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Harold S. Black, Negative
Feedback and the History
of Operational Amplifiers
Op amps and negative feedback circuits are ubiquitous
today, and you would be forgiven for thinking that they
have been around forever. But there was a time when
electronics was still developing, and such devices had
not yet been invented. That changed in 1927 with the
bright idea of one clever man…
by Roderick Wall & Nicholas Vinen
O
Fig.1: Harold Black’s original hand-written notes on the principle of using
negative feedback for distortion cancellation.
38
Silicon Chip
Australia’s electronics magazine
ne of the most significant early
circuit ideas was Harold Steven
Black’s invention of negative feedback. In 1927, Harold S. Black (18981983) was on a ferry heading towards
his office in the West Street Labs of
Western Electric, the forerunner of
Bell Telephone Laboratories in New
York City.
An idea popped into his head that
would dramatically change electronic
communications, which continues to
be used to the present day.
His idea was for a negative feedback
amplifier, where the gain is accurately
set and distortion limited by feeding
part of the output signal back into the
amplifier.
Black sketched his idea on a misprinted page of his copy of the New
York Times, the only paper that he had
on him. When Black got to his office,
he had a colleague witness and sign
it – see Fig.1.
Black’s job had been trying to
improve three- and four-channel telephone amplifiers based on carrier
telephony for the last six years. For
long-distance telephone calls, repeaters had to be added to cover the distance. But these repeaters had too
much distortion, so by the time the
audio signal reached its destination,
it was unintelligible.
Black realised that amplifier distortion and noise could be reduced using
negative feedback, at the expense of
reduced amplifier gain. He later said
that he did not know what made his
idea pop into his head; it just came.
siliconchip.com.au
Fig.2: a page from one of Harold Black’s many patents regarding
negative feedback. This one is from patent 2,102,671, showing some
possible ways of building an amplifier with negative feedback using
valve(s).
Black used his new idea to design
low-distortion broadband repeater
amplifiers that were finally suitable
for long-distance telephone calls.
That allowed more channels over a
pair of wires.
His patents
Harold S. Black was granted 62 patents during his career, 18 of which
relate to negative feedback; these are
listed in Table 1. His most famous patent is number 2,102,671, which you
can view at https://patents.google.com/
patent/US2102671A
If you replace the number in that
link with the other patent numbers
(plus the “US” prefix), you can view
the relevant PDF.
This patent, titled “WAVE TRANSLATION SYSTEM”, was filed in 1932
and granted in 1937. It comes to 87
pages and includes many detailed
drawings (including circuits and plots)
and plenty of explanatory text.
One of the most important sets of
circuit diagrams (but far from the only
one!) in this patent, appearing on page
four, is reproduced in Fig.2. It shows
four different ways of implementing
his idea using ‘tubes’ or valves, the
technology of the day.
Other important plots in the patent
include gain curves, stability criteria,
equivalent circuits and several practical implementations of the technique.
control, battery monitoring, instrumentation and sometimes RF too.
The principle is used in TVs, radios,
computers, medical equipment, control circuits, measuring instruments
and mobile phones. You would find it
very hard to find an electronic appliance that does not use negative feedback.
You will see negative feedback being
used with operational amplifiers and
in discrete circuits in most issues of
Silicon Chip.
Operational amplifiers
This paved the way to the development of operational amplifiers (op
amps); essentially, a monolithic implementation of a circuit which applies
negative feedback.
Thousands of different types of op
amps are available to suit just about any
application; low-power types, highspeed types, high-gain types, precision
types, singles, doubles, quads etc.
The term “operational amplifier”
goes back to about 1943, when this
name was mentioned in a paper written by R. Ragazinni with the title
“Analysis of Problems in Dynamics”.
The paper was the work of the US
National Defence Research Council
(1940), was published by the IRE in
May 1947 and is considered a classic
work in electronics literature.
George A. Philbrick Researches introduced the K2-W valve-based generalpurpose op amp in 1952, more than a
decade before the first transistorised
version appeared (Figs.3 & 4).
The first solid-state transistor was
successfully demonstrated on December 23, 1947, but it took a while before
transistors were in widespread use.
