BIOELECTRONICS 1 Lec7: CHAPTER 3 Bio-Amplifiers By 6 October University Faculty of Applied Medical Sciences Department of Biomedical equipment and systems Lec7: CHAPTER 3 Bio-Amplifiers   By Dr. Eng. Hani Kasban Mahmoud 2017 BIOELECTRONICS 1

Biopotential Amplifiers Basic function Bio-Amplifiers Basic function • To increase the amplitude of a weak electric signal of biological origin Amplifier Amplified Biopotential Biopotential The biopotential amplifier function is to take a weak electric signal of biological origin and increase its amplitude so that it can be further processed, recorded, or displayed. Author : Dr. Eng. Hani Kasban A. Mahmoud / 2017

Bio-amplifier requirements • The measured signal should not be distorted • The amplifier should provide the best possible separation of signal and interferences • The amplifier has to offer protection of the patient from any hazard of electrical shock • The amplifier itself has to be protected against damages Author : Dr. Eng. Hani Kasban A. Mahmoud / 2017

Operational Amplifiers (Op Amp )

Operational Amplifiers (Op Amp ) + - Non-inverting input Positive voltage supply Negative voltage supply Output Symbol Inverting input At a minimum, op amps have 3 terminals: 2 input and 1 output. An op amp also requires dc power to operate. Often, the op amp requires both positive and negative voltage supplies (V+ and V-).

Operational Amplifiers The Op Amp Model + - Inverting input Non-inverting input Rin v+ v- A(v+ -v- ) vo Ro The op amp is designed to sense the difference between the voltage signals applied to the two input terminals and then multiply it by a gain factor A such that the voltage at the output terminal is A(v+-v-). The voltage gain A is very large (practically infinite). The gain A is often referred to as the differential gain or open-loop gain. The input resistance Rin is very large (practically infinite). The output resistance Ro is very small (practically zero).

Operational Amplifiers Ideal Op Amp Circuit model (ideal) We can model an ideal amplifier as a voltage-controlled voltage source (VCVS) The input resistance is infinite. The output resistance is zero. The gain A is infinite.

Operational Amplifiers + - Inverting input Non-inverting input Rin v+ v- A(v+ -v- ) vo Ro For A741, A = 100dB=105， if vo=10V， Then

Inverting Amplifier:Voltage Gain The negative voltage gain implies that there is a 1800 phase shift between both dc and sinusoidal input and output signals. The gain magnitude can be greater than 1 if R2 > R1 The gain magnitude can be less than 1 if R1 > R2 The inverting input of the op amp is at ground potential (although it is not connected directly to ground) and is said to be at virtual ground. But is= i2 and v- = 0 (since vid= v+ - v-= 0) and

Inverting Amplifier: Input and Output Resistances Rout is found by applying a test current (or voltage) source to the amplifier output and determining the voltage (or current) after turning off all independent sources. Hence, vs = 0 But i1=i2 Since v- = 0, i1=0. Therefore vx = 0 irrespective of the value of ix .

Inverting Amplifier: Example Problem: Design an inverting amplifier Given Data: Av= 20 dB, Rin = 20kW, Assumptions: Ideal op amp Analysis: Input resistance is controlled by R1 and voltage gain is set by R2 / R1. and Av = -100 A minus sign is added since the amplifier is inverting.

The Non-inverting Amplifier: Configuration The input signal is applied to the non-inverting input terminal. A portion of the output signal is fed back to the negative input terminal. Analysis is done by relating the voltage at v1 to input voltage vs and output voltage vo .

Non-inverting Amplifier: Voltage Gain, Input Resistance and Output Resistance Since i-=0 and But vid =0 Since i+=0 Rout is found by applying a test current source to the amplifier output after setting vs = 0. It is identical to the output resistance of the inverting amplifier i.e. Rout = 0.

Non-inverting Amplifier: Example Problem: Determine the output voltage and current for the given non-inverting amplifier. Given Data: R1= 3kW, R2 = 43kW, vs= +0.1 V Assumptions: Ideal op amp Analysis: Since i-=0,

The Summing Amplifier Since i-=0, i3= i1 + i2, Scale factors for the 2 inputs can be independently adjusted by the proper choice of R2 and R1. Any number of inputs can be connected to a summing junction through extra resistors. This circuit can be used as a simple digital-to-analog converter. This will be illustrated in more detail, later. Since the negative amplifier input is at virtual ground,

The Difference Amplifier Since v-= v+ For R2= R1 This circuit is also called a differential amplifier, since it amplifies the difference between the input signals. Rin2 is series combination of R1 and R2 because i+ is zero. For v2=0, Rin1= R1, as the circuit reduces to an inverting amplifier. For general case, i1 is a function of both v1 and v2. Also,

Difference Amplifier: Example Problem: Determine vo Given Data: R1= 10kW, R2 =100kW, v1=5 V, v2=3 V Assumptions: Ideal op amp. Hence, v-= v+ and i-= i+= 0. Analysis: Using dc values, Here Adm is called the “differential mode voltage gain” of the difference amplifier.

Many thanks Dr. Eng. Hani Kasban Questions?????