Lab 3 Filters and Amplifiers

1. Introduction

The purpose of this lab is very simple: ampilfy a small signal (500 mVpp, 1 Hz, 1 V DC offset) to the 0-5 V range.

We prefer the 0-5 V range is beacuse we assume the power supply for the entire system is 5 V. The dynamic range of the Analog-Digital Converter (ADC) is 0-5V.

Task 1: Generate the signal shown in Fig. 1 from a function generator and display it on the scope. Make a screenshot of the view for your report.

Fig. 1

Task 2: Use the 'AC' option in the channel's menu to remove the DC offset in the view. Make a screenshot of the view for your report.

Fig. 2

Task 3: Add a 3.3 V DC voltage from the 3.3V-5V DC module to the sinewave you used above. Show the DC coupling (not shown in Fig. 4) and the AC coupling of the final signal separately. Make a screenshot of the view for your report.

The 3.3-5V DC modlue (ignore the wire connections in the following figure):

Fig. 3

Fig. 4

You will notice that the sinvewave looks a little more noisy than the one doesn't have anything added to the sinewave. The 60 Hz noise comes from the outlet. The 3.3-5V module is powered up by the outlet and this noisy is being added to the circuit in series.

Task 4: Add a Low-Pass Filter to remove the 60 Hz noise. Display the input and the output at the same time and identify the differences of the input and the output signals. Make a screenshot of the view for your report.

If fc is designed to be 3.4 Hz, C=458 nF, what is the appropriate R?

Fig. 5

Task 5: Add a High-Pass Filter at the end to remove the DC offset from the signal. Display the input and the output at the same time and identify the differences of the input and the output signals. Make a screenshot of the view for your report.

If fc is designed to be 0.5 Hz, C=4.7 uF, what is the appropriate R?

Fig. 6

However, the signal is being attenuated. We need some gain to be applied to the signal so we can utilize the whole dynamic range of the 0-5 V ADC.

Task 6:  Repeat the results in Fig. 9, Fig. 11, and Fig. 13. Make screenshots of the results for your report.

Let's add some gain to the signal. Use the 741 Op Amp (low quality and low cost) as the amplifier. Add a 2 V DC offset to the signal from a 'reference voltage'. The reference voltage is provided by a 680 ohm resistor and a 2 V zener diode (1N4370 Zener). If you do not have it, you can use some alternative ones, such as the 3 V ones.
The typical V-I curse for the Zener diodes looks like the following figure. An Reversed Biased zener diode can stablize the voltage at a specific level. The following one shows the voltage is stablized at 3.4 V for reverse biasing.

Fig. 7

Use a 1N4370 zener diode to provide a 2 V reference voltage. You may use an alternative one to stablize it at 3 V too. Use the same R's and C's in your passive filter for the following circuit. Use a 741 Op Amp as the amplifier to provide the gain.

Fig. 8

However, the bottom of the output signal is cut off.

Fig. 9

The 741 is a low-cost and fair quality Op Amp, it is not a rail-to-rail Op Amp and it is amplifying the DC tiny difference between the two inputs and causing a large DC offset at the output. We can fix it by inceasing the rail-to-rail power supply to 10 V and lift the signal to 5 V as the DC reference (offset) voltage.

Fig. 10

Now it works for the same signal:

Fig. 11

However, you have to provide an extra 10 V power in your system to amplify/filter the signal. If we only have 5 V available in the system, we have to use a higher-end and better quality Op Amp - the instrument amplifier.

Fig. 12

The output is shown below. Keep in mind, change the output coupling mode to DC to verify that your signal is being amplified within the 0-5 V range without any distortion.

Fig. 13

The final circuits look like this:

Fig. 14

The end of the lab