The first series of solid-state op amps
were introduced by Burr-Brown
Research Corporation and GA Philbrick Researches Inc in 1962.
Fig.3: a popular
early valve-based op
amp, the Philbrick
Research K2-W.
The importance of
negative feedback
Almost all analog equipment manufactured today uses negative feedback. This includes circuits that handle audio signals, analog video, motor
siliconchip.com.au
Australia’s electronics magazine
August 2021 39
Table.1: Harold S. Black’s patents relating to negative feedback (patent numbers are hyperlinks)
When filed
UNKNOWN
8 August 1928
3 December 1929
3 December 1929
26 March 1930
3 April 1931
22 April 1932
30 September 1932
29 December 1932
29 March 1933
29 March 1933
25 September 1934
6 October 1934
5 December 1936
5 December 1936
23 March 1937
10 November 1937
27 May 1938
20 December 1938
30 July 1940
28 February 1942
Serial number
UNKNOWN
298,155
411,223
411,224
439,205
527,371
606,871
635,525
649,252
663,316
663,317
745,420
747,117
114,391
114,390
132,559
173,749
210,333
246,791
348,433
432,860
The first solid-state monolithic op
amp IC, designed by Bob Widlar and
offered to the public in 1963, was the
uA702 manufactured by Fairchild
Semiconductors.
But it required strange supply voltages such as +12V and -6V and had
a tendency to burn out. Still, it was
the best in its day, and sold for about
US$300 (a fortune today!). It was used
mainly by the US military due to its
high cost.
Then the uA709 from Fairchild
Semiconductor was released in 1965.
It was introduced at about US$70, and
was the first to break the $10 barrier,
then not much later, the $5 barrier.
By 1969, op amps were selling for
around $2 each. From then on, multiple manufactures produced op amps in
When issued
Patent number Title
7 February 1928
CA277770A Wave signalling system
Split into serial numbers 411223 & 411224 below
21 December 1937
2,102,670 Wave Translation System
4 June 1935
2,003,282 Wave Translation System
NA
Not granted Wave Translation System
1 August 1933
1,920,238 Wave Translating System
21 December 1937
2,102,671 Wave Translation System
28 May 1935
2,002,499 Wave Translation System
20 August 1935
2,011,566 Wave Translation System
9 July 1935
2,007,172 Wave Translation System
27 September 1938
2,131,365 Wave Translation System
16 November 1937
2,098,950 Vacuum Tube Circuit
17 March 1936
2,033,917 Electric Wave Amplifying System
27 September 1938
2,131,366 Electric Wave Amplifying System
6 August 1940
2,209,955 Wave Translation System
18 April 1939
2,154,888 Wave Translation System
3 December 1940
2,223,506 Wave Amplification
17 June 1941
2,245,565 Wave Translating System
7 October 1941
2,258,128 Wave Translating System
26 May 1942
2,284,555 Signaling System
20 July 1943
2,324,815 Stabilized Feedback System
many varieties, up to the present day.
One particularly popular model was
the uA741, which has been improved
since it was first introduced in 1968.
Some variants of it, such as the LM741,
are still being produced today! Its
equivalent circuit is shown in Fig.5.
Modern op amps mostly use the same
principles, but differ in some implementation details, such as the method
of internal frequency compensation.
One big benefit of the op amp is
its flexibility. It can perform a wide
range of analog ‘functions’ with the
addition of a few passive components.
These functions include signal mixing, amplification, filtering (low-pass,
high-pass, bandpass, notch etc), integration, differentiation, multiplication, simulated inductance and more.
Fig.4: the K2-W uses a similar configuration to transistor-based op amps, with
an input pair (one 12AX7 twin triode) followed by a voltage amplification/
buffering stage made from another 12AX7 twin triode plus two neon lamps.
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Australia’s electronics magazine
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You can think of op amps as the
building blocks for most analog circuits.
Negative feedback
So how is negative feedback used to
control an op amp to reduce the distortion and set a fixed gain?
The output voltage of an op amp is
the non-inverting input voltage minus
the inverting input voltage times a
large factor (in some cases, over one
million). If we say the gain is exactly
one million, this means that:
• If the + input is 100μV and the
− input is 99μV, the output will
be +1V.
