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

2. Bioelectric Amplifiers
Definition: An amplifier is used to process biopotential
Adjust gain
DC coupled: needed when signal is very slowly changing or is dc i.e. O2level may change pressure mmHg per min or hour
AC coupled: need to overcome electrode offset
Frequency Response: range that amplifier can work over i.e. ECG is 0.05 to 100 Hz
Low Frequency Response or High Frequency Response: frequencies where gain drops 3 dB below its mid frequency value.
Gain Types of Amplifiers:
Low Gain Amplifier: gain between 1 and 10 where unity or 1 is most common (used for Isolation, buffering and possibly impedance transformation between signal source and readout device)
Medium Gain Amplifier: gain between 10 and 1000 used for ECG, EMG etc.
High Gain Amplifier: gain greater than 1000 used for EEG

3. Important Design Parameters
Noise: want to keep noise as low as possible so keep thermal and shot noise low by choosing proper components and shielding wires
Drift: want to keep drift at a minimum where drift is spurious changes in output signal caused by change in operating temperature rather than input signal chagnes
High Input Impedance: Needed with all three types of amplifiers because almost all bioelectric signal sources have high source impedance of 103 to 107 Ώ
Engineering Rule of thumb is to have input impedance at least 1 order of magnitude higher than source impedance

4. Operational Amplifier
Circuit Diagram where power supply is bipolar (positive voltage to ground and negative voltage to ground) +V - + -V
Inverting
Noninverting
Output
Inverting Input Produces an output signal
180o out of phase of the input signal
Noninverting Input Produces an output
Signal in phase with the input signal
Inverting and Noninverting input have
Same gain but different phase effects
+V and –V can be DC or AC (note this circuit has no ground terminal -> ground connection is between 2 power supplies) +V + -
Ground + -
Notice how –V is negative to ground -V

5. Differential Signals: when E1 ≠ E2 then op amp will see a differential input of E2 –E1 thus output ~ gain (E2-E1) +V - + -V E1 E2
Output E3
Common Mode Signals: Signal Voltages are common or the same to both inputs or E1 = E2 such that the input = 0 so the output = 0
Common Mode Rejection Ratio (CMRR): expression of how close an op amp is to ideal where the common mode signal has no effect on output terminal voltage

6. 741 Properties of an Ideal Operational Amplifier
Infinite Open Loop (no feedback) voltage gain Avol = infinity
Zero Output impedance (Zo = 0)
Infinite Input impedance (Zi= infinity)
Infinite Frequency Response
Zero noise Contribution
Both Inputs follow each other in feedback circuits

7. Implications of Ideal Properties on Operational Amplifiers
Infinite open loop gain : gain without any feedback; thus the closed loop characteristics of the circuit are determined entirely by the properties of the feedback loop network and are independent of amplifying device
Output impedance = 0 : ideal voltage source
Input impedance = infinity : input terminals neither sink nor source any current thus don’t load any circuit to which they are connected
Input tend to follow each other: thus can treat inputs as if they were the same (if voltage is applied to noninverting input then we treating inverting input as if it were the same in other words have same voltage at other input)

8. Inverting Followers - +
Noninverting Input is grounded thus inverting input is ground by definition of property 6 (slide6) and is called a virtual ground
Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation
Ohm’s Voltage Law = V = IR R2 - +
Voutput
Vinput R1 A
Point A = Summing Junction
R1 = Input Resistance
R2 = Feedback Resistance R2 R1
Voutput
Vinput I2 I1
Transfer Function =>

9. Example of Inverting Amplifier
What is the gain of the inverting amplifier if R2 = 120KΏ and R1 = 12KΏ
120KΏ - +
Voutput
Vinput
12KΏ A
Note: R1 and R2 determine the magnitude of the Gain and the sign tells if the output
Is in phase (+) or 180o out of phase (-)

10. Non Inverting Amplifier
Noninverting Input is grounded thus inverting input is ground by definition of property 6 (slide6) and is called a virtual ground
Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation
Ohm’s Voltage Law = V = IR - +
Voutput
Vinput R1 A R2
Point A = Summing Junction
R1 = Input Resistance
R2 = Feedback Resistance R1 R2
Voutput I2 I1
Vinput

11. Non Inverting Amplifier
Calculate the voltage gain of the following noninverting amplifier when R2 = 10KΏ and R1 = 2 KΏ - +
Voutput
Vinput R1 A R2
Can this circuit ever be an attenuator? (meaning have the gain be less than 1)

12. Unity Gain Noninverting Followers
Connect output directly to input using a wire. Circuit is used for output buffering and impedance matching between a high source impedance and a low-impedance input circuit - A +
Voutput
Vinput

13. Multiple Inverting Input Circuits} Rf R1
Ohm’s Law: I = V/R
Vinput R2 -
Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation A R3 +
Voutput R1
Vinput I1 R2 I2 R3 Rf
Voutput I3 If

14. Multiple Non Inverting Input Circuits} Rf R1
Ohm’s Law: I = V/R R2 -
Kirrchhoff’s Current Law = Σ of all currents entering = Σ leaving the summation A R3 +
Voutput
Vinput R1 I1 R2
Vinput I2 R3 Rf
Voutput I3 If

15. Homework Problems Read rest of Chapter 7 and begin Ch8 Problems 1-10
Problem 6 described on page Where the input resistances (“R1 or Rin”) must be 10 times larger than the source impedance looking into the amplifier
Problem 7 and 8 refer to figure 7-19 not figure 7-20

16. Review Three key points on how to solve Op Amp Gain Equations
Derivation of Gain Equation
Inverting Op Amps
Non Inverting Op Amps