# Biomedical Engineering lab report Essay Example

Table of Content

Biomedical Instrumentation Laboratories

Lab 5 – The Op-Amp

Lecturer:

## ABSTRACT

In this lab, an operational amplifier was connected in order to investigate the operational amplifier and how it could be used to as an amplification of the signal conditioning in biomedical instrumentation systems. Circuits for Non-Inverting Summing Op-Amp was both simulated and connected physically in order to determine the difference between the theoretical and the experimental values. Tests were done to determine the outputs for both the simulated and the practical circuits. The percentage errors for was determined using the difference between the simulated output and practical output. The results obtained in this experiment are summarized in the table 1 below.

Table 1: Summary of the results

 Input V1 (V) Input V2 (V) Simulated Output Vout (V) Practical Output Vout (V) 0 0

ABSTRACT 2

4INTRODUCTION

4BACKGROUND THEORY

5Inverting Amplifier

6Non-inverting amplifier

7DESIGN / THEORETICAL ANALYSIS

7Non-inverting summing configuration

8Inverting Summing Op-Amp

9PROCEDURE

9Simulation

11Practical part.

12RESULTS AND DISCUSSION

12Simulated op amp

13Physical and simulated circuit

14CONCLUSION

REFERENCES 15

## INTRODUCTION

An operational amplifier (op amp) is an electronic circuit that is used to control the current and voltage in the circuit. It is widely used in electrical circuits such as accelerometers, thermistors, microphones, and antennas. Op amp is used in biomedical instrumentation systems to amplify small signals (measurand) emanating from the body. An Op amp has five main terminals that include negative and positive power supplies (Vs- and Vs+), output (Vout), non-inverting input (V+) and inverting input (V-) like as shown in the figure 1 below.

Figure 1: Schematic diagram for an op amp [1]

The terminal with the label ‘+’ is non-inverting input terminal, the terminal with the label ‘-‘is inverting input terminal, and the output terminal is on the right. This lab is to explore some more advanced op-amp circuits

The objective of this laboratory was explore an operational amplifier and how it can used as an inverting amplifier or non-inverting amplifier for signal conditioning in biomedical instrumentation systems. Both simulation and experimental work were made on inverting and Non-Inverting Summing Op-Amp. An op amp together with resistors in a circuit can perform useful functions such as scaling, summing and subtracting. When used with capacitors and inductors, it produce a differentiating and integrating circuits.

## BACKGROUND THEORY

Internally, an op amp is sophisticated circuit, but it is easy understand the external operation without referring to the internal circuit. A functional diagram for an op amp is shown in the figure 2 below.

Figure 2: A functional circuit for an Op Amp

R1 is very large, hence very small current flow into the op amp. When the value of R1 is infinite and the input current is approaching zero, the op amp is ideal. For an ideal op amp, the current that flows into the inputs is zero (Natarajan, 2016). When the value of one of the inputs change, the other input also change to match. This produce linear operation and ensures that the output voltage is constant. The negative feedback maintains the balance through the feedback loop. The feedback loop brings feedback to the inverting input terminal, thus the output voltage is driven to the inverting input, which brings an equilibrium in the circuit [1].

For non-ideal operation amplifier there is no input voltage difference. This results in the operational amplifier to leave the linear region. However, the negative feedback in the circuit reduce the difference between the inputs, and in most cases we assume linear operation [1, 2].

## Inverting Amplifier

In this configuration, the non-inverting input is connectd to the ground and the input volatge is connected to the non-inverting input, which causes negative gain. The feedback loop sends the output volatge to the inverting input through Rf. The circuit for this configuration is shown in the figure 3 below.

Figure 3: Inverting Amplifier configuration

Using the the KIrchoff’s current law, the current going to the node is equal to the current going out.

At node A,

Vout/Vin = Gain=-(Rf*I)/(RI*I) = -Rf/R1

The –ve sign shows that the gain is inverted.