• If the + input is 100μV and the −
input is 100μV, the output will
be 0V.
• If the + input is 100μV and the −
input is 101μV, the output will
be -1V.
From this, you can see that if the
difference between the input voltages
is more than a few microvolts, the
output voltage will be ‘pegged’ at one
supply rail or the other. So unless we
are using the op amp like a comparator (a possible op amp function), the
inputs will almost always be at a very
similar voltage. The negative feedback
is typically configured to ensure that
this is the case.
Let’s say we feed 10% of the output
voltage back to the inverting input and
apply 1V to the non-inverting input.
siliconchip.com.au
Fig.5: the internal circuitry of perhaps
the most ubiquitous op amp, the uA741
(actually, National Semiconductor’s
equivalent). It contains 20 transistors,
12 resistors and one ‘Miller’
compensation capacitor for stability.
When the output is less than 10V,
the voltage difference between the
inputs will be positive, so the output
voltage increases. When the output is
greater than 10V, the voltage difference
between the inputs will be negative,
so the output voltage will decrease.
Thus, the output voltage will tend
towards 10V.
The only real sources of error in a
DC context are the input offset voltage
(the output not being exactly 0V with
both inputs at the same voltage) and
the finite gain, which adds a few additional microvolts of error. But that’s
just one part per million or so.
So it is pretty close to an ideal amplifier with fixed gain; that is certainly not
the case with a typical single-transistor
or single-valve amplifier! Due to manufacturing tolerances, it is challenging
to set up (bias) a single transistor or
valve to provide an exact gain. Even
if you achieve it (eg, by trimming), it
will likely change with temperature
and over time.
Note how the exact gain of the op
amp is not important; it only affects
the (tiny) gain error. The overall gain
is set by the feedback divider, usually made of resistors (and sometimes
capacitors), so it’s easy to set it close
to the desired value. It can be trimmed
to be almost exact if required, and it’s
unlikely to drift.
Negative feedback also gives close
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to ideal results for AC signals, as long
as they are well below the op amp’s
bandwidth (usually specified as gain
bandwidth, which must be divided by
the configured gain). Thus, an op ampbased amplifier can give an essentially
flat gain curve across a range of frequencies, whereas a transistor or valve
will typically be far from flat unless it
is a special type.
Here are some basic op amp circuits:
1) Unity-gain buffer
Fig.6 shows an op amp arranged
as a unity-gain buffer. The output is
fed back to the inverting input, so
the output voltage tracks the noninverting input. As the output of an
op amp has near-zero impedance (due
to feedback), but the input has a relatively high impedance, this configuration is useful to avoid the circuit
feeding the input from being loaded
Fig.6: using an op amp to buffer a
signal can be as simple as connecting
its output to its inverting input.
However, resistor Rf is a good idea
to balance the input currents if
the source impedance for the noninverting input is relatively high.
Australia’s electronics magazine
by the circuit the output is driving.
Often, the output will be connected
directly to the inverting input. But
in some cases, the resulting source
impedance mismatch between the
inputs can cause temperature drift
and other problems. Resistor Rf can
be chosen to match the non-inverting
source impedance to avoid this.
2) Non-inverting amplifier
Fig.7 shows an op amp providing
non-inverting gain. The output voltage
is an AC signal with the same shape
as the input signal but an increased
magnitude, by a factor of Rf ÷ R1 + 1.
As with the buffer, this circuit can be
connected to a signal source that has
a high impedance, but it still provides
a low-impedance output.
Capacitor C1 may be omitted, but
it’s usually a good idea to keep it. It
reduces the circuit’s gain at higher frequencies, thereby increasing stability
and preventing the amplification of
unwanted high-frequency signals.
You might see a high-value capacitor
at the bottom of the feedback divider,
between the bottom end of R1 and
ground, shown as an alternative connection for R1 in Fig.7. This sets the
circuit’s DC gain to unity regardless
of the AC gain, so it is mostly used
when amplifying AC signals; also refer
to Fig.19.
By reducing the DC gain of the circuit, it prevents the output from pegging at the positive rail on positive
signal excursions, and also reduces
the amplification of the input offset
error voltage.
