Week 9: Amplification

Question 1

Define amplitude

Answer 1

Amplitude refers to increasing the magnitude of a signal, sometimes the polarity, it has no units as only gain changes the voltage of the transducer.

The amplifier only amplifies the signal into something which is readable and can be displayed

Question 2

Define amplification

Answer 2

Amplification is the process of increasing the strength, size, or magnitude of a signal without changing its basic shape or information content.

Question 3

Define Gain

Answer 3

Gain refers to the efficiency with which power is transferred from a source to the load, a measure of how efficiently the transducer amplifies the available power.
To achieve optimal transfer, the impedance of the source and the load needs to be matched, so power amplification can occur without any loss.

Gain does not change the natural frequency, damping or hydraulic behaviour. It is a property of the amplifier

Gain has no units
Gain= V(out)/ V(in)

Question 4

Define sensitivity

Answer 4

Sensitivity refers to output V/ input V, it is a property of the transducer, units of μV/mmHg or μV/cmH2 O. It indicates how much a transducers output changes in response to a change in input voltage.

A more sensitive transducer only produces larger voltage changes.
This may make the oscillations look more visible, but the shape of the response does NOT change.
Sensitivity describes how much output you get per mmHg.
Gain describes how much the amplifier scales that output.
Sensitivity does NOT increase the likelihood of underdamping.

Question 5

Describe positive feedback and negative feedback

Answer 5

NF: makes a amplifier stable and predictable as it returns the system to a set level & opposes the input. Feedback is returned to the inverted input (-)

PF: reinforces changes, it enhances the original stimulus and reinforces the input

Question 6

Describe the effect on gain without feedback

Answer 6

No feedback results in a large gain known as open loop gain,

Gain is large and unpredictable, gain becomes highly sensitive to: Temperature changes, Component ageing & Power supply variations

Without feedback, the amplifier gain becomes very high but unstable, non-linear, noisy, and easily distorted, making the amplifier unsuitable for accurate or predictable use.

Question 7

Define a transistor

Answer 7

A form of amplifers which is used as a switch or amplifies signals.
A semiconductor device that uses a small input current to control a much larger output current

Seen in basic and integrated circuits.

Question 8

Define a integrated circuit

Answer 8

A small electronic device that contains many interconnected components such as transistors, resistors & capacitators, built into a single semi-conductor.

Key features of:
1. Miniaturised: thousands to billions of components in a few millimetres.
2. Reliable: factory-made, consistent performance.
3. Fast: short distances between components = high speed.
4. Low power: much less energy than older discrete circuits.
5. Used everywhere: amplifiers, computers, ECG machines, medical monitors.

Question 9

Describe a operational amplifier

Answer 9

A high-gain electrical amplifier that amplifies inputs and 1 output.
A type of integrated circuit that has many transistors that amplify the difference between the input voltages.

It needs negative feedback to be stable & predictable

Relevance: Op-amps amplify tiny biological signals (ECG, pressure transducers) into readable voltages while rejecting noise.

Question 10

Describe an inverting amplifier

Answer 10

A type of Op-Amp amplifier where the input signal is applied to the inverting input (-) causing the output to be inverted.

A positive input gives a negative output, a negative input gives a positive output

• Input resistor RiR_iRi feeding into the inverting (–) input
• Feedback resistor RfR_fRf connected from output back to the – input
• Non-inverting (+) input connected to 0 volts
• Op-amp powered by positive and negative supply rails

Question 11

Describe how a inverting amplifier circuit works

Answer 11

1. The signal enters the inverting input through the input resistors
2. The Op-Amp uses negative feedback through the feedback resistors
3. Feedback forces the Op-Amp to adjust it’s output so the input stays at 0V
4. As the signal enter the input , the signal at the output becomes inverted

Question 12

Define the gain calculation in relation to an inverting amplifier

Answer 12

Gain= -Rf/ Ri

Rf= feedback resistor
Ri= input resistor

If input voltage is positive (+), the output voltage will be negative (-)

Question 13

Describe a non-inverting amplifier

Answer 13

A type of Op-Amp circuit which the signal is applied to the non-inverting input (+), giving the output the same polarity as the input
The output is a larger version of the input, no inversion

The input voltage connected directly to the + (non-inverting) input
A feedback resistor RfR_fRf from output back to the – input
A resistor RiR_iRi from the – input to 0 V

Question 14

Describe how a non-inverting amplifier circuit works

Answer 14

1. The input signal enters the non-inverting input
2. Op-Amp uses negative feedback to stabilise the gain
3. The output adjusts until the inverting input (-) becomes equal to the + input
4. As the input is on the + terminal, the output is non-inverted.

