Electronics

Describe open loop operation (comparator mode)

1) The op amp compares the two inputs and provides an output if they are different 2) The output is limited by the supply voltage 3) Due to losses, saturation voltage will be approximately 85-95% supply voltage 4) The output will saturate positive or negative depending on which input is biggest (i.e most positive) 5) If the inverting input is bigger then output saturates negative. If non inverting input is bigger, then output saturates positive

What is the most common operational amplifier?

741

What does feedback occur via for an integrator amplifier?

A capacitor

Describe what is meant by hysterisis for a transducer

A measure of the accuracy and repeatability of the device over the dynamic range from a minimum to a maximum and vice versa

Describe what is meant by monotonicity for a transducer

A measure of the transducer ability to maintain an accurate measurement over the dynamic range without missing a value

What will be made if the input resistors are different value in a summing amplifier? What can this be used to do?

A scaling summing amplifier is produced which will output a weighted sum of the input signals. This can be used to convert a binary number to a voltage in a digital to analog converter

When converting to and from digital numbers what can a truly analogue value only be represented as?

A series of discrete values

What will be made if the input resistors are all equal in a summing amplifier?

A unity gain inverting amp will be made

List some transducer terminology

Accuracy Repeatability Resolution Sensitivity Linearity Deadzone Hysterisis Stability Zero Stability Monotonicity

What is meant by a transducer?

An element that converts any process or kinetic variable into electrical energy and vice versa

What is an integrated circuit?

An integrated circuit is a complete electonic circuit containing transistors, maybe diodes, resistors and capacitors all made from and on a chip of silicon

Basically what type of amplifier is an op amp and why?

An op amp is basically a differential amplifier i.e it amplifies the difference between the voltage V1 and V2 at its inputs

What does an op amp only respond to?

An op-amp only responds to the difference between the voltages on its two inputs terminals known commonly as the differential input voltage

Why does a virtual earth arise in a summing amplifier?

As a consequence of the high value of Ao

Why is there a virtual earth. Describe the resulting voltages

Because of the very large open loop gain of the op-amp there is a virtual earth therefore the voltage across equals the input voltage V1 and that across C is the output voltage Vo

What are the main uses and applications of integrated circuits?

High gain dc Ac voltage amplifiers

Describe what is meant by stability for a transducer

Sometimes referred to as drift. This is a measure of th ability of the device to maintain a constant output over a period of time when subjected to either a static or dynamic input

What is meant by a sensor

Specifically an element that converts any process or kinetic variable into electrical energy

What are the disadvantages to the differentiator/

Suffers from instability at high frequenciess The capacitive input makes it susceptible to random noise signals and any noise in the source circuit will be amplified more than the input signal itself

What can we deduce from the idealised characteristics?

That the input resistance is infinite so no current flows into either input terminal and the differential input offset voltage is zero

Describe what is meant by zero stabilityfor a transducer

The ability of the transducer to return repeatedly to its zero level when the input is zero

Give a use for a unity gain voltage follower

The advantage of the unity gain voltage follower is that it can be used when impedance matching or circuit isolation is more important than amplification as it maintains the signal voltage

Describe how the counter ramp method ADC works

The basic idea is to connect the output of a free running binary counter to the input of a DAC, then compae the analog output of the DAC with the analog input signal to be digitalised and use the comparators output to tell the counter when to stop counting and reset.

What will happen if a constantly changing input signal such as a square wave is applied to to the input of the integrator amplifier? What doe this result in and why?

The capacitor will charge and discharge in response to changes in the input signal. This results in the output signal being that of a sawtooth waveform whose output is affected by the RC time constant of the resistor/capacitor combination because at higher frequencies the capacitor has less time to fully charge.

What does the current do to the capacitor in the integrator op amp?

The current through the feedback loop charges or discharges the capacior as the required negative feedback occurs through the capacitor

What happens the faster or larger the change to the input voltage signal in a differentiator?

The faster or larger the change to the input voltage signal, the greater the input current. The greater will be the output voltage change in response

Describe briefly how the input signal to the differentiator amplifier works

The input signal to the differentiator is applied to the capacitor. The capacitor blocks any DC content so there is no current flow to the amplifier summing point - resulting in zero output voltage. The capacitor only allows AC type input voltage changes o pass through and whose frequency is dependant on the rate of change of the input signal

What does the virtual earth do to the input voltages in a summing amplifier?

