Biomedical Instrumentation I Chapter 7 Bioelectric Amplifiers from Introduction to Biomedical Equipment Technology By Joseph Carr and John Brown Part 2

Differential Amplifiers Infinite Input impedance thus current passes from R3 to R4 and from R1 to R2 R2 R1 E1 A - Vinput R3 + Voutput E2 R4 Book Assumes: Vinput = E2-E1 And R1 =R3 and R2=R4 R1 A R2 E1 I1 I2 Voutput E2 A R3 R4 I3 I4

Advantages of Differential Amplifier In differential mode you can cancel noise common to both input signals R2 1V R1 E1 A - 2V R3 + 3V Voutput E2 R4

Instrumentation Amplifier + R5 R4 - E1 - R2 + R1 Vinput Voutpt R3 R6 R7 E2 - + E2 Give you high gain and high-input impedance. Composed of 2 amplifiers in noninverting format and a 3rd amplifier as a differential amplifier

Derivation of Gain for Instrumentation Amplifier step 1 - + E2 R1 R3 E1 E2 E3 I1 I2

Derivation of Gain for Instrumentation Amplifier step 2 + E4 - E1 R2 I2 I1 R1 E2 R1 R2 E2 E1 E4 I1 I2

Derivation of Gain for Instrumentation Amplifier step 3 - + R4 R7 R6 Voutput E4 E3 I7 I6 I4 I5 Book Assumes R4 =R6 and R5=R7 A A

Derivation of Gain for Instrumentation Amplifier step 4 Book Assumes R3 =R2

Example of Instrumentation Amplifier Find the gain of the previous instrumentation amplifier if R2 = 10K; R1=500; R4 = 10K ; R5 = 100K 

Problem 1 Design a differential amplifier where the feedback resistors are equal and the input resistors are equal. The gain should be equal to 10. One input voltage is 1 V and the second input voltage is 2 V. What is the output voltage? If the input resistance is 4 K what is the feedback resistance?

Solution 1

Problem 2 An instrumentation amplifier has a gain of 20. Using the schematic discussed earlier in the lecture, R5 = R7; R4=R6; R2 = R3. If R5 = 10K and R4 = 1K. The current across R2 is 4 mA and Vinput1 is 1V. Vout1 = -2V. Draw Schematic Find R2 & R1.

Solution 2 - + - + - + E1 R5 Vout1 R4 E1 R2 Vin1 IR2 R1 Voutpt R3 R6 Vinput Voutpt R3 R6 R7 E2 - + E2

Solution 2 cont

Review for Exam 1 Review all Homework Problems Review Wheatstone Bridge Lab & Amplifier Lab Review Studio exercises (precision & accuracy and aliasing exercises) Bring Calculators Closed book Equation sheet given previously will be given out at exam

Example of a Low pass Filter C Vout = output potential in volts(v) Vinput = input potential in volts(v) R = input resistance C =feedback capacitance T = Time (sec) Vic = initial conditions present at integrator output at t =0 Analog Integrator using a 1M resistor and a 0.2F capacitor. Find the output voltage after 1 second if the input voltage is a constant 0.5V? R - A Vinput + Voutput R Cf Voutput Vinput IR IC

Example of a Low pass Filter C R - A Vinput + Voutput R Cf Voutput Vinput IR IC

Low Pass Active Filters = Integrator Cf Attenuates High frequency where cutoff frequency is =RfCf Rf Ri - A Vinput + Voutput Cf ICf Ri Vinput Rf Voutput Ii IRf

High Pass Active Filters=Differentiator Rf Voutput = differentiator output voltage (v) Vinput = input potential in volts (v) Rf = feedback resistor ohms () Ci = input capacitance farads (F) Find the output voltage produced by an op-amp differentiator when Rf = 100K and C =0.5F and Vin is a constant slope of 400 V/s. Ci - A Vinput + Voutput Cf Vinput Rf Voutput Ii IRf

High Pass Active Filters Rf Ci - A Vinput + Voutput Cf Vinput Rf Voutput Ii IRf

High Pass Active Filters Attenuates High frequency where cutoff frequency is 1/(2) =1/ 2RiCi Rf Ci Ri - A Vinput + Voutput Cf Ri Vinput Rf Voutput Ii IRf

Band Pass Active Filters Cf Attenuates High frequency and low frequencies where cutoff frequency is =RfCf Rf Ci Ri - A Vinput + Voutput Cf ICf Ri Ci Vinput Rf Voutput Ii IRf