EE3300/EE5300 Electronics Applications
Week 4 Tutorial

Written by A/Prof Bronson Philippa
College of Science and Engineering, James Cook University
Last updated 6 March 2025

Workshop warm-up

  • From this week’s notes, what material were you most comfortable with? What material were you the least comfortable with?
  • Is there anything that you’d like clarification on?

Workshop discussion question

The Arduino Uno R3 is one of the most well-known microcontroller development boards. Studying its schematic, what power supply protection features can you identify?

Discussion prompts:

  • Easy warmup: how is the barrel jack protected against reverse polarity?
  • How is the Vin pin on the header protected against reverse polarity?
  • What’s the power path when the board is powered via USB?
  • Is it possible for power to flow backwards into the USB port if the board is also powered via the barrel jack?
  • Bonus extension question: The USB 2.0 standard allows devices to draw a maximum of 500 mA, but this is a development board aimed at hobbyists who might connect any manner of high current peripherals. How does the Arduino design ensure compliance with the USB specification?

Tutorial questions

Figure 1
Figure 1:

An emitter follower circuit.

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  1. Follow the steps below to see the reasoning that leads to the invention of the push-pull stage. Recall that an emitter follower has a low output impedance, suggesting that it should be good for driving “heavy” loads (i.e. loads with low impedances).

    Figure 1 is an emitter follower driving an 8 Ω speaker (i.e. ). The power supplies are and .

    1. Given that the gain of an emitter follower is

      choose such that the amplifier’s gain is 0.9. Perform this analysis under the condition that .

    2. Derive the equation for the DC load line ( vs ) and find the Q point (when ).

    3. Derive the equation for the AC load line.

    4. Using the load line, estimate the voltage swing of this amplifier.

    5. The voltage swing you obtained in part (d) is inadequate. What could be done to improve it? Hint: assume that is implemented as a transistor; can you make it more useful than a fixed current source?

      (Solution)

  2. (This is Razavi Exercise 14.1.) Consider the emitter follower shown in Figure 1 with , and .

    1. Determine to achieve a small-signal voltage gain of 0.8.

    2. Suppose that the positive peak of the output waveform corresponds to an instantaneous output power of 0.5 W. Writing , calculate the voltage gain of the amplifier that occurs at this moment. The change in voltage gain from the average value you calculated in part (a) indicates the non-linearity in the amplifier.

    (Solution)

  3. (This is Razavi Exercise 14.6.) The emitter follower of Figure 1 is is driven with a sinusoidal input with a peak amplitude of 1 V. Assume and .

    1. Calculate for and .

      Hint: Analyse the transistor behaviour using the Ebers-Moll model and solve numerically.

    2. Sketch the output waveform.

    (Solution)

  1. Prove that the maximum efficiency of the push-pull output stage (Figure 2) is by following the steps below.

    1. Notice by symmetry that the power in the positive half cycle is supplied by whereas in the negative half cycle it is supplied by The two cycles are symmetrical. Calculate the average current drawn from each supply and hence calculate the total power. Assume is a sine wave with the largest possible peak voltage of .

    2. Calculate the AC power delivered to .

    3. Hence, calculate the amplifier’s efficiency.

    (Solution)
Figure 2
Figure 2:

The push-pull output stage showing the current flows in the positive half cycle () and negative half cycle ().

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Figure 3
Figure 3:

Amplifier design where the output voltage is the voltage delivered to .

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  1. This question refers to the circuit shown in Figure 3.

    1. What is the class of this amplifier?

    2. Prove that the small signal voltage gain is

    Hints:

    • Assume that the transistors have a high transconductance.

    • Assume that the transistors have a high so that the impedance seen looking into the base () is much larger than .

    • Assume that the capacitors are large enough that their AC impedances can be neglected.

    • Assume that the small signal resistance of the diodes is negligible.

    • Recall that the gain of a degenerated common emitter amplifier is

      where is the resistance seen at the collector and is the resistance seen at the emitter.

    (Solution)

  1. This question refers to the circuit shown in Figure 4.

    Even with the diode biasing trick, the push-pull stage still exhibits non-linearity and distortion. Therefore practical audio systems use feedback.

    is a current source (presenting a high input impedance) so that the common-emitter amplifier has high gain and good voltage headroom.

    1. Let the gain of the common-emitter and emitter-follower stages be . Find the loop gain in terms of , , and the resistor values.

    2. Write down the closed-loop gain.

    3. Use the assumption to simplify the expression for the closed-loop gain.

    (Solution)
Figure 4
Figure 4:

An amplifier with feedback. Make sure that you can recognise the type of feedback.

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  1. In lecture notes, we discussed a polarity protection circuit that uses a P-channel MOSFET and is placed on the high side of the power supply. However, N-channel MOSFETs generally have superior performance including a lower on-resistance.

    1. Draw the circuit diagram for a polarity protection circuit that uses an N-channel MOSFET. Explain how it works.

    2. Suppose that the circuit requirements are to withstand a maximum reverse polarity bias of 24 V and and peak forward current of 3 A.

      Given the MOSFETs listed in AoE Table 3.4a (p. 188), select a suitable device with the minimum , and calculate the expected voltage drop across its on-resistance when conducting 3 A.