EE3300/EE5300 Electronics Applications
Week 1 Practical

Written by A/Prof Bronson Philippa and Luke McCarthy
College of Science and Engineering, James Cook University
Last updated 7 February 2025

Pre-lab preparation

Before the scheduled lab session, use circuit simulation software of your choice (e.g. LTSpice or another program) to create models of these circuits. Carry out the intended measurement procedures in the simulation software. In exercise 1, the model is especially useful to explore the impact of the oscilloscope probes on your ability to measure the input impedance. You should simulate the difference between 1x probes (~ 1 MΩ || 20 pF) and 10x probes (~ 10 MΩ || 20 pF).

Equipment (per student)

  • 1 x 2N3904 (small signal npn)
  • 2 x 2N3906 (small signal pnp)
  • 2 x DMMT3904W (matched pair of npn transistors)
  • plus standard electronics lab equipment and components.

Instructions to students

  • Work individually on these activities.
  • Focus on neat circuit breadboarding. You must establish good habits now to avoid problems later when you are working on more complex circuits.
  • Include power supply decoupling capacitors, which are capacitors placed between each power rail and ground, in order to stabilise the voltage on that rail. The exact value of capacitance is usually not important. It’s typical to use ceramic capacitors in the range of 100 nF - 1 μF. Circuits that switch higher currents will require additional capacitors such as 10 - 100 μF electrolytics, added in parallel with the smaller capacitors.

A suggested layout for dual supply rails on a breadboard is shown in Figure 1.

Figure 1
Figure 1:

A suggestion for how to use dual supply rails on a breadboard.

Not shown here, but important to include, are ceramic capacitors placed close to sensitive components. Ceramic capacitors have good high-frequency performance.

The capacitors shown here are electrolytic, providing bulk capacitance of tens - hundreds of μF. Capacitance of this size would only be important if the circuit was switching high currents. If you use electrolytic capacitors, be careful to note that they are polarised, so pay attention to the negative signs on the case when connecting them.

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Exercise 1: Measure input and output impedances of an emitter follower

Motivation

Consider the circuit scenario shown in Figure 2. You have designed a circuit to produce a voltage signal; unfortunately that circuit stage has a relatively high output impedance.

Can the first stage of the circuit drive the second? Predict the expected voltage attenuation (using a voltage divider formula). Would this be acceptable?

Figure 2
Figure 2:

An example to illustrate how input and output impedances must be considered when breaking down a circuit design into smaller fragments.

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The circuit solution

To improve the ability of a “weak source” to drive a “heavy load”, we can use an emitter follower.

The circuit is shown in Figure 3. When you build this circuit, don’t forget power supply decoupling!

Figure 3
Figure 3:

(a) The emitter follower circuit. The resistor is placed to intentionally degrade the output impedance of the function generator . The capacitor is needed so that the load does not alter the DC bias point of the transistor. (b) If circuits had eyes, we imagine that the first stage “sees” a load . (c) Similarly, we imagine that the load “sees” a Thévenin equivalent circuit representing the upstream stages.

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Another perspective on this circuit is that the emitter follower provides current gain. The load requires more current than the source can supply. The follower allows a small current to be amplified to produce a larger current. From this perspective, the load becomes easier to drive (its impedance increases by a factor of ), and the source becomes stronger (its output impedance decreases a factor of ).

Your task

Your task is to accurately measure the input impedance and output impedance . By comparing these to the source and load impedances, you can calculate the current gain of the transistor. If you have made careful measurements, then your two calculated values of should be approximately equal.

Measurement of input impedance

  1. Measure the input impedance of the emitter follower circuit.
  2. Hence, calculate the current gain ( value) of the transistor. You might find it helpful to refer to AoE section 2.2.3 B (p. 80) for a discussion of the input and output impedances of the emitter follower and the relationship with .

In your lab prep, you should carry out these steps in simulation. You can create AC coupling in a transient simulation by using a large coupling capacitor and pull-down resistor (but be careful that the resistor is large enough that it doesn’t affect the measured input impedance).

Show hint 1 (How to measure the input impedance?)
Measuring the input impedance

Use two channels on the scope to simultaneously measure the voltage at and the voltage at the transistor base. Use a voltage divider formula to calculate .