The practical gain limit depends on
the op amp’s gain bandwidth and the
maximum signal frequency. For example, an op amp with a gain bandwidth
Fig.7: you only need two resistors
to set up an op amp as a fixed gain
voltage amplifier. As the signal is fed
directly into the non-inverting input,
the input impedance is high. Optional
capacitor C1 limits the bandwidth
for stability, while C2 can be used
to reduce the DC gain to unity while
having a higher AC gain.
August 2021 41
Fig.8: the inverting amplifier
configuration also uses two resistors
and one optional capacitor. While it
has the advantage that the gain can
be less than unity, the disadvantage
is that the input impedance is equal
to Rin, rather than the usually much
higher figure for the op amp’s inputs.
Fig.9: the virtual ground mixer is
an inverting amplifier with multiple
signal sources. As both op amp inputs
are held very close to 0V, there is no
way that the signals being fed in can
interact with each other, except at the
output of the mixer.
Fig.10: the basic differential
amplifier calculates the difference
between two voltages, multiplied by
a constant, plus an offset. It needs
good resistor matching.
of 3MHz has a maximum practical gain
of 30 times for signals up to 100kHz
(3MHz ÷ 100kHz). Noise and distortion in the output increase with gain,
as there is less feedback (closed-loop
bandwidth) for the op amp to work
within.
3) Inverting amplifier
By feeding a signal into the inverting input rather than the non-inverting
input, via a resistor, the signal is
inverted and gain can still be applied,
as shown in Fig.8. The gain is -Rf ÷
Rin, so unlike the non-inverting version, gain values less than unity (ie,
attenuation) are possible without a
separate input attenuator.
An unfortunate consequence of this
configuration is that the typically high
input impedance of the op amp is
reduced to the value of Rin, so the circuit feeding the input is loaded more
heavily. This can be solved by adding
a unity-gain buffer between the signal
source and the inverting amplifier.
One advantage of this configuration
is that both op amp inputs are held at
a constant voltage (Vbias), so there is
no common-mode signal and therefore
no common-mode distortion (often
the dominant distortion mechanism).
In this circuit, capacitor C1 performs
a similar role as in Fig.7, although it is
arguably more effective here since it
reduces the gain at very high frequencies to zero rather than unity.
4) Virtual ground mixer
Fig.9 shows a circuit that is basically an inverting amplifier with
multiple resistors feeding different
signals into the inverting input. As
the inverting input is held at a fixed
DC voltage by the negative feedback,
there is no possibility of cross-talk
between the signals (which might be
significant in a mixing console, where
they are fed to multiple locations).
5) Differential amplifier
This is a very useful circuit used
in many different forms. While you
can build it using regular op amps,
it is probably more widely used in
monolithic instrumentation amplifiers
(albeit in modified form), difference
amplifiers and current shunt monitors.
Fig.10 shows the basic version of
this circuit. It provides an extremely
useful function; it takes the difference
between two voltages, multiplies it by
a constant (determined by the resistor values) and then possibly adds
a positive or negative offset voltage.
However, Vref is often set to 0V so the
output voltage is referenced to ground.
This circuit needs precise resistor
matching for a good common-mode
rejection ratio (CMRR). Even with
0.1% tolerance resistors, a CMRR of
more than 60dB is difficult to guarantee. Trimming can give good results,
although the procedure can be tricky.
It’s generally better to use lasertrimmed monolithic devices like
instrumentation amplifiers (‘inamps’)
that can have CMRRs over 100dB.
Most instrumentation amplifiers
use a slightly different internal circuit
that includes three op amps; besides
having a very good CMRR, this has
the advantage that the gain can be set
using a single external resistor. However, the basic principle is the same.
A difference amplifier is basically
an instrumentation amplifier where
the input voltages can be well outside
(usually above) the device’s supply
range. A current shunt monitor is a
specialised version of an instrumentation amplifier. All are based internally
on op amps or similar circuits.
A shunt monitor allows you to place
a low-value shunt resistor in the positive supply to a section of the circuit,
Fig.11: this full-wave rectifier circuit uses op amps to effectively cancel out
the forward voltage of the diodes. As a result, for positive voltages at Vin,
Vout tracks very closely (within microvolts, given sufficiently high precision
op amps) while for negative voltages at Vin, Vout = −Vin (again, within
microvolts). This is ideal for circuits that need to sense peak signal levels,
such as audio clipping meters.