The input signals are effectively isolated from each other by the virtual each node at the inverting input of the op amp

Describe what is meant by linearity for a transducer

The measure consistency of sensitivty over the dynamic range of the device

What is the integrator operational amplifier?

The op-amp integrator is an op-amp circuit that performs integration.

Describe what is meant by accuracy for a transducer

The relationship between the output of the device and the actual value of the measurand. This is normally stated as a standard deviation or percentage tolerance

What does the time between updates depend on in a counter ramp ADC method?

The time between updates (new digital output values) changes depending on how high the input voltage is.

What is open loop?

There are no feedback components used around the amplifier so the feedback path or loop is open

How many terminals does an operational amplifier have?

3 terminals. It consists of two high impedance inputs, one called the inverting input and the other called the non inverting input

What does the DB scale define?

A ratio,

Describe how a schmitt trigger is a square wave osciallator

An astable producing a continuous train of pulses with fast rise time an fall times suitable for use as a clock in a digital circuit. Oscillations occurs because the capacitor starts to charge up at switch on and when its voltage reaches the upper tig point the output of the schmitt triggger goes rapidly low (since its input is high) C then starts to discharge through it. When the voltage drops to the lower trip point the schmitt output quickly switches to high allowing C to recharge to the upper trip point and so on

How is the output voltage equal to the input voltage in a non inverting voltage follower? Why is the voltage follower circuit ideal as a unity gain buffer circuit?

As the input signal is connected directly to the non inverting input of the amplifier the output signal is not inverted resulting in the output voltage being equal to the input voltage. This then makes he voltage follower circuit ideal as a unity gain buffer because of its isolation properties.

What will happen to the differentior op amp at high frequencies?

At high frequencies the rectance of the capacitor is much lower resulting in a higher gain and higher output voltage. But the op amp becomes unstable and will start to oscillate

Give some applications of summing amplifiers

Audio mixer (summing several signals with equal gains) If the input resistances of summing amplifiers are connected to potentiometers then individual input signals can be mixed together by varying amounts. Eg produce an audio mixer for adding or mixing together individual waveforms from different source channels before sending them combined to an audio amplifier. Digital to analogue converter Also used to perform the mathematical process of addition in analogeu computing

Give an example of a piezoelectric sensor

Buzzers, piezo oscillators

How can the performance of an amplifier be changed?

By feeding back part of the output back to the input

How would the summing amplifier be used to add together the voltage signals without any amplification?

By putting all of the resistances of the circuit to the same value of R1, the op-amp will have voltage gain or unity and an output voltage equal to the direct sum of all the input voltages. Vout = -(v1+v2+v3)

How is the integrator produced (different from an op amp)

By replacing the feedback resistance with a capacitor, we now have an RC network connected across the operational amplifiers feedback path producing another type of op-amp circuit - commonly called an op-amp integrator.

What can analogue computing use and then do in turn?

Can use positive and negative feedback with an assortment of additional components to yield voltage functions

Describe how a schmitt trigger is a rise time improver

Converts a slowly changing input into one with a fast rise time. Would also turn a sine wave into a square wave

List the ADC conversion methods from slow converion times to fast

Counter ramp method Tracking ADC Successive approximation

What type of conversion types does Successive approximation ADC have?

Even faster conversion times

How is feedback achieved in the non inverting amplifer?

Feedback is achieved by applying a small part of the output voltage back to the inverting input terminal via voltage divider netwrok producing negative feedback

Describe what the updates are like for low signal levels and high signal levels in the counter ramp method ADC

For low signal levels, the updates are rather close spaced. For high signal levels, they are paced further apart in time

What is the gain for inverting amplifier?

Gain = -R2/R1 R2 is the feedback resistor

What is the gain for a non-inverting op-amp?

Gain = 1 + R2/R1

What is the gain of an operational amplifier?

Gain = Vout/Vin Gain in this instance refers to voltage only

What is gain?

Gain is just a ratio of the output and input

Describe what is meant by repeatability for a transducer

Given a set condition, this is the closeness of a group of measurements from a specific set input

What is the non-inverting input connected to in the inverting op amp?

Ground

Give some appliations of opamps

High speed switches Signal detectors Voltage comparators

Ho w is a voltage follower circuit produced?

