st12 – operational amplifier 1 operational amplifier lecturer: smilen dimitrov sensors technology...

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ST12 – Operational amplifier

Operational amplifier

Lecturer:Smilen Dimitrov

Sensors Technology – MED4

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Introduction

• The model that we introduced for ST

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Introduction

• We have discussed

– The units of voltage, current and resistance, from both a microscopic and macroscopic (electric circuits) perspective

– The definition of an elementary electric circuit, Ohm’s law and Kirschoff Laws

– Solving and measurement of voltage divider circuit and more complicated circuits - and applications in sensors

– Resistive based sensors– AC current, capacitors, and capacitive based sensors– Semiconductor structures – diode and transistor (and sensor

applications)

• This time we discuss the operational amplifier as an electronic element, and a basic example of an integrated circuit

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Differential amplifier

• If R1=R2 and that the two transistors are identical → when v1=v2 it follows that I1=I2, hence I1 = I2 = IE/2 (balanced)

• v1 ↑ => I1 ↑ => VE1 (=VE) ↑ => VBE2 ↓ (since V2=const.) => I2 ↓ (but IE ≈ const)

BECE VHI

EB VVHI 11 EB VVHI 22

2121 vvIIH

2122 vvIIH E

212 2

1vv

hi

ib

212

2vv

h

Rv

ibout

H parameter - links voltage and current (dimension 1/Ohm = Siemens)

Only interested in changes – 'delta' or differentiation,

here with lower case letters

Output is related to difference between the inputs

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• Important – the basis of a differential amplifier

• Amplifies differential signals, rejects common signals• When differential signal is applied to the inputs: this will incrementally

increase and decrease the base voltages to VB1 + ΔV and VB2 - ΔV – Because Q1 conducts a little more and Q2 a little less, IE now splits

unevenly creating IC1 > IC2 → forces the voltage at VC1 to decrease and VC2 to increase. The result: a voltage change at each output

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• suppose a common-mode input signal is applied: it incrementally increases both inputs to

VB1 + ΔV and VB2 + ΔV– Because the conduction level of neither

transistor has changed (both bases and emitters moved by the same amount), the collector currents did not change.IC1 = IC2 ≈ IE / 2.

– Subsequently, the voltages at VC1 and VC2 remain the same! Therefore, the circuit has rejected a signal common to both inputs.

– The bias condition assumes equal voltages at VB1 and VB2 → forcing the bias current IE (set by RE) to split equally between the transistors → resulting in IC1 = IC2.

– With RC1 = RC2, equal voltages develop at VC1 and VC2.

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• Note that a differential amplifier is a more general form of amplifier than one with a single input; by grounding one input of a differential amplifier, a single-ended amplifier results

• The voltage of each output: – grows when the input in its own branch grows– drops when the input in the opposite branch grows. .

• When both Vbe's are equal → the potential of both bases is also equal → which means that the voltage between the inputs is zero.– Vbe of a single transistor will typically change in fractions of a

volt (ex. from 0.620V to 0.638V) → for all practical purposes, the voltage between inputs stays zero, even when the circuit amplifies.

• The input resistance of a differential amlifier input is resistance seen into a base of a transistor – resistance of current source (theoretically infinite), times hFE - (close to) infinity

• -Vcc is kept on zero potential (ground) → Vi1 or Vi2 must >= VBEON

(typically 0.6V); if -Vcc is -5V → Vi1 or Vi2 must >= -4.4 (-5+VBEON); so by using a negative potential as a power supply, we can amplify both positive and negative signals

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ST12 – Operational amplifierIntegrated circuits [IC] - operational amplifier construction (741)

• The operational amplifier is based on the differential amplifier - it is a whole circuit, and we cannot really treat it as a 'basic' electronic element (of the likes of a resistor or a capacitor).

• However, it is quite commonly produced as an 8-pin integrated circuit (IC), with a standardized layout for the pins; and in such a form, the op-amp finds many uses - so it is an 'element', in the sense of being a basic building block found in many circuits.

