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Experiments Instructions Sheet

Characterisation of Op-Amp Circuits

Electrical Circuits and Devices (ELCD)

International Year One in Engineering

1. Introduction

This is software and hardware experiments exercise forms one of your assessed submissions and will be  used to assess your  practical and theoretical  progress  in this  module.  It will also assess your theoretical  and  practical  skills  in  addition  to  your  technical  report  writing  skills.  The  report is weighted 20% of your total grade of this module.

2. Experiment Brief

This  report  is  based  on  the  work  you  do  to  study  the  performance  of  some  typical  op-amp circuits  using   Multisim   software   tool   and/or   advanced   solderless   breadboard.   In   particular, you  are instructed to build and analyse a comparator, follower, non-inverting amplifier, inverting amplifier,   differentiator,    and    integrator    using    software    and    hardware    and comment  on the results.

3. Software Tool

You will need to use the Multisim software tool to complete these experiments. There are some options for you to access this software, check the Technical Requirements section on the Module Information page of the ELCD Canvas site.

4. Procedure

Connect the following circuits:

a. Comparator (Figure 1)

Apply a 1 kHz sine wave signal of about 1 Vp−p  to the noninverting + input, with the inverting − input grounded. Observe this signal on the oscilloscope, triggering on external. Plot the output signal against the input signal. Does it change if you reduce the input voltage to 0.1 V? What is the effect of grounding the + input and connecting the signal to the − input?

Figure 1: Comparator circuit

b. Follower (Figure 2)

Apply the same signal to the input as in Part a . Note that with negative feedback (to the inverting input)  the  op-amp  adjusts  its  output  so  that the  inverting  input  is  at  the  same  voltage  as  the noninverting input. What is the signal at the output? Plot it against the input signal. What is the input impedance of the circuit? What happens with a square wave input?

Figure 2: Follower circuit

c. Non Inverting Amplifier (Figure 3)

The two 1 k resistors divide the signal at the output of the amplifier by a factor of two. In order for the voltages at the two inputs to the amplifier to be equal the circuit must have a gain of two. Apply the same signal as in Part a to the input. What is the signal at the output? Plot the output signal against the input signal. What is the signal at the inverting input? What is the gain of the amplifier? What happens when the input voltage is too large? What happens with a square wave input? How would you change the circuit to make the gain equal to 10?

Figure 3: Non inverting amplifier circuit

d. Inverting Amplifier (Figure 4)

In this circuit the noninverting input is at ground. In order to keep the inverting input near ground the amplifier must "balance" a positive input with a negative output, and vice versa. Measure the voltage at the inverting input. The inverting input is said to be at a "virtual ground." Observe the performance of the amplifier on the oscilloscope. What is the gain of the amplifier? Plot the output signal against the input signal. What happens if you connect the inverting input to a “real” ground? The input impedance of a circuit is the ratio of the input voltage to the input current. With a 1 V input, calculate the current flowing through the 1 k input resistor. What is the input impedance of the circuit? How would you change the circuit to make the gain equal to −10?

Figure 4: Inverting amplifier circuit

e. Differentiator (Figure 5)

This  circuit  differentiates the  input voltage.  Since the  inverting  input  is  at  a virtual ground, the current flowing through the 1 kQ  resistor,  and  hence the  output voltage,  is  proportional to the derivative of the voltage across the 0.2 uF capacitor.

Apply the same signal as in Part a to the input. What is the signal at the output? What is the output with a triangular wave input? Plot the output signal against the input signal.

Figure 5: Differentiator circuit

f. Integrator (Figure 6)

In this circuit the capacitor and the resistor are interchanged and the signal is integrated instead of differentiated. In addition, a 100 kQ resistor has been added to prevent any input offset voltage that might be present from driving the output of the op-amp to one of its extreme limits. Apply the same signal as in Part a to the input. What is the signal at the output? What is the output with a square wave input? Plot the output signal against the input signal.

Figure 6: Integrator circuit

Assignment Submission

The report should be Word processed, on A4 paper size. Your written report must be submitted to the Canvas site of the module by the specified deadline on Canvas.

5. Report Length and Contents

The word limit for the report is 2,000 words (±10%), excluding figure and table captions, equations and reference list. Write the word count on the title page. The report must include:

Title page – with module name, report title, candidate number and word count.

Contents  list  –  with  page  numbers  to  include  introduction,  circuit  types,  testing,  error source, applications, conclusions and reference list.

Results of measurements – with answer to each question, explanations of what you did and why.

Circuits  Characterisation  -  analysis  of  results. You  should  support  your  report  with your circuit diagram and circuit response plots using Multisim software tool and/or practically using advanced solderless breadboard and oscilloscope.

Conclusions – containing no new material but summarising what has been achieved.

Reference list – numbered source details corresponding to the cross-reference numbers in the report.

6. Assessment Criteria

The following criteria apply:

Mark Range     Criteria

80-100            An exceptionally well written and presented report which covers all of the criteria below and is indicative of individual initiative and originality.

70-79              A very well written and presented report, which covers all of the aspects below and also discusses the background and objectives of the experiments and shows good understanding of building, testing and analysing op-amp circuits.

60-69              A well written and presented report which covers all of the aspects below. In addition, the candidate presents the source errors with explanation.

50-59             A complete record of the test undertaken with an evaluation of the outcomes. The account demonstrates that circuits were correctly built and specified according to the experiments instructions sheet, that it could be constructed, that the circuit was tested correctly and that its performance was appropriately analysed.

40-49             A clear record of the test undertaken and an evaluation of the outcomes. The account demonstrates that circuits were built according to the experiments instructions sheet, although there may be some errors in simulating the circuits, that it could be constructed and that the circuit was tested appropriately.

30-39            An adequate record of the test undertaken. The account demonstrates that the circuits were built according to the experiments instructions sheet and that some progress was made towards testing the circuit.

0-29             There will have been no submission or a seriously inadequate report which indicate that the experiment objectives were not addressed.