With the base resistor of 33 kΩ, you’ll see a relatively small voltage attenuation. You might find it useful to try a larger base resistor so that there is more attenuation. If you have an accurate measurement of the two amplitudes (e.g. you use your scope’s measurement features), then you can calculate the input impedance. Alternatively, if you’re more of a trial-and-error type, try installing progressively larger resistors until you see an attenuation of approximately 50%. If you achieve exactly 50%, then you have matched the input impedance of the emitter follower.

Show hint 2 (Measurement tips)
Measurement tips
  • It’s easier to accurately measure the signal amplitude if you use AC coupling on your scope.
  • The input impedance is large, so you need to be careful that your scope probes aren’t interfering with your measurement. Make sure that your scope probe is set to 10x. (Why?)
  • Use a small input sine wave signal (e.g. less than 1 V amplitude). (Why?)
  • Make sure that any parasitic capacitance and the capacitance of your oscilloscope probe is not affecting your measurement. If your input signal frequency is too high, then capacitance (including capacitance of your oscilloscope probe) will dominate the input impedance. Adjust the input signal frequency until you see no phase shift. (Why?)

Measurement of output impedance

  1. Restore the base resistor to its original value (33 kΩ).
  2. Measure the output impedance of the emitter follower circuit. Again, calculate the implied value and compare it to your earlier result.
Show hint 3 (How to measure the output impedance?)
Measuring the output impedance

A simple approach is to do this in two steps. First, measure the open-circuit output voltage. Then install the load (4.7 μF + 1 kΩ) and repeat the measurement. Use a voltage divider to analyse the circuit.

A more sophisticated approach is to inject a test current at output side and measure the attenuation, similar to what you did in step 1.1.

Interpretation

Interpret these results and explain them to a friend. Here are some conversation prompts:

  • Could a 33 kΩ source drive a 1 kΩ load? What does the follower do for us?
  • In general, do you want an input impedance to be high or low? What about output impedance? Does your answer change if your signal is a current instead of a voltage? (e.g. some sensors produce a current.)
  • What does this teach out us about modular design in electronics? What conditions must be met for us to be able to design circuit stages in isolation?
  • How close is your value to that measured by a friend? Are your measurements consistent with the range in the transistor’s datasheet? If you have a multimeter with a transistor tester, how does your value of compare to the value reported by the tester? How do you think that your bias conditions compare to those that might be used by a transistor tester?

To be marked off on this exercise, explain your results and interpretation to the tutor.

Exercise 2: Build an op-amp from scratch

This exercise will answer the question: “are op-amps magic?”

Build the circuit

A basic three-stage op-amp is shown in Figure 4. Take a moment to familiarise yourself with this circuit. You should be able to recognise most of it. The input stage is a differential pair. Next comes a pnp common emitter amplifier, which if you are a npn enjoyer will look upside-down to you. Transistors and are what is called a push-pull stage, which is a circuit pattern that we will return to later in the context of power amplifiers.

The capacitor is called a “compensation capacitor.” Most real op-amps include a similar capacitor. Its purpose will become clearer when we study feedback stability in the coming weeks. For now, you can think of it as reducing the gain at high frequencies to prevent the op-amp from oscillating. You should first try the circuit without If you see spontaneous oscillation, then you can add a very small capacitor (e.g. 1 - 10 pF) to suppress high frequency gain and make the feedback look stable.

The load is split into two resistors because the point will be used to feed back a reduced version of the output signal. After all, this is (more or less) an op-amp, and most op-amp circuits use voltage dividers in the feedback path.

Figure 4
Figure 4:

A basic three-stage operational amplifier. The op-amp’s input pins are and .

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You should first investigate this circuit in simulation, then build it on your breadboard. Having a simulation model will help you spot problems in your physical circuit because you’ll have a reference to compare against.

Use the following parts:

  • For transistors that must be a balanced pair, use the DMMT3904W. This is a package with two identical transistors on the same substrate, allowing discrete circuit designs to use balanced transistors.
  • For other transistors, use 2N3904 and 2N3906 for npn and pnp, respectively.

This is a complicated circuit for a breadboard, so it’s essential to be very neat and tidy and test as you go. Below is a suggested sequence.

  1. Set up your power rails including decoupling capacitors. We don’t show decoupling capacitors on the schematics because we want you to get into the habit to think about power quality in everything you build.

  2. Design an implementation of . For example, you might choose a current mirror as shown in Figure 5. Test it in simulation and then on the bench to confirm that it sinks approximately the desired amount of current.

