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Grading, out of 10

0 : not solved or major mistakes, such as missing units, or plot labels, or non-realistic values.
5 : result are realistic
6 to 8: result are realistic, but explanation is missing parts, unnecessarily long, or partly incorrect.
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10 : result is correct and explanation is excellent, short and including all the information.

1. Background

The aim of this lab is to understand how differential and instrumentation amplifiers behave for a differential voltage and common mode voltage. First, you will analyse this behaviour in the theoretical domain. Consecutively, you will build the circuit and analyse the same behaviour in a real implementation.

You are using theTL074 from Texas Instruments, powered at ±15V . You can ﬁnd the datasheet here. The final aim of the Instrumentation amplifier is to amplify the surface skin biopotential produced by the cardiac activity.Therefore,do not remove it from the protoboardwhen this lab session finishes.

Question 1a. What is the typical amplitude peak to peak of a ECG signal on the skin surface? Include the reference to the source where you found this information.

Question 1b. What is the maximum amplitude peak to peak that we can produce in our amplifier if using the TL074 powered at ±15V ?Explain your answer?

2. Theoretical analysis

2.1 Differential amplifier

Following is the circuit of the differential ampliﬁer that you will build. We chose the values for the resistors as follow: R1 = 3.9kΩ, R2 = 15kΩ, R3 = 3.9kΩ, R4 = 15kΩ;

Fig 1. Differential amplifier.

Question 2.1a. Circuit in Fig 1 expresses the output depending on Vin1 and Vin2. Redraw the circuit so that the inputs are expressed as differential voltage (Vd) and common mode voltage (Vcm).

Calculating the differential gain of the differential amplifier.

Question 2.1b. Based on the circuit in 2.1a, use circuit analysis to derive the formula for the differential gain (Gd). Use this formula to calculate the differential gain (Gd) for this circuit.

Calculating the common mode gain of the differential amplifier.

Question 2.1c. Based on the circuit in 2.1a, use circuit analysis to derive the formula for the common mode gain (Gcm). Use this formula to calculate thecommon mode gain (Gcm) for this circuit, if the resistors are ideal.

Question 2.1d. Based on formula in 2.1c, calculate the worstpossible (the highest) common mode gain (Gcm), if we use resistors with ±5% error?

Question 2.1e. What is the CMRR measured in decibels? (useGcmfrom question 2.1.d)

Question 2.1f. Propose a modification in the circuit to make the common mode gain zero Gcm=0.

Question 2.1g. What is the input impedance of each input in the differential ampliﬁer?Explain why is that value?

2.2Instrumentation amplifier

You are going to use the circuit in the previous exercises to design an instrumentation amplifier by adding the 2 followers at the input configured as a first stage amplifier. We chose the values for the resistors as follow: R1 = 3.9kΩ, R2 = 15kΩ, R3 = 3.9kΩ, R4 = 15kΩ; R5=3.9kΩ ; R6=3.9kΩ; Rg=120Ω.

Fig 2. Instrumentation amplifier.

Question 2.2a. What is the differential gain (Gd) for this circuit? (don’t need to do circuit analysis, you can take the formula and indicate the source)

Question 2.2b. What is the input impedance of the instrumentation ampliﬁer?Explain why is that value?

3. Real circuit analysis

3.1 Differential amplifier

Build the differential amplifier circuit from Fig.1 in the breadboard:
Feed the OpAmp with a supply of Vcc+=+15V and Vcc-=-15V

3.1.1 Measuring the differential gain of the differential amplifier.

Question 3.1.1a. Explain how to connect lab instruments and circuit in order to measure the differential gain. Sketch a circuit showing this configuration. Justify the values selected for Vd and Vcm (amplitude and frequency). Tip: configuration is as the one used in 2.1b.

Take screenshots/pictures of the input and output signals for your report. Pictures MUST show clearly the amplitude values. (tip: for example you can set input in channel 1 and output in channel 2 of the oscilloscope, and set the measurement toll to show Vpp or amplitude of channel 1 and 2).

