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Feature article
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.
68
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 tele
phone 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.
Practical Electronics | March | 2025
All About Op Amps
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.
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
Practical Electronics.
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 control,
Practical Electronics | March | 2025
69
Feature article
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
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, highpass, 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.
70
Title
Pages
7 February 1928
CA277770A Wave signalling system
Split into serial numbers 411223 & 411224 below
2,102,670 Wave Translation System
21 December 1937
2,003,282 Wave Translation System
4 June 1935
NA
Not granted Wave Translation System
1,920,238 Wave Translating System
1 August 1933
2,102,671 Wave Translation System
21 December 1937
2,002,499 Wave Translation System
28 May 1935
2,011,566 Wave Translation System
20 August 1935
2,007,172 Wave Translation System
9 July 1935
2,131,365 Wave Translation System
27 September 1938
2,098,950 Vacuum Tube Circuit
16 November 1937
2,033,917 Electric Wave Amplifying System
17 March 1936
2,131,366 Electric Wave Amplifying System
27 September 1938
2,209,955 Wave Translation System
6 August 1940
2,154,888 Wave Translation System
18 April 1939
2,223,506 Wave Amplification
3 December 1940
2,245,565 Wave Translating System
17 June 1941
2,258,128 Wave Translating System
7 October 1941
2,284,555 Signaling System
26 May 1942
2,324,815 Stabilized Feedback System
20 July 1943
NA
NA
21
12
NA
17
87
10
7
6
12
5
5
5
29
5
7
5
9
8
7
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.
Practical Electronics | March | 2025
All About Op Amps
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
Practical Electronics | March | 2025
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
amp-based 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.
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.
71
Feature article
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. Howev-
er, 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
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.
72
Practical Electronics | March | 2025
All About Op Amps
►
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.
supply to a section of the circuit, 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 openloop 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 SallenKey low-pass filter shown in Fig.12.
This has a -12dB/octave slope, compared to -6dB/octave for the RC filter,
Practical Electronics | March | 2025
using just one op amp. It also allows
gain to be applied.
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 pemag.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 pemag.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 highpass 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 pemag.au/link/ab8w
9) Active bandpass filter
A second-order bandpass filter can
be created by combining active second-
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 pemag.
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 can be useful in circuits like
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.
73
Feature article
graphic 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 the 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).
74
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.
Practical Electronics | March | 2025
All About Op Amps
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 in-
terchangeable, 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
PE
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.
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