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

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

3. 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

4. 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

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

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

7. 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

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

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

10. 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?

11. Solution 1

12. 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.

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

14. Solution 2 cont

15. 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

16. Example of a Low pass FilterC
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

17. Example of a Low pass FilterC R - A
Vinput +
Voutput R Cf
Voutput
Vinput IR IC

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

19. High Pass Active Filters=DifferentiatorRf
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

20. High Pass Active FiltersRf Ci - A
Vinput +
Voutput Cf
Vinput Rf
Voutput Ii
IRf

21. 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

22. Band Pass Active FiltersCf
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