Report on electronics.

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ELEC2320 Lab 1 Supplementary Note

Due to the unfortunate event of COVID-19, this lab is to be re-arranged to be simulation based. This

note explains how to do this.

I. MICROPHONE MODEL

Fig. 1 shows a typical electret microphone capsule and its preamp circuit using an FET in a common

source configuration. The two-terminal electret capsule contains a FET which must be externally powered

by supply voltage V+. The resistor sets the gain and output impedance. The audio signal appears at the

output, after a DC-blocking capacitor. (Source: Wikipedia, Electret microphone)

To enable the simulation of the microphone capsule, we use a pre-recorded audio sound (Arnold

Schwarzenegger’s “I’ll be back” in the Terminator). The sound is in the file Ibeback2.wav. You can play

this sound using any audio playing software (e.g., Quick Time Player). The simulated microphone capsule

is shown in Fig. 2. The LT-Spice file is mic.asc. You can run this model to see the audio waveform at

MIC OUT. The value of resistor R1 can be changed to adjust the “volume” of MIC OUT. If you are

curious about how the sound file is linked to the voltage source, this is simply done by rick-clicking the

voltage source and typing in “wavefile=Ibeback2.wav” (without the quotation marks) in DC Value[V].

Note that MIC OUT corresponds to vS in Fig. 5 in Lab 1 Note, which is duplicated below in Fig. 3.

Also note that V+ and OUTPUT in Fig. 1 correspond to VCC and vI in Fig. 3.

Fig. 1. Electret Condenser Microphone Capsule and Circuit Diagram

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Fig. 2. LT-Spice Model for Microphone Capsule

Extra Note: You can replace Schwarzenegger with your own voice if you like. Simply record some

sound and convert your sound file to “.wav” format and use it instead. Try to keep it short to save your

simulation time. You may need to adjust the simulation time (.tran) and adjust R1 to make MIC OUT

roughly ±200mV.

Extra Note: Your amplifier circuit uses LM324. If necessary, you may find extra information on LM324

simulation in the supplementary note for Lab 2 (Import LM324 in LT-Spice.pdf).

Extra Note (Important): Task 3 in Lab 1 requires you to work out the Thevenin’s equivalent resistance

RTh between the terminals of the mic for your choice of R4. Since we do not know the internal resistance

Rmic of the microphone explicitly, we can not use the simple formula RTh = Rmic//R4 directly. Instead,

please refer to the note “ELEC2320 Lab 1 solution online.pdf” for an alternative solution.

II. ASSESSMENT INFORMATION

The previous arrangement for assessment was demonstration to Lab Demonstrator. But this needs to

be revised to cope with the online nature of the lab work. Instead, each group is required to submit a

lab report electronically. A brief guideline is as follows:

• Report must be submitted to Turnitin (under Turnitin Assignments in Blackboard) by Friday night,

10 April, 2020.

• To make report writing and marking easier, please think of the report as an assignment. That is, only

the answers for the respective questions are necessary. There is no need for introduction, conclusion,

list of figures, etc..

• Make sure to include circuit screenshots or plots from LT-Spice if they are relevant to the respective

question (possibly as a justification for your answers).

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Fig. 3. Fig. 5 in Lab 1 Note (duplicated)

• During the design process or any other calculations, use letters (symbols) to indicate the components

(such as R1, R2 for resistors or C1, C2 for capacitors, for example). Only substitute the values at

the last step when calculating desired values for the question. Also, please briefly include the steps

in your design process.

If you have any questions, pease discuss with the teaching team (Course Coordinator and Lab Demon-

strators).

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