ENGR337 Lab 2020 Spring
Lab 3
Name: Kurt Emslie
Email: kdemslie@fortlewis.edu

1. Filters and Amplifiers

2. Introduction
The purpose of this lab is to construct circuits that are able to amplify a small signal, 500 mVpp, 1 Hz, and 1 V DC offset to the 0-5 V range. The 0-5 V range is an optimal range because we are able to assume the power supply for the system is 5 V. The Dynamic range of the Analog-Digital Converter (ADC) is 0-5 V. The is the signal we are matching by amplifying the small input signal.

3. Materials and Methods
The first step in this lab was to produce a sine wave signal with  500mVpp, 1Hz, and 1 V DC off set with the signal generator. This signal was then observed on the oscilloscope in both DC and AC modes. The next step was to build a circuit was built on a bread board and add a 3.3 V DC voltage from the 3.3-5V DC module to the sine wave that we were all ready generating. After this noise was added to the signal, the next step was to filter it out. The original circuit was modified by adding a Low-Pass filter to remove the 60 Hz noise added to the circuit. The next step was to add a High-Pass Filter to the circuit to remove the DC offset from the signal. The cutoff frequency was desired to be 0.5 Hz and the value of the capacitor was 4.7 uF. The next step was to add gain to the signal. This was done by using a 741 Op Amp and adding 2 V DC offset as the reference voltage. This was provided by a 2 V zener diode and a 680 ohm resistor. The next step was to improve the signal gain by increasing the rail to rail voltage seen by the 741 OP Amp and increasing the reference voltage to 5 V. This does provided the desired signal gain to the circuit, but with an in creased voltage. The next step is to do try and produce the replicate the signal increase with only 5 V rail to rail voltage being delivered to the Op Amp. In the next step the 741 Op Amp was replaced with a 1NA 128 PA Instrument Amp.
4. Results

Figure 1: The 500 mVpp, 1 Hz, 1 V DC sine wave signal being observed on  the oscilloscope in DC mode.


Figure 2: The 500 mVpp, 1 Hz, 1 V DC sine wave signal being observed on  the oscilloscope in AC mode.


Figure 4: The resulting noisy signal after adding the 3.3 volts  to the  sine wave signal.


Figure 5: The signal after the 60 Hz noise was removed.


Figure 6: The signal with the DC off set removed.


Figure 7: The circuit with the 741 Op Amp, not operating reel to reel cutting off the bottom of the output.


Figure 8: The circuit with the two power sources in series delivering the 10 V to the rails of the op amp.


Figure 9: The signal gain in the circuit by increasing the rail to rail voltage of the Op Amp and the increasing the reference voltage.


Figure 10: The signal gain in the circuit with the 1NA 128 PA Instrument Amp.


Figure 11: The circuit that was built with the 3.3 V DC current added to the signal that was being generated.


Figure 12: The had calculations for the resistor values used in the High and Low Pass Filters.

5. Discussion
The first steps in this lab demonstrate that the oscilloscope has the ability to remove a DC offset on a signal. The next set of steps of the lab were to demonstrate that we could generate a noisy signal and effectively make filters to filter out the noise. We also demonstrated that the DC offset of the circuit could be removed as well. To build the filters we had to calculate by hand the values of the resistors to be used to generate the desired cutoff frequencies. Passive filters are not the most effective way to remove noise because of the attenuation they can create.
The problem of the attenuation can be remedied by using an Op Amp. However, the quality of the Op Amp is another factor when trying to correct the attenuation and demonstrated by the differing results between the 741 and NA 128 PA Instrument Amp. The NA 128 PA Instrument Amp provided the cleanest signal with the lower input voltage to the rails of the Op Amp.