    Hint: for a quick current measurement, find the voltage across the resistors and use Ohm’s law.

    Figure 5
    Figure 5:

    A plausible implementation of the current source . Make sure that you use a balanced pair of transistors.

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    Show hint (I need help designing a current mirror)
    How to design a current mirror

    1. Choose an appropriate voltage to drop across the emitter degeneration resistor . Typical values would be to . Lower values improve the compliance of the current source (because less voltage headroom is wasted on the resistor) while higher values give better temperature stability and tolerance to variations in transistor properties (because the emitter degeneration resistor acts to stabilise the vs behaviour of the circuit).

    2. Since a current mirror has equal currents on both sides, you know the current that you want to flow in . Calculate the size of using Ohm’s law.

    3. Calculate the desired . You can either inspect the transistor datasheet for a plot of vs (saturation voltage) or you can use the Ebers-Moll model,

      If you are using the DMMT3904W, then . (You can find this value by looking in the properties for the 2N3904 in LTSpice, and assuming that the DMMT3904W is similar to the 2N3904.)

    4. Determine the required voltage at the transistor base (which equals the voltage at ’s collector), and therefore calculate the required size of to set the desired voltage.

    Don’t build any more of the circuit until you have tested your current sink and confirmed that it works.

  3. Build the differential pair and test it with a differential input signal. For now, use very slow sine waves (~ 1 kHz). How strongly does it react to differential signals? Is its DC quiescent point suitable for a pnp common-emitter referenced to Vcc?

    Figure 6
    Figure 6:

    Setup for introducing a differential signal (driving one input higher or lower than the other).

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    Make sure that your differential pair is working before you build any more of the circuit. The quizzes below help you confirm your understand so that you can think about what to look for.

    Self-Check Quiz 1

    Suppose is low enough that is completely cut off (conducting no current). What is the voltage output from the differential pair? The voltage output is taken from the collector of , and once you go further, will be connected to the next stage of the circuit.

    Self-Check Quiz 2

    Suppose is high enough (or low enough) that is taking all of the tail current. What is the voltage output from the differential pair?

    Self-Check Quiz 3

    Based on the quiz answers above, which of the following statements is true?

  4. Build the gain stage.

    Test that the circuit works before proceeding. Apply a differential sine wave input signal and confirm that there is amplification. By now, the circuit will have so much gain that you’ll need to use a very tiny input signal to avoid clipping.

  5. Build the push-pull output stage (including 11 kΩ load). Again, apply a differential input signal and experiment with varying the input’s amplitude and frequency. Wince at the crossover distortion.

  6. Connect your baby op-amp in the non-inverting amplifier configuration. Take the feedback from the point so that you have an overall gain of 11. Does your circuit amplify the input signal by approximately 11 times? Marvel at the reduction in crossover distortion, at least for slow sine waves.

Solution

Show solution (example of final circuit and measured performance)

Figure 7 shows the final circuit constructed neatly on a breadboard, and Figure 8 shows the measured performance of the circuit.

You should aim to build your circuits with a similar level of neatness. Click here to see what happens when the construction is not as neat.

Figure 7
Figure 7:

Example breadboard construction, showing the stages which can be built and tested sequentially.

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Figure 8
Figure 8:

Measured performance of the circuit shown in Figure 7 when operating as a non-inverting amplifier with a gain of 11. Channel 1 (yellow) is a 500 kHz, 1 V peak sine wave produced by a function generator. Channel 2 (blue) is the output from the home-made op-amp. The vertical scale is 5 V/div.

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Interpretation

Here are some questions to consider:

  • How does this circuit handle higher frequency sine waves? What about a square wave? Would you expect square waves to be more or less problematic?
  • Why did the crossover distortion in the output improve when you connected the feedback loop, at least for slow sine waves (e.g. ~ 1 kHz)? Strong hint: put an oscilloscope probe on the base of to observe the signal being fed into the push-pull stage, and compare it to the signal seen at the output. Explain what you see.
  • Are op-amps magic?

Conclusion

Your tutor will mark you off for completing this activity and being able to discuss the results. If you do not finish on time, you have one week to complete it. Bring your completed circuit or evidence of your work to a subsequent lab session for marking.

When you leave, make sure that the lab is just as neat or even neater than when you arrived.

Acknowledgements

This activity was adapted from Labs 4 - 5 in Learning the Art of Electronics by Hayes and Horowitz.