►
Fig.12: this Sallen-Key low-pass filter provides ►
a reduction in amplitude at -12dB/octave above
its -3dB frequency, and multiple stages can be
cascaded for an even steeper slope. Changing the
resistors to capacitors and capacitors to resistors
makes it a high-pass filter instead.
42
Silicon Chip
Australia’s electronics magazine
siliconchip.com.au
►
Fig.14: this active bandpass
filter blocks signals outside of a
given frequency range, although
the slopes are only -6dB/octave.
For steeper slopes (eg, -12dB/
octave), one of the active lowpass filters described above can
be connected in series with a
similar high-pass filter.
►
►
Fig.13: this multiple feedback filter does the same job as the Sallen-Key filter,
but is more effective at higher frequencies. That’s important for low-pass filters
as otherwise, it can pass signals that the filter is supposed to block. As only
one extra resistor is needed, it’s a worthwhile upgrade, and the gain can be set
without any more resistors (although it does invert the signal).
Fig.15: this Twin-T active notch filter attenuates signals at a specific frequency. Both that frequency and the
steepness/depth of the notch can be controlled by careful selection of the passive component values.
and obtain a ground-referenced voltage to feed to an analog-to-digital converter (ADC) or similar. They have a
high CMRR to reject supply ripple.
6) Precision rectifiers
A precision rectifier acts like a diode
or bridge rectifier, but without the forward voltage drop. This is important
for rectifying low-level signals (too low
to forward-bias a diode), or for accurately rectifying AC signals in order to
measure their magnitude etc. They are
commonly employed in devices like
VU meters or AC current monitors.
Fig.11 shows the full-wave version,
similar to a bridge rectifier. The halfwave version is basically just one of
the op amp/diode/resistor sections.
The op amps reduce the effective
forward voltage of the diodes by a factor of their open-loop gain, meaning
the ~0.7V drop of a standard silicon
diode is effectively less than 1μV for an
open-loop gain of around one million.
The resistor values shown result
in unity gain. This circuit originally
came from National Semiconductor
who specified R = 100kW, although
other values can be used. The values
could be changed to give a fixed gain
if necessary.
7) Active low-pass filter
The simplest way to implement a
low-pass filter with an op amp is to
combine a basic RC low-pass filter
with a unity-gain buffer. However, a
more economical arrangement is the
Sallen-Key low-pass filter shown in
Fig.12. This has a -12dB/octave slope,
compared to -6dB/octave for the RC
filter, using just one op amp. It also
allows gain to be applied.
siliconchip.com.au
Fig.13 shows a multiple-feedback
low-pass filter. This provides precisely
the same function as the Sallen-Key filter, but it is less prone to signal feedthrough, which means it performs
much closer to an ideal filter at frequencies approaching the op amp’s
bandwidth. The only disadvantage is
the use of one more resistor.
To calculate the required resistor
and capacitor values for a given cutoff frequency, go to siliconchip.com.
au/link/aajq
Note that it is possible to build a
third-order Sallen-Key active low-pass
filter using a single op amp. This will
give you an 18dB/octave roll-off with
one op amp, 30dB/octave with two
etc. This is shown at siliconchip.com.
au/link/ab8v
8) Active high-pass filter
To convert the low-pass filters
shown in Figs.12 & 13 into high-pass
filters, simply transpose the resistors
and capacitors. As with the low-pass
filters, these will provide a 12dB/
octave slope per op amp.
For both the low-pass and high-pass
filters, by adjusting the resistances and
capacitances, it is possible to design
filters with characteristics other than
Butterworth. Butterworth has minimal
(essentially no) ripple in the passband,
but different filter types such as Chebyshev trade off increased passband
ripple for a steeper roll-off beyond it.
To calculate the required component values, see siliconchip.com.au/
link/ab8w
9) Active bandpass filter
A second-order bandpass filter can
be created by combining active secondAustralia’s electronics magazine
order low-pass and high-pass filters.
