Designing Bio-potential Amplifiers with Transistors and Opamps

What are Bio-potentials and how are they measured?

You should submit your answers type-written on paper, along with printed screens (screenshots) of the Multisim circuit and screen snippets of the meter and oscilloscope

readings obtained during the simulations.

The circuit Electrophysiological Bio-potentials are generated from volume conduction of currents made by collections of electrogenic cells. For example, the electro encephalogram (EEG) is the electrical potential induced from collective activities of a large number of neurons in the brain. Various other bio-potentials (EOG, ERG, EGG, etc.) also result from collective effects of large numbers of electrogenic cells or ionic distribution.

Almost all biopotentials, including EEG, ECG, and EMG, range over very low frequency, typically less than 1 kHz and are very low in amplitude, ranging tens to hundreds of μV when measured by an electrode.

We are aiming to record bio-potentials from a small animal, for this purpose we require designing a simple transistor-based amplifier to read such signals.

Answer the following questions:

(i) What is the name of the type of amplifier chosen for this application?

(ii) What components in the circuit determine the gain of the amplifier and how this can be increased?

(iii) Given that the signal voltage needs to be amplified for both the positive and negative parts of the signal justify your biasing considerations.

(iv) Given the results of your resistor calculations, justify the obtained resistor and capacitor values.

(v) Table 1 is an extract from the datasheets for the 2N4401 transistor. Identify the two most important ratings to be considered in the design of this circuit and explain why

Simulation exercise

(i) Construct your choice of transistor-based amplifier in Multisim and include a print-screen of the circuit with your submission.

(ii) Measure the following parameters: Collector emitter voltage and Collector current. Does this agree with your design (calculations vs. simulations) why or why not?

(iii) Input a sinusoidal signal of frequency = 400 Hz and an amplitude of 10mVp and validate the gain of your design by using an oscilloscope to measure the input and output signals. Please provide a screenshot of the oscilloscope showing the voltage scales and comment on why or why not the circuit required gain has been met.

(iv) Validate that your design is working for the specified cut-off frequency by varying the input frequency while recording the amplifiers gain, you can do this by choosing 10 frequency values and plotting these manually or alternatively you can use the bode plotter.

Designing a Transistor-based Amplifier for Bio-potentials

Present this in a table, plot and discuss your results.

5. The following exercise aims to develop a Bio-potential amplifier using opamps As before the essential function of a bio-potential amplifier is to take a weak electric signal of biological origin and increase its amplitude so that it can be further processed, recorded, or displayed. Usually such amplifiers are in the form of voltage amplifiers, because they are capable of increasing the voltage level of a signal.

(i) Identify the two different amplifier configurations forming this circuit, determine the gain for each of these two configurations independently and discus what is the functionality of each amplifier configuration.

(ii) Derive an expression for the total gain of the circuit and calculate the total gain using the given resistor values.

(iii) Draw the circuit using multisim and provide a screen shot of it. Use LM741 opamp and a ±15 V power supply.

(iv) Using the previous total gain expression determine what is the function of Rg. Hint: you can replace RG for a variable resistor.

(vi) Set the biopotential amplifier gain to 80, then validate that the overall gain obtained in your calculations corresponds to the simulated gain using the following signals as inputs:

• Input AC signal V1 parameters: frequency = 10 Hz Amplitude 10mVpk phase = 0°

• Input AC signal V2 parameters: frequency = 10 Hz Amplitude 10mVpk Phase = 180° Present the input and output signals using the oscilloscope and discuss the differences found between your calculations and simulation considering the opamp ideal and non-ideal behavior.

Please use the Course work template available in canvas.

Word length – there is no minimum/maximum as long as all questions are answered.

The report must include the following:

Describing briefly, but clearly, the objectives of the project and the basic techniques involved; states the principal results and conclusions; omits all inessential detail; stands alone i.e. must make sense without reading any other part except the title; is on a single page.

Number the major sections and sub-sections.

Circuit design – including the design calculations.

Circuit simulations – showing and explaining the Multisim simulations performed; including screenshots of the Multisim circuits and instrument readings.

Choice of components – explaining and justifying the selection of components used in your prototype.

Conclusions – a brief summary of how far the objectives of the Group project have been achieved and why; considers what could be done in addition, or differently.

Report presentation

• Use a clear font of at least 11 point;

• number all pages;

• number all equations;

• number all figures and tables and include figure and table captions;

• refer to all equations, figures and tables in the text;

• only include images which are essential to understand the information you are providing;

• make sure that all images, figures and diagrams are clear and easy to view;

• write in accurate, grammatical, technical language as explained in lectures;

• use the Numerical referencing system;

• save your document in one of the supported file formats for e-submission;

• name the document file “H7076 Report your candidate number”.