Question 3.1.1b. Show here the picture. Use the signals amplitude from the pictures to calculate the real differential gain.

Question 3.1.1c. Compare the measured and theoretical differential gain in questions 2.1b and 3.1.1b. Calculate the error in percentage. Explain the source of possible discrepancies.

Before continue show me how you have done this.

3.1.2 Measure the “maximum output voltage swing” of the OpAmp.

Still on the previous configuration, increase the amplitude of the input until it clearly saturates the output of the OpAmp (clearly cuts the peaks of the sine signal).
Take screenshots/pictures of the input and output signals for your report. Pictures MUST show clearly the amplitude values. (tip: for example you can set input in channel 1 and output in channel 2 of the oscilloscope, and set the measurement toll to show Vpp or amplitude of channel 1 and 2).

Question 3.1.2a. . Show here the picture. What is themeasured “maximum output voltage swing”? Calculate the error in percentage with the theoretical value from 1b.

3.1.3Measuring the common mode gain of the differential amplifier.

Question 3.1.3a. Explain how to connect lab instruments and circuit in order to measure the common mode gain. Sketch a circuit showing this configuration. Justify the values selected for Vd and Vcm (amplitude and frequency). Tip: configuration is as the one used in 2.1b.

Take screenshots/pictures of the input and output signals for your report. Pictures MUST show clearly the amplitude values. (tip: for example you can set input in channel 1 and output in channel 2 of the oscilloscope, and set the measurement toll to show Vpp or amplitude of channel 1 and 2).

Question 3.1.3b. Show here the pictures. Use the signals amplitude from the pictures to calculate the real common mode gain.

Question 3.1.3c. Compare the measured and theoretical common mode gain in questions 2.1d and 3.1.3b. Calculate the error in percentage. Explain why error is so large.

3.1.4 Cancelling the common-mode gain of the differential amplifier.

As seen in question 2.1.f, a way to increase the CMMR is to cancel out the mismatch of the non-ideal resistors by trimming the value of one of them. For example with a potentiometer in resistor R4.

Fig 3. Differential amplifier with CMRR correction trimmer.

Replace R4 for the new resistor+trimmer.
On the configuration used in 3.1.3a, turn the potentiometer to reduce as much as possible the common mode gain.
Take screenshots/pictures of the input and output signals for your report. Pictures MUST show clearly the amplitude values. (tip: for example you can set input in channel 1 and output in channel 2 of the oscilloscope, and set the measurement toll to show Vpp or amplitude of channel 1 and 2).

Question 3.1.4a. Show here the pictures. Use the signals amplitude from the pictures to calculate the new common mode gain.

Question 3.1.4b. Use the new common mode gain to calculate the CMRR. Compare this to an on-chip commercial instrumentation amplifier such as INA118. Datasheet here. tip: page 7 CMRR changes with frequency and gain, find the right one.

Question 3.1.4c. The INA118 does not use a trimer to reduce the common mode gain. What method does it uses?

3.2Instrumentation amplifier

3.1.4Measure the differential gain of the instrumentation amplifier.

Add the first amplification stage to the differential amplifier, as seen in Fig2.

Question 3.1.4a. Explain how to connect lab instruments and circuit in order to measure the differential gain. Sketch a circuit showing this configuration. Justify the values selected for Vd and Vcm (amplitude and frequency). Tip: configuration is as the one used in 3.1.1a.

Take screenshots/pictures of the input and output signals for your report. Pictures MUST show clearly the amplitude values. (tip: for example you can set input in channel 1 and output in channel 2 of the oscilloscope, and set the measurement toll to show Vpp or amplitude of channel 1 and 2).

Question 3.1.1b. Show here the picture. Use the signals amplitude from the pictures to calculate the real differential gain.

Question 3.1.1c. Compare the measured and theoretical differential gain in questions 2.2b and 3.1.4b. Calculate the error in percentage. Explain the source of possible discrepancies.