 20 V.

• Explain how the circuit works.
• Why does the output waveform have this shape?
• Use KVL and KCL along with the diode
  characteristics to explain the output voltage
  versus time.
• Account for the voltage drop across the diodes
  when they conduct in the forward direction.