– Opamp is a DC-coupled high-gain electronic voltage amplifier with differential inputs and, usually, a single output.

– the output of the op-amp is controlled by negative feedback (external elements), which almost completely determines the output voltage for any given input.

– originally designed to perform mathematical operations

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• A monolithic integrated circuit (also known as IC, microcircuit, microchip, silicon chip, or chip) is a miniaturized electronic circuit that has been manufactured in the surface of a thin substrate of semiconductor material.

• The first integrated op-amp to become widely available, in the late 1960s, was the bipolar Fairchild μA709, created by Bob Widlar in 1965;

• it was rapidly superseded by the 741, which has better performance and is more stable and easier to use.

• The μA741 is still in production, and has become ubiquitous in electronics — many manufacturers produce a version of this classic chip, recognisable by part numbers containing '741.'

• Better designs have since been introduced, some based on the FET (late 1970s) and MOSFET (early 1980s). Many of these more modern devices can be substituted into an older 741-based circuit and work with no other changes, to give better performance.

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• 741

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Opamp

• Op-Amp is nothing more than a differential amplifier that amplifies the difference between two inputs. One input has a positive effect on the output signal, the other input has a negative effect on the output.

• The Op-Amp requires two power supplies; a positive voltage supply and a negative voltage supply, both with respect to our circuit ground/Earth/chassis connection.

• The theoretically perfect Op-Amp has an infinite voltage gain, an infinite bandwidth and infinite input impedances. In this way it just senses an input voltage level without actually interfering with that voltage in any way.

• The perfect Op-Amp also has a zero-Ohm output impedance. It may therefore be used to drive heavy (in electronic terms) circuits.

• A typical operational amplifier has input impedances of 100M-ohms, 1-ohm output impedance and will drive up to 20mA of output current. Supply voltages may be as high as +20V and -20V (total 40 volts from +ve supply to -ve supply).

• The typical voltage gain/bandwidth of a normal Op-Amp is about 1 000 000. This is to say that at DC it will have a voltage gain of one Million, but at 1MHz it will only have a voltage gain of 1.

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Opamp

• The basic function of the Op-Amp is to multiply a voltage level by the gain of the amplifier. If you were to couple a DC level of +1v into the + input of our Op-Amp then the output would be 1v X 1000000 or one Million volts. The output, however, cannot exceed the supply voltage, so the output will be +20v DC.

• If you were to couple a DC level of +1v into the - input of our Op-Amp then the output would be -1v X 1000000 or MINUS one Million volts. The output, however, still cannot exceed the supply voltage, so the output will be -20v DC.

• If you were to couple a DC level of +1v into both the - and + inputs of our Op-Amp then the output would be (-1v X 1000000) plus (1v X 1000000) = 0v.

• In other words, both inputs act on the output simultaneously and the output is the sum of both input functions. If both inputs are identical then the output should always be zero. – This is a good test for an Op-Amp. If you connected both inputs

to the same input, then the output SHOULD be zero volts. In reality, there are small differences in the circuit's characteristics and components, this will result in a small 'offset' voltage.

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• Symbol

• Ideal opamp– large forward transfer function,– virtually nonexistent reverse transfer function,– large input impedance (any signal can be supplied to the op-amp without loading

problems)

RIN → ∞

– small output impedance, (the power supplied by the op-amp is not limited), ROUT → 0

– wide bandwidth, and– infinite (open-loop) gain . A → ∞

Operational amplifier as an electronic element

linear amplifier – the output is directly proportional to the amplitude of input signal.

open-loop gain, A - the voltage gain without feedback (~106)

closed-loop gain, G - the voltage gain with negative feedback

negative feedback - the output is connected to the inverting input forming a feedback loop (usually through a feedback resistor).

