EE40, Spring 2015, Pre-Lab 6

Capacitor and Filters

Logistics

You should submit your prelab assignments on Gradescope before your lab section. This will be different than the Gradescope page of the course where you submit your homework and you should already be able to see the page of your lab in addition to the main course in your Gradescope account. If not, you can email your lab GSI to do that for you.

You will complete the lab assignments during the lab sessions. You should ask your GSI to verify it and check you off the list at the end of each lab session before you leave.

If you complete your lab at home before beginning of the session, please show up to your lab session and make sure your GSI verifies that you have completed all the required tasks correctly and checks you off.

Objective of Lab 6

In this prelab you will first a few simple simulation to understand the basics of a capacitor. Based on your understanding of capacitor, you will be guided to analyze some basic RC filter circuit. Then you will revisit the microphone front end circuit with a focus on its frequency response.

Prelab assignment (to be completed before your lab session) o Simulations: S.1, S.2, S.3 o Questions: Q.1, Q.2, Q.3

Lab assignment (to be completed during your lab session) o Build simple high pass/low pass filter and measure frequency response o Measure frequency response of microphone front end

Outline

1. Prelab Assignment 2. Lab guidance

1. Prelab Assignment

You should complete your prelab assignment and submit it to the Gradescope account of your

lab section before beginning of your lab session.

Prelab assignment consists of two parts. In the first part you will do a few simulations using

Multisim and in the second part you should answer a few intuitive questions about those

simulations.

1.1) Fundamentals of Capacitor

Understanding the current and voltage relationship of a capacitor is important. In this

part, you will simulation two simple circuits as below. In figure 1.(a), a capacitor is driven

by a bipolar pulse current source. In figure 1.(b), a capacitor is driven by a sinusoidal

current source. Both capacitors are 0.1uF. You could watch module 4.1, 4.2 and 4. 4 to

revisit some basics of capacitor.

(a) (b) Figure 1 a) Capacitor driven by a sinusoidal current b) Capacitor driven by a bipolar pulse current

The bipolar current source could be under the directory:

Place/Components/Signal_Current_Sources/Bipolar_Current;

Set the “Positive pulse current” to 100uA; “Negative pulse current” to 100A;

“Frequency” to 1k Hz; “Duty cycle” to 100%.

The sinusoidal current source could be found under the directory:

Place/Components/Signal_Current_Sources/AC_Current;

Set the “Current (Pk)” to 100uA; “Frequency” to 1k Hz.

In this simulation, you will perform transient analysis, which means you plot signal in

time domain. To do that, go to Simulate -> Analyses -> Transient Analysis

In the “Analysis parameter” tab: change “End time” to 0.002s.

In the “Output” tab: Add the variables you want to check from left column to right

column. Then hit “Simulate” on the bottom.

S.1: Perform transient analysis for both circuits. Plot VC1 and VC2 and measure the

maximum and minimum voltage of VC1 and VC2. Now change the frequency to 2 KHz,

Plot VC1 and VC2 again and measure the maximum and minimum voltage of both.

RC filter

In this part, you simulate two simple RC filters as shown in figure 2. (a) and (b). You

could watch Module 4.5 and 4.6 to revisit the basic concepts of RC circuits

(a) (b)

Figure 2

In the simulation, you will perform AC analysis to look at the frequency response of the

RC filter. In AC simulation, we usually use sinusoidal input source, then the simulator will

sweep the frequency of AC source and measure the magnitude of any response (which

is usually also sinusoidal). To do that, go to Simulate -> Analyses -> AC Analysis.

In the “Frequency parameters” tab: change “Number of points per decade” to 100;

In the “Output” tab: use “Add expression” to add “Vo1/Vi1” and “Vo2/Vi2” separately;

Then hit “Simulate” to view the result

Set the input voltage source in both cases to be 1Vpk and 1 kHz.

S.2: Plot the frequency response of Vo1/Vi1 and Vo2/Vi2 respectively. Identify which is

the low pass filter and which is the high pass filter. Please measure Vo1/Vi1 and Vo2/Vi2

at 10Hz, 1 kHz, and 100 kHz. Now please perform transient analysis, plot Vo1, Vo2.

Measure the magnitude of Vo1 and Vo2. Calculate Vo1/Vi1 and Vo2/Vi2 from transient

simulation. Dose transient simulation at 1 kHz match AC simulation?

1.2) Revisit Microphone Front End

Now with the knowledge of RC filter, let’s look at the microphone front-end circuit again

as shown in figure 3, with the focus on its frequency response. You could watch module

4.8 to help you analysis this circuit.

Figure 3 Microphone Front End Circuit

Different from previous lab, right now we are interested in the frequency response of

this circuit.

S.3: Plot the frequency response of Vout/Vin. What type of filter this circuit is? Please

measure the gain Vout/Vin at 800Hz. Please measure the frequency when the gain drop

to 1

√2 of the gain at 800 Hz.

1.3) Questions In this part you will answer a few questions about the simulation that you did in the

previous part, hence, it is important for you to finish all three simulations before starting

with the questions.

Q.1: In the first simulation (S.1) explain intuitively how the amplitude (= max − min) of

the voltage changes with respect to the frequency. (Hint: pay attention to the

integration relationship between voltage and current for a capacitor)

Q.2: Calculate the voltage gain of both RC filters in figure 2 (a) and (b) at 1 kHz (Hint: you

may need refer to phasor models introduced in Module 4.7)

Q.3: In the third simulation, what’s gain (Vout/Vin) when frequency is infinitely high?

What type of filter this circuit is? Explain why we need it?

2. Lab Guideline

(a) Build the simple RC filter circuits as shown in figure 2 (a) and (b). Measure

the gain at 100 Hz, 1 kHz, 10 kHz and 100 kHz.

(b) Measure the gain of the microphone front-end circuit at 800Hz. Decrease the

frequency and find the point when the gain drop to 1

√2 of the gain at 800 Hz.