If we made the feedback resistor equal to zero and R2 equal to infinity then the circuit would have a fixed gain of 1 as all the output voltage would be present on the inverting input terminal. This would then produce a voltage-follower circuit.

What are the advantages of negative feedback?

Increased stability of the overall gain i.e the gain is less affected by changes of transistor parameters (eg hfe) and by variations of temperature and supply voltage. Less distortion of the output Greater bandwidth i,e a wider range of frequencies is amplified by the same amount Alteration of the input and output impedances of the ampifier to almost any desired value

List the characteristics for an ideal opamp

Infinite open loop gain Infinite input impedance Zero output impedance

Describe the op-amp characteristic input impedance and state its idealised state.

Infinite! Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry.

Describe the characteristics of negative feedback

Inherently more stable Amplifiers Control of gain

Describe the characteristics of positive feedback

Inherently unstable Astable devices Tends to saturate

Describe how a schmitt trigger is a switch debouncer

It can act as a switch debouncer due to the discharging action of the capacitor, the schmitt trigger prevents contact bounce at the switch

What does the feedback connection between the output and the inverting terming in an inverting operational amplifier cause? What does this produce as a result?

It forces the differential input voltage towards zero. This produce a closed loop circuit to the amplifier resulting in the gain of the amplifier now being called its closed loop gain. The closed loop inverting amplifier uses negative feedback to accurately control the overall gain of the amplifier but reduces the amplifiers gain.

What does the integrator amplifier do? What is its action the same as?

It inverts, amplifies and integrates the input voltage over any period of time. Basically its action is the same as the CR integrator circuit but the op-amp provides the greater output voltages required for analogue computing

What is the magnitude of the output signal in an integrator determined by?

It is determined by the lenght of time a voltage is present at its input.

What is the output voltage in the integrator opamp proportional to?

It is proportional to the integral of the input voltages

What does a differentiator amplifier do?

It produces a voltage output which is directly proportional to the input voltages rate of changes with respect to time.

What happens as a result of the closed loop feedback in the inverting amplifier?

It results in the voltage potential at the inverting input being equal to that at the non inverting point producing a virtual earth (summing point) because it will be at the same potential as the grounded reference input

What will the summing operational amplifier do?

It will now amplify each individuaal input voltage and produce an output voltage that is proportional to the algeraic sum of the three individual input voltages

Give an example of a resistive sensor

LDRs, thermistors

Give an example of an indutive sensor

Linear variable differential transformers

Give an example of an electromechanical sensor

Mechanical actuators, relays, solenoids

Give an example of a capacitive sensor

Microphone

What type of feedback is obtained from an inverting op-amp?

Negative feedback

What type of feedback to inverting and non inverting amplifiers have?

Negative feedback

What is meant by updates in the counter ramp method ADC

New digital output values

List the important points about the inverting amplifier

No current flows into the input terminal The differential input voltae is zero as V1 = v2 = 0 virtual earth) gain = -R2/r1

For a summing ampifier, is the opamp inverting or non inverting?

Non inverting.

Describe the op-amp characteristic open loop gain and state its idealised state.

Open loop gain - ideally infinite. The main function of an operational amplifier is to amplify the input signal and the more open loop gain is has the better. Open loop gain is the gain of the op-amp without positve or negative feedback

What is the output for an inverting amplifier?

Output = -input

What is the output or a non inverting amplifier

Output = input

Is the output in phase or in antiphase with the input for a non inverting op amp?

Output signal is inphase with the input

Give an example of a photoconductive sensor

Photo transistors

list the typical DAC considerations

Precision Speed of conversion Accuracy Type of input (TTL, CMOS) Type of output (voltage or current) Power supply requirements Reference voltage (internal or external)

What is a schmitt trigger?

Predefined voltage level triggering to saturation. The schmitt trigger is a bistable multivibrator in which feedback is via a resistor that is common to the emitter circuits of both transistors, rather than from the collector of one to the base of the other s in an ordinary bistable

List the generic sensor types

Resistive - LDRs, Thermistors Capacitive - Microphones Electromagnetic - Speakers, Hall effect probes, microphones Electronmechanical - Mechanical actuators, relays, solenoids Inductive - Linear variable differential transformers (LVDT) Photoconductive - Photo transistors Photovoltaic - Solar panesl Piezoelectric - Buzzers, piezo oscillators

What feedback components is the operational amplifier designed to be used with?

