KickStart
by Mike Tooley
Part 10: Getting to grips with low-power operational amplifiers
Our occasional KickStart series aims to
show readers how to use readily available
low-cost components and devices to
solve a wide range of common problems
in the shortest possible time. Each of the
examples and projects can be completed
I
n recent years, several chip
manufacturers have introduced
operational amplifiers (aka ‘op
amps’) in single, dual and quad
packages designed specifically for
low-power and low-voltage operation.
Since they can operate with 3.3V and
5V supplies, these devices are ideal
for use with modern microcontrollers.
The MCP6400x family of lowpower operational amplifiers
A good example of these chips is the
MCP6001/4 family of op amps from
Microchip. These widely available,
inexpensive devices are designed
specifically for use in general-purpose,
low-power applications, including
analogue signal processing circuits and
instrumentation amplifiers.
As with many similar devices, the
MCP600x family offers a (fairly standard)
gain × bandwidth product of 1MHz.
However, unlike many earlier devices,
they can support rail-to-rail input and
output voltage swings, remain stable
in the presence of moderate capacitive
loads, and operate from supply voltages
extending from as little as 1.8V to a
maximum of 5.5V. Table 10.1 shows how
the MCP600x compares with several
other popular types of op amp.
Applications
To give you plenty of food for thought,
we have provided you with a handy
in no more than a couple of hours using
‘off-the-shelf’ parts. As well as briefly
explaining the underlying principles and
technology used, the series will provide
you with a variety of representative
solutions and examples along with just
enough information to be able to adapt
and extend them for their own use.
This tenth instalment introduces
the latest generation of low-power
operational amplifiers that can be used
with supply voltages of less than 5V.
collection of sample
applications for modern
low-power op amps.
These circuits have
been designed to be as
simple as possible to
use and all of them will
operate successfully
with a low-current 5V
supply (see later for
details of how this can be
realised). We will start
with the classic fixedgain inverting amplifier
Fig.10.1. Classic fixed-gain inverting amplifier.
shown in Fig.10.1.
If you are unfamiliar with op amps,
gain will be −10. Due to the inverting
the triangular symbol for these devices
action, the output waveform will be 180°
shows two inputs, one output and
out of phase with the input (as depicted
two supply connections (positive and
by the two waveforms shown). The input
ground). Notice that one of the inputs is
impedance of the amplifier is nominally
marked ‘−’ and the other is marked ‘+’.
1kΩ (determined by the value chosen for
These polarity markings have nothing
R1). Fig.10.1 can be easily modified for
to do with the supply connections.
different gains and input resistances by
Instead, they indicate the overall phase
simply changing the resistance values.
shift between each input and the output.
The lower cut-off (−3dB) frequency (f1)
The ‘+’ sign indicates zero phase shift
is Part
determined
by to
the
values
foroperational amplifi
10: Getting
grips
with chosen
low-power
while the ‘−’ sign indicates 180° phase
C1 and R1 and can be determined from
shift. Since 180° phase shift produces
the relationship:
an inverted (ie, turned upside down)
1
0.159
=
f1=
waveform, the ‘−’ input is often referred
2p C1R1 C1R1
to as the ‘inverting input’. Similarly,
the ‘+’ input is known as the ‘nonHere, C1 is in farads and R1 is in ohms.
inverting’ input.
The upper cut-off (−3dB) frequency (f2)
The voltage gain of the inverting
is determined by the gain × band width
1
0.159
amplifier is determined by the ratio of R2
product
f 1 = for the =op amp. With a gain of
2p Cthe
2 R 2caseC 2when
R 2 R2 = R1), the
to R1. With the values shown, the voltage
unity (in
Table 10.1 Typical performance comparison of popular operational amplifiers.
Device
Open-loop
gain
Gain-bandwidth
product
Slew
rate
Input
resistance
1
0.159
Input offsetf 1 = Supply
=
current
2pvoltage
CR
CR
Supply
current
741C
106dB
1MHz
0.5V/µs
2MΩ
20nA
±15V
1.2 to 3.3mA
LM324
100dB
1.2MHz
0.5V/µs
2MΩ
3nA
3 to 32V
1.2 to 3mA
TL081
106dB
4MHz
16V/µs
103GΩ
5pA
±15V
1.4mA
MCP6001
112dB
1MHz
0.6V/µs
104GΩ
1pA
1.8 to 5.5V
100µA
58
Practical Electronics | October | 2022
Table 10.2. Voltage gain and upper cut-off frequency for
combinations of R1 and R2 in Fig.10.