This give a high input impedance & stable, predictable gain

Question 15

Define the gain calculation in relation to an non-inverting amplifier

Answer 15

Gain= 1+ +Rf/Ri

Rf=feedback resistor
Ri= input resistor

The gain is always positive, greater than 1 as adding feedback increases amplification
Gain has no units

Question 16

Describe a Op-amp supply

Answer 16

Op-Amps cannot run on input signal alone, it needs an outside power source to produce a larger voltage output then the input.
It provides both negative and positive power supplies for the amplifier to produce positive and negative output signals.

The op-amp can output both positive and negative voltages.

Relevance: As some input signal cross 0, without a negative supply the Op-Amp output cannot go below 0V

Question 17

Describe the 3 components of a Op-Amp supply

Answer 17

1. Positive supply: marked +, allows the Op-Amp to swing up to a positive limit depending on the circuit
2. Negative supply: marked as -, allows the Op-Amp to swing down to a negative limit e.g., -9V
3. 0 Volts: referred to as ground, it is the reference point of a system, input and output voltages are measured relative to this

Question 18

Define Moore’s law

Answer 18

refers to the number of transistors in a integrated circuit doubling approximately every 2 years, leading to increased computing power and decreased costs.

Question 19

Describe a differential amplifier

Answer 19

A differential amplifier in a Op-Amp circuit amplifies the difference between the 2 input voltages whilst rejecting any voltage that is common to both inputs, common mode signals are cancelled.

In ECG, electrodes pick up noise, differential amplifier removes common noise as it appears equally at both inputs, so only the desired signal is amplified.

Question 20

Describe how a differential amplifier works

Answer 20

A differential amplifier contains 2 input resistors & 2 feedback resistors, as well as a Op-Amp

Gain for differential signal is:
Gain= R2/R1

Question 21

Describe the effect of impedance on a differential amplifier

Answer 21

Too high: the amplifier takes no current from the electrodes/transducer, preserving the true signal
Modern amps use 10,000 MΩ

Extremely high results in increased sensitivity to small currents, noise may be picked up

Correct impedance: clean & good quality signal, accurate measurement

Too low: Current is drawn from the source of the signal resulting in attenuation and distortion of the signal. Amplifier loads the source reducing the measured differential voltage

Question 22

Define common mode voltage

Answer 22

Refers to a voltage that appears identically at both inputs of a differential amplifier, so is the same at both inputs, it is NOT part of the desired signal!

Same voltage at both inputs so therefore differential amp produces no output

A differential amplifier is designed to reject CMV, so noise is not amplified.
The greater the rejection, the better quality of the measured ECG signal

Question 23

Define differential voltage

Answer 23

Refers to the differences in voltages at the two inputs of the differential amplifier.

Important as signals produced by ECG are tiny, measured between 2 points and easily overwhelmed by noise

So amplifying the difference allows the amplifier to extract the desired signal while rejecting common-mode interference.

Signals that are both the same at both inputs are amplified

Question 24

Define the differential voltage formula

Answer 24

V(diff)= V+ = V-

Question 25

Define common mode rejection ratio (CMRR)

Answer 25

CMRR refers to a measure of how well a differential amplifier rejects common mode noise/signals, compared with how well it amplifies the desired differential signal A high CMRR is needed to prevent signal distortion A high CMRR, strongly amplifies the difference between the 2 inputs & barely amplifies common mode noise This is important as the desired signals e.g., from an ECG are tiny in mV, compared to noise

Question 26

Describe the CMRR equation

Answer 26

CMRR= differential gain/ common mode gain Gain has no units