Alternatively, you can use the configuration shown in Fig.14, where a single
op amp can act as a first-order bandpass filter with adjustable gain and a Q
of up to 25. This configuration inverts
the signal phase; however, if chaining
multiple filters, it can be re-inverted
by another stage.
10) Active notch filter
Fig.15 shows a “Twin-T” active
notch filter. One interesting aspect
of this design is that the Q, and thus
the depth of the notch, changes based
on the resistor and capacitor values
selected. See the online calculator at
siliconchip.com.au/link/ab8x
11) Gyrator
Fig.16 shows a ‘gyrator’, an active
element that behaves similarly to an
ideal inductor at low current values. It
does this by using the op amp’s negative feedback to effectively invert the
behaviour of capacitor C.
This is useful in circuits like graphic
Fig.16: the gyrator is a particularly
clever circuit. It uses negative
feedback to make a capacitor behave
like an inductor. It is superior to
an actual inductor in many signal
processing applications.
August 2021 43
equalisers, where resonant (LC) elements are needed with accurate resonance frequencies, low distortion
and small size. Inductor tolerances
are typically much wider than capacitors, and high-value inductors can
be very bulky, so in signal-processing
circuits, the gyrator is almost always
better than a resonant circuit based on
an actual inductor.
12) Baxandall active filter
Fig.17 shows a basic version of the
widely-used Baxandall active tone
control. It has many good properties,
such as the ability to have as many
or as few bands as you want, with no
interaction between the controls, and
no special requirements for the potentiometers. This one shows bass and
treble pots, but one or two midrange
pots can easily be added.
Fig.18 is the Baxandall active volume control. The traditional volume
control method is a logarithmic potentiometer, but dual versions usually
have poor tracking at the low end, so
they are not great for stereo circuits.
The Baxandall active circuit provides logarithmic-like control with
a linear potentiometer for superior
tracking. It can also offer significantly
better noise performance as the pot
adjusts the gain over a wide range,
from zero up to many times (as set by
the fixed resistors).
13) Audio amplifiers
Fig.19 is a simplified version of the
circuit from our SC200 audio amplifier. It is essentially a high-power op
amp with large output transistors that
can source and sink plenty of current
(and that are well heatsinked).
Most Class-A, Class-AB and similar
amplifiers are variations on this theme.
Even Class-D amplifiers typically use
some form of negative feedback to
avoid gross distortion.
14) Other uses for op amps
An op amps can be used as a basic
comparator by operating it in openloop mode, or with positive feedback (hysteresis). A comparator IC
is essentially just an op amp with
the frequency compensation component(s) removed for a faster swing at
the output.
An op amp can also be used to build
an ‘integrator’ or ‘differentiator’. An
integrator produces an output ramp
proportional to its input voltage, while
a differentiator produces an output
voltage that’s proportional to its input
ramp (rate of change).
A log amp takes the exponential
nature of a bipolar transistor and turns
it on its head using negative feedback
to provide a logarithmic transfer function. As a result, its output voltage is
a constant multiple of the natural logarithm of its input voltage.
This can be used as the basis of a
multiplier circuit; by taking the natural loge(x) of several voltages, summing
or averaging them, then exponentiating the result, the output voltage is the
product of the input voltages.
Other mathematical functions can
be applied to voltages by an op amp,
including addition, subtraction, division and inverse logarithm (the exponentiation mentioned above).
Op amps can also be used to build
controlled current sources/sinks,
including constant loads, by combining op amps with large transistors that
can handle lots of power with sufficient heatsinking.
The generalised impedance con-
Fig.17: the Baxandall tone control was initially designed with a
valve or transistor as the active element, but it works even better
with an op amp. It is elegant and expandable, with virtually no
interaction between the stages (in this case, two: bass and treble
adjustments). No matter how many bands it has, only one op amp
is required per channel (ie, two for stereo).
44
Silicon Chip
verter uses two op amps to present
a load impedance proportional to
another impedance. The ratio can be
set using fixed or variable resistors (or
even other impedances!).
Many op amps are designed to drive
relatively low load impedances, such
as 600W. These work quite well as
basic headphone drivers, with relatively low distortion figures driving
typical headphone loads, even as low
as 16W. They can’t supply a tremendous amount of power, but enough for
most headphones to deliver decent
volume, using one low-cost IC.