## Non-inverting amplifier

In this configuration, non-inverting input is connected to the voltage source and to the ground, a resistor is connected between the inverting input to the output, causing a positive gain. The output voltage is send back to the inverting input through Rf. The feedback loop send the output volatge to non-inverting terminal to bring the circuit to equlibrium.

Figure 4: Non-inverting terminal configuration

The positive gain shows that the gain is non-inverted.

Given that VA = Vin

Adding Vout / R2 to both sides and re-arranging gives:

Dividing both sides by Vin gives and multiplying through by R2 gives:

Finally gain, Acl is calculated as follows

## Non-inverting summing configuration

A non-inverting summing configuration such as shown in the figure below, has two input voltages connected to the inverting input and non-inverting input terminal. The voltage sources are connected to the ground and to the output through resistors. Therefore, the gain will be positive.

Figure 5: Non-inverting summing configuration

The output increase if Vin increase.The feedback loop sends the output voltage into the inverting input and into non-inverting input. In other words, the feeback will cause the difference between the positive and negative inputs to be zero. By applying node analysis, we get:

It is then simplified to give

Since VA =VB, then

## Inverting Summing Op-Amp

In inverting Summing Op-Amp the two input voltages are connected to the inverting input through resistors as shown in the figure below.

Figure 6: the Inverting Summing Op-Amp Configuration

The circuit is analyzed by replacing the input network on the inverting input by the Thévenin equivalent circuit. The figure below shows how Thévenin equivalent is applied in the circuit through nodes b and a.

Figure 7: The Thévenin equivalent of the Input Network

The value of VTH is found using superposition theorem. If V2 is short circuited, the voltage difference between a and b, VTH’ is obtained using the potential divider equation. VTH” is also found by short circuiting V1. Thus:

It is simplified to:

The Thévenin resistance RTH, is measured between a and b with all sources de-activated. Thus:

## PROCEDURE

This practical is divided into two parts. The first part involves simulation in order to explore theoretical aspects of the op amp. The second part is the practical part where the circuit simulated was physically assembled and tested. Simulation was divided into three tasks that is investigating the Fourier series by use of AC sine wave voltage sources together with an inverting op-amp to produce square wave; the use of non-inverting op-amp set up to explore different aspects of op-amp; and finally, the simulation of the non-inverting summing op-amp set up to put up a design that will be tested in a practical section.

## Simulation

1. Creating a square wave using AC voltage sources and an op-amp

Simscape was used to construct the circuit as shown in the figure below.

Figure 8: Square Wave Generation Using an Inverting Summing Op-Amp

After being constructed the following steps was followed so as to produce the sine wave.

• One of the voltage sources was set to a frequency of 10Hz and an amplitude of 1V.

• Then the first odd harmonic was added through setting the frequency and amplitude of the subsequent source of voltage to the required values.

• That process was repeated for other voltage sources

• The amplitudes of every voltage source was then set back to the 1V and the results was noted.

• The circuit image and those of the signals was saved for the log book.

1. Exploration of the Ideal Op-Amp versus Practical Op-Amp

For this simulation the finite-gain op-amp was used, the op-amp was available at Simscape, the circuit was constructed as shown below.

Figure 9: Circuit for Investigation of the Ideal versus practical Op-Amp

The following steps was then followed

• The resistors values was set to 1kΩ.

• The op-amp was double clicked and its gain was then set to 1e99 (i.e. 1×1099).

• The input voltage was set to 5V and the results of the voltages that was shown in the displays that was measuring Vout and V+ — V- was then recorded.

• The op-amp was double clicked again and the gain was again set to 10,000.

• The voltages displays was again recorded.

1. Design of a Non-Inverting Summing Op-Amp for the Practical Session.

Simscape was used to construct the Non-Inverting Summing Op-Amp. By experimenting with the use of resistor values provided in the simulation and following the theory provided, we tried to find the values that will make the value of Vout to be equal to 3(V1+ V2). It was again experimented with resistor values of 1kΩ or several resistors of 1kΩ, e.g. 2kΩ, 3kΩ or 4kΩ.