AVVGVVV openloopOUT

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Operational amplifier as an electronic element

• Two rules can be used to analyze op-amp circuits:– Rule 1: The input currents I+ and I- are zero, ( I+ = I- = 0

{RIN→∞} )

– Rule 2: The voltages (input potentials) V+ and V- are equal, ( V+ = V- {A→∞} )

• To apply these rules requires negative feedback.

• Real op-amps can only approximate to the ideal – but in analysis we can take them as ideal in first approximation

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Measuring (testing) opamps

• We can't measure to find out the pin function – must read datasheet

• The input impedance of an OP-AMP is very high and probing either input with a multimeter or CRO [cathode ray oscilloscope] will change the voltage on the input and alter the state of the output. The reason is this: The voltage on either input is extremely critical.

• It is also impossible to measure the difference in potential between the inverting input and non-inverting input. Thus the normal method of probing and testing an OP-AMP with a multimeter or CRO DOES NOT WORK!

• Don't be tricked by a CRO. It puts a load on the OP-AMP and if the line under investigation is HIGH IMPEDANCE, the CRO will affect the amplitude of the signal. The amplitude on the display will be reduced (attenuated) as the frequency increases.

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Measuring (testing) opamps

• If you suspect an op-amp is faulty, check it by substitution but first:

– Is each pin of the op-amp connected? - Check visually that none of its pins have become wrapped under its body instead of being inserted into the board.

– Is the output finite? - If the output is within a volt or so of either power rail then either it has failed, or there is an excessive voltage at its inputs.

– Is the input consistent with the output? - Measure the voltage between the inverting and non-inverting inputs, it should be within millivolts of zero

• There is actually very little that can go wrong in an opamp based circuit. Opamps usually work or they don't - intermittent states can occur, but are very uncommon.

• Almost all faults with a newly built opamp based circuit will be the result of wiring mistakes.

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• Type of a power supply - having a ground, positive and negative terminal is known as a symmetric, split, bipolar or dual voltage (power) supply. – On the other hand, using a single battery would

be considered using a single or unipolar supply.

• Positive and negative terminals of a symmetric source are also known as supply rails.

• Opamp needs a symmetric power supply - to account for amplifying negative voltages, as well as for compensating for the transistor turn-on voltage (0.6V) for input voltages close to 0.

Symmetric (dual, split) power supply

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• In opamp design, this issue is adressed in two ways - either a design of a symmetric power section; or designing the opamp circuit to work with a single supply


– Voltage divider could be used as a symmetric converter, but isn't stable

– Redesigning for single supply means changing the operating point, and the values of the signals involved

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• A converter using an opamp (based on voltage divider, but more stable)


– Split supply – Single supply

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• The simplest use of an operational amplifier is as a comparator - Without external components the op-amp functions as a comparator.

• Equality of input values is very difficult to achieve in practice.• The speed at which the change in output results from a change in

input (often called the slew rate in operational amplifiers) is typically in the order of 10ns to 100ns, but can be as slow as a few tens of μs.

• A dedicated voltage comparator chip, such as the LM339, is designed to interface directly to digital logic (for example TTL or CMOS). – Circuits specially built for use as comparators, such as the

LM311 and the LF311, are not op-amps, and cannot be used as op-amps. Among other things, they would not be stable.

Basic circuits - comparator

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• Note that you cannot compare with ground, in a single supply opampcomparator

Basic circuits - comparator

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• Voltage follower implements a negative feedback by connecting the output to the inverting input.

• This is a 'total' feedback, the amplification is 1 (also known as unity gain) - that is, the input is duplicated (followed) at the output.

• the benefit is that the original source of the input voltage now 'sees' the near infinite input resistance of the opamp

Voltage follower (buffer)

0 II

0 dUVV VVU d

VV i

VV o

io VV

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• The circuit works because

• Growth of Vi as amplified as AV+, which tends to infinity, but – as whole of the output is brought to the inverting input, which

figures with a minus sign - the infinite gain is compensated (as Vd tends to zero).

• However, if the inverting and non-inverting input switch places, there is no more voltage following

– growth of input Vi goes to inverting input, and shows up as -AVi in the output expression.

