Op Amps For Everyone

negative feedback

- circuit gain dependent on passive feedback components rather than the active amplifier

How Real Op Amps Differ diverge from ideal OA's

1. Input Offset Voltage 2. Gain is a function of frequency. Gain decreases at high frequency (ideally, OA has no frequency- dependent elements)

2 Main Types of Resistive Sensors

1. Potentiometer 2. Strain Gauge

4 Different Displacement - Sensitive measurement Methods

1. Resistive 2. Inductive 3. Capacitive 4. Piezoelectric

Ideal OA Assumptions

1. input offset voltage = zero 2. zero current flowing into leads 3. gain is INFINITE (assumes OA voltage can obtain any value) 4. voltage between input leads is zero... aka input impedance is infinite (gain drives output voltage until voltage between input leads is zero) 5. output impedance is zero (ideal OA can drive any load without an output impedance dropping the voltage across it) 6. frequency response is flat (gain doesn't change as frequency changes)

Inverting vs Non inverting Amplifier

A non-inverting amplifier produces an output signal that is in phase with the input signal, whereas an inverting amplifier's output is out of phase. Both the inverting and non-inverting op amps can be constructed from one op amp and two resistors, just in different configurations.

A sensor converts _____ to ______

A sensor converts a physical parameter to an electrical output

An actuator converts _____ to _____

An actuator converts an electric signal to a physical output

How do Inductive Sensors Work

An inductance, L, can be used to measure displacement by varying the number of turns of the coil, geometric form factor, or permeability of the medium There is 1. Self- inductance 2. Mutual inductance 3. differential transformer Pros of Inductive Sensors: - not affected by the dielectric properties of its environment COns - may be affected by external magnetic fields How it works: - alterations in the self- inductance of a coil may be produced by changing the geometric form factor or the movement of a magnetic core within the coil - with mutual inductance transformers in measuring changes in dimensions of internal organs, the induced voltage in the secondary coil is a function of the geometry of the coils, number of turns, and frequency/amplitude of excitation voltage. the induced voltage in the secondry coil is a nonlinear function of the separation of the coils. in order to maximize output signal, a frequency is selected that causes the secondary coil to be in resonance. the output voltage is detected with standard demodulator and amplifier circuits Practical Applications: - radiotelemetry because of the devices low power requirements and its ability to produce wide range of inductance variance - measure cardiac dimensions, monitor infant respiration, and calculating artery diameters

Transistor

Can act as a switch or gate, opening & closing an electronic gate many times per second

How do Capacitive Sensors Work

Change in position of two capacitive plates proportionally changes the output voltage Real world example: - Capacitance microphone (DC excited circuit, so no current flows when the capacitor is stationary. A chnage in position changes the voltage) Limitations: - typically, the resistor in a capacitive sensor is 1 MegaOhm or higher, so the readout device must have 10 MegaOhm or higher input impedance Example: What is the plate spacing required to pass sound frequencies above 20 Hz for a 1cm^2 capacitance sensor with R = 100 MOhm - Calculate corner frequency: Fc = 1/2*pi*f*R =(1/2)(pi)(20Hz)(10^8) = 80pF - Calculate x: x = [(dielectric constant of free space)*(dielectric constant of insulator) * (Area)] / (Fc) = [(given)(given)(.0001m^2)] / 80pF = 11.1 micrometers

Active Transducer

Do not require any power source for their operation, and work solely on the energy conversion principle. Example: Thermocouple

How does capacitor work in filtering? High Pass

How does a capacitor work as a filter? for low frequency, the capacitor offers extremely high resistance, and for high-frequency signals, it proves less resistance. So it acts as a high pass filter to allow high-frequency signals and block low-frequency signals. How filter capacitors work is based on the principle of capacitive reactance. Capacitive reactance is how the impedance (or resistance) of a capacitor changes in regard to the frequency of the signal passing through it. Resistors are nonreactive devices. This means that resistors offer the same resistance to a signal, regardless of the signal's frequency. This means, for example, that a signal of 1Hz and a signal of 100KHZ, will pass through a resistor with the same resistance. Frequency isn't a factor. However, a capacitor is not like this. A capacitor is a reactive device. Its resistance, or impedance, will vary according to the frequency of the signal passing through. Capacitors are reactive devices which offer higher resistance to lower frequency signals and, conversely, lower resistance to higher frequency signals, according to the formula XC= 1/2πfc. Being that a capacitor offers different impedance values to different frequency signals, it can act effectively as a resistor in a circuit.

