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Digital and Interfacing Systems Lab 9 The Differential Amplifier CENG 215 Name …………………….…. Objective: To investigate the properties of a Differential Amplifier using an Op-Amp. Equipment: 1. DVM, Scope, Function Generator, Parts kit Introduction: The differential amplifier (Diff Amp) is useful for increasing signal to noise ratios. This circuit amplifies a difference signal connected to the 2 inputs. The Diff Amp, for example, is used to amplify the output from a Wheatstone Bridge circuit. It will not amplify two signals that are the same (common mode signals) even if those signals are very large. The result is that desired signals are amplified and noise voltages are actually attenuated, giving a very desirable signal to noise ratio. This characteristic of a Diff Amp is expressed as the Common Mode Rejection Ratio (CMRR) and is usually expressed in dB. Procedure: Part 1- Differential Mode Gain 1. Set up the circuit shown at right. Use 1% precision resistors available from your instructor. 2. Calculate the theoretical differential mode voltage gain AV,DM. AV, DM = ........................ 3. Connect the left (open) end of R1 (VIN,1) to ground. Set the function generator for about 0.5VP-P sine wave at 1 kHz with no DC offset, and connect it to R2 at VIN,2 4. Using AC coupling on the scope, connect CH 1 to VIN,2 and CH 2 to VOUT. Measure the VP-P values of both waveforms and using these values record the measured differential mode gain AV,DM VIN,P-P = …………… VOUT,P-P = ……….….. AV,DM = ........................ Does this value agree with step 2 above ….…... 5. Draw sketches of VIN,2 and VOUT showing exact voltage levels and phase relationships. Are VIN,2 and VOUT in or out of phase?……………………. Should they be in or out of phase and why? ………………………. Procedure: Part 2 - Common Mode Voltage Gain of the Amplifier 6. To measure the common mode voltage gain AV,CM, of the amplifier disconnect the left end of R1 from ground and connect VIN,1 and VIN,2 together. Connect the function generator to this common mode input now referred to as VIN. The function generator signal represents a common mode noise voltage. 7. Increase the amplitude of VIN to 6 to 8 volts. Measure the VPK PK values of both input and output signals. Turn the CH 2 scale to a very small setting in order to view and measure VOUT. Determine the measured Common Mode Gain AV,CM VIN,P-P = …………… VOUT,P-P = ……….….. AV,CM = ……………… 8. Draw sketches of VIN and VOUT for Step 7 showing exact voltage levels and phase relationships. 9. The two gain readings of AV,DM and AV,CM can now be used to calculate the Common Mode Rejection Ratio (CMRR) of the amplifier. This ratio can be expressed as a linear gain ratio or, more commonly, as a dB ratio. First, the linear ratio is: CMRR (Linear) = AV,DM / AV,CM = …………..………….. This means the desired signal will be amplified by that factor compared to the noise signals. The ratio expressed in dB is: CMRRdB = 20 log CMRR (Linear) = ……………………….. dB This dB reading means the desired signal voltage level is that many dB above the noise voltage level. Remembering that each 3dB drop reduces the voltage to approximately 0.707 of what it was (or 1/2 power) it can be seen that this gives a very good signal to noise ratio. Procedure: Part 3 - Unbalanced Circuit 10. To achieve the best CMRR the differential op-amp relies on the value of RF being identical to R3, and R1 being identical to R2. Any deviation from this rule will reduce the CMRR and increase the undesired voltages at the output. To illustrate this point we will deliberately increase the value of R1 by 10% making it 1kΩ more than R2. This means the non-inverting I/P will have a larger (10%) signal than the inverting I/P. 11. Add an additional 1 kΩ resistor in series with R1 to increase its value 11 kΩ. Reconfigure the amplifier as you did in step 3 above so that you can measure the differential mode gain once more. 12. Repeat steps 3 to 9 above to measure AV,DM and AV,CM again. Record the results with this 10% R1/R2 ratio imbalance: What is the new calculated value of AV,DM? ………………………. Measurements VIN,P-P = ….… VOUT,P- P = …..….. AV,DM = .................... VIN.P -P = ….… VOUT,P- P = …..….. AV,CM = ………….. Calculate the new linear CMRR. CMRR (Linear) = AV,DM / AV,CM = …………..……….. Calculate the new CMRR in dB. CMRRdB = 20 log CMRR (Linear) = dB 13. Calculate the difference in CMRR caused by the 10% variance in R1 = dB Difference CMRRdB = dB Original CMRRdB New CMRRdB = dB Question The VIN voltage in Common Mode operation can be thought of as representing a noise voltage. What happens to the magnitude of the noise voltage at the output of the Differential amplifier when the CMRRdB decreases? Signed ………………………………………… Date……………………….