– That negative voltage then comes back on the non-inverting input, so again we have -AVi in the output expression.

– So it turns out that Vo = -2AVi, which basically means that the opamp will immediately show the negative supply voltage as output

– you cannot make an inverting follower, by simply switching the inverting and non-inverting inputs in the regular voltage follower.

• in general true for all opamp circuits - the non-inverting and inverting input are not reversable.


0VVAVVAUAV ido

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• Most sensor interfaces take a range of voltages as input, and convert them into a digital representation.

• If you make your own sensors, or use sensors not perfectly matched to your system, you can usually improve the quality of your data by scaling the voltages into the precise range needed by your interface.

• The voltage follower is an extremely simple circuit that simply outputs a low impedance voltage that is identical to the input.

• This would be fairly useless, except that it changes high impedance inputs to low impedance, and makes the signal stronger. Used in conjunction with an inverting op amp, it can be a simple way to condition your signals.

• A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another. If the voltage is transferred unchanged (the voltage gain is 1), the amplifier is a unity gain buffer; also known as a voltage follower.


a noticeable distortion for single supply: the output (red) stays at around 0.2V, when the input is below 0.2V ; the reason is the base-emitter turn-on voltages of the differential section.

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• The inverting amplifier:

Opamp amplifiers - inverting

0 II

0 dUVV VVU d

– Since I+ = 0, then I1 = I2 ( KCL: +I2 -I- -I1 = 0 )

– Since V+ is connected to ground, and the differential voltage Vd is zero, then V- must also keep the ground potential (zero).

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• The non-inverting amplifier:

Opamp amplifiers - non-inverting

– Since I+ = 0, then I1 = I2 ( KCL: +I2 -I- -I1 = 0 )

– Since Vi is connected to V+, and the differential voltage Vd is zero, then V- must also keep the potential Vi.

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• no hydraulic analogy for opamps as such - there is still another physical analogy for inverting opamp amplifier

• Changing the resistor ratio R2/R1 changes the gain of the amplifier circuit, just as – changing the fulcrum position on the lever changes its

mechanical displacement 'gain.'

Analogy for divided feedback in opamps

– If we draw a lever diagram next to the amplifier schematic, with the distance between fulcrum and lever ends representative of resistor values,

• the motion of the lever will signify changes in voltage at the input and output terminals of the amplifier.)

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• The summing amplifier:

Summing amplifier (summer, mixer)

– Since I- = 0, then I2 = I11+I12+I13+… ( KCL: +I2 -I- -I11 -I12 -I13 - ... = 0 )

– Since V+ is connected to ground, and the differential voltage Vd is zero, then V- must also keep the ground potential (zero).

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• Differential opamp amplifier:

Differential opamp amplifier

– (KCL node V-)

– (KCL node V+)

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– V+ is set by a voltage divider from V2

– since If = -I1

– since V- = V+

– When R1=R2 and Rf=Rg, we have amplified difference:

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• Instrumentation amplifier:

Instrumentation amplifier

– points Vo1 and Vo2 are inputs of the differential amplifier, so write directly the expression for the output voltage

–I+ and I- must be zero, single current Ix flows through R1, Rgain and second R1. We can write three expressions for this current

– we can express Vo through Ix

– we can express V2-V1 through Ix

– replace this Ix, into the expression for Vo; and we obtain dependency directly between Vo and V2-V1

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• The operational amplifier does not have a 'sensor' conterpart, it is almost unavoidable for sensor circuits.

Sensing application

– High impedance transducers such as piezoelectric sensors, hydrophones, and some accelerometers require an amplifier that converts a transfer of charge into a voltage change.

– when interfacing strain gauges, commonly a Wheatstone bridge is used – it must be amplified with a differential amplifier; more commonly, an instrumentation amplifier is used with a strain gauge Wheatstone bridge.

st12 – operational amplifier 1 operational amplifier lecturer: smilen dimitrov sensors technology...

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