## Practical part.

• The circuit that is simulated was physically assembled and tested. The op-amp used in the experiment was OP275. Below is a partially Completed Layout Diagram for the Non-Inverting Summing Op-Amp Circuit

Figure 10: Partially Completed Layout Diagram for the Non-Inverting Summing Op-Amp Circuit

• The partially completed circuit in the breadboard was completed by positioning the 4 resistors in the right positions in order to implement the wiring diagram. When the wiring is over it was then checked with the demonstrator.

• We started with the 3 black connectors by plugging OP275 into the middle of left hand block of the breadboard.

• Then the positive supply pin which pin 8 was connected to the topmost track of the breadboard shown by red wire, while the negative supply pin that is pin 4 was connected to the lowest track indicated with blue wire.

• To insert resistors and decoupling capacitors, the wiring diagrams in fig 1.6 was followed, short insulated wires were used at where it was seen necessary.

• The breadboard was then connected to the base plate terminal using a similar color scheme. We used the white terminals for output voltage and yellow terminals for input voltages.

• The power supply used in the experiment was Rohde & Schwarz HMC 8043, before connecting the power to the breadboard the power input and output was set first, the voltage was set to 12V while the current was set to 0.1A

• The power was connected to the breadboard using black, red, and blue colored wires whereby red = +12V, black = 0V, blue = -12V.

• The output was measured and recorded on a table.

## Simulated op amp

We used a Simscape to construct a model for the circuit and used it to evaluate gain with difference inputs. The model for Non-Inverting Summing cifiguration made is shown below.

Figure 11: Simscape Model of Inverting Configuration

Figure12: Simscape Simulation of Oscilloscope Reading

For the practical experiment, the simulated circuit was assebled physically and tested. OP275 op amp was used in this circuit.

## Physical and simulated circuit

The results from the simulation and the results obtained from the practical part were combined and the following table 2 were made.

Table 2: The results obtained and the percentage errors

 Input V1 (V) Input V2 (V) Simulated Output Vout (V) Practical Output Vout (V) 0 0

From the results obtained from the inverting configuration for the simulation and for the practical, it can be seen that they are generally related to each other. The percentage errors for some combinations are small and were within 2% of the theoretical value. This combinations include 2V and 1V, -4V and 6V, and -6V and 6V. The highest percentage error is 47.15%. The possible sources of errors are human and machine errors. The oscilloscope produced distorted gain to as much as 0.3%, and the accuracy of the resistors were upto 5%. In addition, the V1 and V2 also contributed to the errors. Source resistance together with errors in the resistors may have combined to produce the error.

Based on the facts above, OP275 is considered to be an ideal op amp to some degree of accuracy. If the error remains the same after using a more accurate equipment, then the source of error may not be attributed to the equipment or the procedure. The source of error will then be considered to be the non-ideal op amp.

## CONCLUSION

In this lab an operation of an op amp was explored. The results for Non-Inverting Summing Op-Amp for the simulation and for the practical part were tabulated. The results obtained in this experiment are summarized in the table below

Table 3: Summary of Gains

 Input V1 (V) Input V2 (V) Simulated Output Vout (V) Practical Output Vout (V) 0 0

The practical results generally similar to the simulated values. The difference is due to human and machine based errors. Errors can be eliminated by use of more accurate resistors, more accurate oscilloscope and steady power supply.

## REFERENCES

1. Natarajan, R. A. (2016). Biomedical instrumentation and measurements, Delhi: PHI Learning Private Limited

2. Huijsing, J. H. (2016). Operational amplifiers: Theory and design, Switzerland: Springer.

3. Rashid M, H., (2016). Microelectronic Circuits: Analysis and Design, Cengage Learning