Resistor and capacitor

What type of conversion types does Tracking ADC have?

Slightly faster conversion times

What type of conversion time is associated with the counter ramp method ADC and why?

Slow conversion times. This is because the circuits need to count all the way from 0 at the beginning of each count cycle

What re the limitations of ICs?

Small size limits the power and voltage. Can't be used for high value resistors and capacitors because they need too much space Inductors and transformers cannot be produced on a silicon chip

What are the advantages of ICs?

Smaller, lighter, cheaper and more reliable

What are used to remove the discret change in voltage associated with digital outputs?

Smoothing capacitors

Give an example of a photovoltaic sensor

Solar panels

Give an example of an electromagnetic sensor

Speakers, Hall effect probes, microphones

Describe the characteristics of a schmitt trigger

The characteristic of a schmitt trigger is shown on a graph with the hysterisis loop. The value of Vi which triggers the circuit to jump from low to high is called the upper trip point. Whilst the low trip point is the value of Vi which causes it to jump from high to low. Since the change from high to the low state occurs at a lower value of Vi than does the reverse action, the circuit exhibits a lagging or hysterisis effect in its switching action

Why is positive fedback unstable?

The gain increases often in an uncontrollable way leading to instability and oscillation

Is the output in phase or in antiphase with the input for an inverting amplifier?

The output is in antiphase with the input

What is the output voltage proportional to in a summing amplifier?

The output voltage is proportional to the sum of the input voltages

What is the golden rule?

The output will move if it can to make both inputs equal i. the output voltage must stay within its saturation range

What is the rate at which the output voltage increases in a integrator amplifier determined by? How can the time in which it takes the output voltage to reach saturation also be changed?

The rate at which the output voltage increases (the rate of change) s determined by the value of the resistor and the capacitor RC time constant. By changing this RC time constant value, either by changing the value of the capacitor or the resistor, the time in which it takes the output voltage to reach saturation can also be changed.

What is negative feedback?

This is the processes of feeding back a fraction of the output signal back to the input, but to make the feedback negative it must be fed back to the inverting input using an external feedback resistor.

Describe what is meant by sensitivity for a transducer

This is the ratio of the change in magnitue of the output for a specific change of the input, of course this is related to gain

Describe what is meant by resolution for a transducer

This is the smallest incremental change in of the measurand the device can measure. Although this is significant to digital devices, it is also relevant to analogue.

Describe what is meant by dead zone for a transducer

This refers to the largest input change of the measurand to which the transducers fails to respond

Describe the layout for the inverting amplifier

Threre is a resistor added to th inverting terminal and there is a resistor in parallel to the operational amplifier

What is the main function of an operational amplifier?

To amplify the input signal

What are the uses of a schmitt trigger?

To convert a sine wave into a square wave Rise time improver Noise remover Switch debouncer

Why has the resisor been added across the op-amp from the output terminal to the inverting terminal of an operational amplifier?

To reduce and control gain making it more stable

Describe how a schmitt trigger is a noise remover

Unwanted high frequency spikes on the input signal are eliminated

What is a summing amplifer?

Used for combining the voltage present on multiple inputs into a single output voltage because of the existence of the virtual earth (virtual earth arises as a consequence of the high value of Ao)

What is an operational amplifier?

Used to perform mathematical operations e.g addition, subtraction, multiplication and division. Used for signal conditioning A voltage amplifier device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals 3 terminal device high consists of two high impedance inputs, one called the inverting input and the other called the non inverting input

What characteristics does a non inverting op amp have?

Very good stability Very high input impedance Low output impedance

What happens as a result of very large gain?

Very large gain makes the op-amp unstable and hard to control

How do you calculate Vout for the integrator amplifier

Vout = -1/RC ∫Vindt

How is vout calculated for the differentiator?

Vout = -Rc (dVin/dt)

How can you calculate Vout for a summing amplifier?

Vout = -Vin1 (R3/R1 + Vin2 R3/R2) OR Vout = -(Vin1 R1/R2 + Vin2 R1/R3 + Vin3 R1/R4)

Describe the op-amp characteristic output impedance and state its idealised state.

Zero! The output impedance is assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it can supply as much current as necessary to the load.