An op amp can also be used as an
error amplifier in feedback control. For
example, to adjust the drive to a motor
to maintain a constant speed despite
a varying load.
An op amp can form the basis of
various oscillators, to generate waveforms at fixed or variable frequencies;
primarily sinewaves, but also triangle
waves or sawtooth waveforms.
An op amp (especially a CMOS
type) can be used as a high inputimpedance buffer amplifier or guard
ring for monitoring sensors that cannot
handle any loading, such as narrowband oxygen sensors and pH sensors.
CMOS op amps can have input impedances in the terohms range (more than
one trillion ohms)!
CMOS op amp buffers can also be
combined with analog switches and
low-leakage capacitors to form sampleand-hold circuits, often used for sampling voltages over small time windows
to feed an ADC or similar.
Signal swing limitations
For a very long time, the signals at
the inputs and outputs of an op amp
Fig.18: the Baxandall volume control also places
the potentiometer in the negative feedback loop.
This gives exponential gain control with a linear
potentiometer and a wide range of gain settings
with a reasonably constant noise level.
Australia’s electronics magazine
siliconchip.com.au
could only have a considerably smaller
swing than the supply range of the op
amp. For example, if you had an op
amp running from 12V, the inputs and
outputs might be limited to a range
of 3-9V. Or, with a dual supply like
±15V, you might be limited to a signal
swing of ±12V.
That’s because the op amp’s internal
circuitry needs some voltage ‘headroom’ to operate.
But more recently, single-supply
and rail-to-rail output op amps started
to become available. Single-supply op
amps typically allow the inputs and
outputs to go down to the negative rail
(eg, 0V). So a single-supply op amp
running from 12V can handle signals
of say 0-9V.
Rail-to-rail output op amps generally have the same input limitations as
standard op amps, but their output can
swing over virtually the entire supply
range. This is especially useful when
applying gain to AC signals, as in that
case, the input swing will never reach
the rails anyway (at least, not without
‘saturating’ the op amp).
These days, rail-to-rail input/output
(RRIO) op amps are very common.
Some can even run down to very low
supply voltages, like 1.8V! These op
amps essentially remove the above
limitations, with input and output signals that can range anywhere between
the supply rails.
Some will even handle input signals
outside the rails, although usually only
in one direction (eg, positive) and by
a limited number of volts.
Note that RRIO op amps sometimes
compromise performance in other
ways, such as having higher noise or
distortion, or just costing more than
‘regular’ op amps.
Multiple op amps
As op amps became cheaper and
more versatile, dual and quad op amps
became popular. These save money
and space; a quad op amp IC often
costs less than twice what a single one
does, and only requires two power
tracks to be routed and one bypass
capacitor. Most dual (8-pin) and quad
(14-pin) op amp ICs use the same pinout so they can be interchanged.
Single op amps are not quite so
interchangeable, as these usually come
in an 8-pin package. After accounting
for the two supply rails, two inputs
and one output, the remaining three
pins can be used for trimming/balancing, external compensation capacitors
or various other functions. Some are
interchangeable (even if they don’t
have exactly the same features), but
not all.
These days, single op amps are also
available in tiny 5-pin SMD packages
for where space is at a premium.
Conclusion
The op amp is an incredibly flexible device, available these days at very
low cost and in a vast range of different versions, optimised for different
tasks. While it is possible to process
analog signals without op amps, generally, the results will be worse. So most
analog designers make extensive use
of op amps in their circuitry.
They are an essential building brick
that most designers would have difficulty doing without. We have Harold
S. Black to thank for making our lives
SC
a lot easier!
Fig.19: a slightly simplified version of our SC200 power amplifier circuit. It’s essentially a big op amp; transistors Q1 &
Q2 are the differential input pair (the inputs are at their bases), Q8 is the voltage amplification stage, Q11 & Q12 are the
output drivers and Q13 and Q15 are the power output transistors. The components highlighted in red form the negative
feedback path, from the output at the emitter resistors of Q13 & Q15 back to the base of Q2, which is the inverting input.
siliconchip.com.au
Australia’s electronics magazine
August 2021 45
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