(Wheatstone) Bridge Circuit

Ideal for measuring small changes in resistance. Wheatstone bridge is composed of two voltage dividers. Resistance sensors are connected in one or more arms of the bridge. The variation in resistance can be measured the change in output voltage with a differential amplifier feeding a ADC. It is common practice to incorporate a balancing scheme into the circuit. Look up further info for equation derivations. AC balancing circuits are more complicated because a reactive as well as resistive imbalance must be compensated.

Microphones in a circuit... AC or DC?

Many times in a circuit, both DC and AC signals need to be both be used in a circuit, at least at a certain stage of the circuit. However, at another stage, in the circuit, we may only want AC signals and the DC taken out. An example of such a circuit is a microphone circuit. We need DC as input to the microphone for it to be able to be powered on and we need AC as input, which represents the voice signal or music, etc. which we want the microphone to record. How do we filter out the DC component of the signal? We use a capacitor to filter out the DC signal. We do this by placing the capacitor in series. In this configuration, which is the circuit you see below, this is a capacitive high-pass filter. Low frequency, or DC, signals will be blocked. Usually, a 0.1µF ceramic capacitor, or value around that range, is placed after the signal that contains both DC and AC signals. And this capacitor filters out the DC component so that only AC goes through. http://www.learningaboutelectronics.com/Articles/Filter-capacitor.php

How Does a Strain Gauge Work

Measure displacement (on the order of nanometers) When a fine wire is strained, the resistance of the wire changes because of changes in the diameter & length Practical Application: - can be used for converting blood pressure to diaphragm movement, then to resistance change, then to an electrical signal Strain gages are classified as either bonded or unbonded - Unbonded: Wheatstone bridge circuits, mounted under stress between frame and movable armature such that preload is greater than any expected external comprehensive load (this is necessary in order to prevent putting the wires in compression - Bonded: Cemented to the strained surface, deviation from linearity is measured in order to measure the displacement of the wire, many practical applications involve deformity of the wire due to changes in temperature (using second strain gauge as a dummy element) - there is a "4 arm bridge" that can be used to measure temperature deformity as a bonded strain gauge. the 4 arm version of these bonded strain gages yield four times greater output if all four arms contain active gages. four bonded metal strain gages can be used on cantilever beams to measure bite force, etc.

How Do Potentiometers Work

Potentiometers measure displacement (translational or rotational) and then output that displacement information into an electrical circuit. Resistive elements in a potentiometer can be excited by either AC or DC voltages. Potentiometers provide a linear output as a function of displacement ( as long as the potentiometer is not electrically loaded). Why potentiometer will not work when it is "loaded" - poten. are rarely used to directly control significant power (more than a watt) since the power dissapation in a poten. is comparable to the power in the load Real life potentiometer examples: - knob volume controls on audio equipment - joystick

How does capacitor work in filtering? Low Pass

Remember that current takes the path of least resistance. Since a capacitor offers very low resistance to high frequency signals, high frequency signals will go through the capacitor. In this way, with the circuit in this configuration, the circuit is a high frequency filter. Low frequency current signals will not go through the capacitor, because it offers too much resistance to low frequency signals. Only high frequency signals go through.

Passive Transducer

Require an external power source for their operation. They produce an output signal from some sort of variation in resistance, capacitance, or any other electrical parameter, which then must be converted to an equivalent current or voltage signal. Example: Photocell (LDR) - varies the resistance of the cell when light falls on it. The change in resistance is then converted to proportional signal with the help of a bridge circuit

What does it mean for resonance to occur in a circuit

Resonance occurs in a circuit when the reactances within a circuit cancel one another out. As a result, the impedance is at a minimum and the current is at a maximum.

Unity Gain Frequency

The frequency at which OA gain reaches 0 dB

Transducers converts energy from ______ to ______

Transducers convert energy from one form to another. Electrical transducer: Can convert physical quantities into proportional electrical quantity like voltage or current. Physical quantity can be pressure, temp, displacement, etc

How do Piezoelectric Sensors Work

Used to measure physiological displacements and record heart sounds. Piezoelectric sensors generate an electric potential when mechanically strained, and conversely an electric potential can cause physical deformation of the material (this happens because of the distortion of a crystal lattice and then the following rearrangement of negative and positive charges) Surface charges can be determined by measuring the difference in voltage between electrodes attached at the surface Practical Applications: - external and internal phonocardiography - detect korotoff sounds in blood pressure measurements - physiological accelerations (estimate energy expenditure)

Ohm's Law

V = IR When current flowing through any part of a circuit is known, voltage dropped across that portion if the circuit is equal to the current multiplied by the resistance across that portion

Superposition

When there are multiple indepedent current or voltage sources, voltages/curernts resulting from each source can be calculated separately & results can be added algebraically

amplifier gain

measure of the ability of a 2 port circuit to increase power or amplitude of a signal from input to output by adding energy converted from the power supply to the signal