R1
(kΩ)
R2
(kΩ)
Voltage
gain
Upper cut-off
frequency (kHz)
1
10
−10
100
1
22
−22
45
1
47
−47
21
1
100
−100
10
1
220
−220
4.5
1
470
−470
2.1
4.7
47
−10
100
4.7
100
−21
48
4.7
220
−47
21
4.7
470
−100
10
4.7
1000
−210
4.8
Fig.10.3. 2.5V reference supply arrangements.
upper cut-off will be approximately 1MHz. As the voltage
gain increases, the value of f2 will decrease in proportion,
as shown in Table 10.2.
The frequency response of the fixed-gain inverting
amplifier is shown in Fig.10.2. The lower and upper
cut-off frequencies are approximately 10Hz and 100kHz
respectively. Note that the output voltage is shown in
decibels (dB) relative to 1V. At the two cut-off frequencies
the output voltage will have fallen to 70.7% of its midband value (ie, 0.707V for a mid-band output voltage
of 1V).
Ensuring stability
Fig.10.4. Communications microphone op amp preamplifier.
To ensure stability (particularly in high-gain applications)
the supply to IC1 must be decoupled close to the chip
using a relatively low-value capacitor (C3 in Fig.10.1)
of typically 10nF to 100nF. An additional larger value
decoupling capacitor (C4 in Fig.10.1) should also be fitted
but this doesn’t have to be placed in very close proximity
to the chip. The value of this capacitor should typically
be in the range 10µF to 220µF.
Supply arrangements
In common with the other sample applications described
in this KickStart, the circuit arrangement shown in
Fig.10.1 requires a reference voltage supply that’s half
that of the main supply (ie, 2.5V for a main supply of
5V). This reference supply does not need to deliver any
Fig.10.5. Frequency response of the communications microphone op
appreciable current and it can be derived from a simple
amp preamplifier.
decoupled potential divider like that shown in Fig.10.3(a).
For larger applications where several op amps are involved, the
be particularly useful where a ‘spare’ (ie, unused) op amp is
arrangement shown in Fig.10.3(b) can be used. This circuit can
available within a dual (MCP6002) or quad (MCP6004) package.
Microphone preamplifier
Fig.10.2. Frequency response of the fixed-gain inverting amplifier.
Practical Electronics | October | 2022
Fig.10.4 shows a development
of the basic fixed-gain inverting
amplifier that we met earlier.
This circuit was designed
for use as a communications
microphone preamplifier to match
an impedance of around 600Ω. The
circuit provides a nominal output
of 1V for an input of 64mV at 1kHz
(a voltage gain of 24dB). The lower
cut-off frequency is determined
by C1 and R1 (as before) while
the upper cut-off frequency (f1)
is determined by C2 and R2 using
the following relationship:
59
f1=
1
0.159
=
2p C1R1 C1R1
f1=
1
0.159
=
2p C 2 R 2 C 2 R 2
Here, C2 is in farads and R2 is in ohms. With the values
used in Fig.10.4 the lower and upper cut-off frequencies are
approximately
230Hz and 2.8kHz. The measured frequency
1
0.159
=
f 1 = of the
response
preamplifier is shown in Fig.10.5.
2p CR microphone
CR
High-gain non-inverting audio amplifier
Our two previous amplifier circuits were based on an inverting
configuration (where the output is in anti-phase with respect
to the input). The circuit shown in Fig.10.6 is a non-inverting
high-gain amplifier which also has the advantage of having a
very high input impedance (determined by the value of R1).
With the values shown, the circuit of Fig.10.6 provides a voltage
gain of around 220 and an input impedance of 1MΩ. Once
again, note that the upper cut-off frequency will be limited
by the 1MHz gain × bandwidth product of the chip (and will
be around 4.5kHz with the values chosen).
Fig.10.7. Simple speaker driver.
Speaker driver
The non-inverting configuration can also make a simple speaker
driver, as shown in Fig.10.7. This circuit has a voltage gain of 1
and an input impedance of 1MΩ. The circuit provides enough
output for use as a simple audio beeper, but the audio quality is
poor and limited to only a few tens of mW (milliwatts) before
distortion becomes significant.
Variable gain high-impedance wideband amplifier
Fig.10.8 shows a variable gain high-impedance wideband
amplifier. The circuit provides a gain that is variable from 1 to
11 (a maximum gain of 21dB) and a minimum bandwidth of
around 75kHz with a lower cut-off frequency of 2Hz. This handy
arrangement is ideal for use in a variety of instrumentation
applications where a high input impedance is required.
Fig.10.8. Variable gain high-impedance wideband amplifier.
Part 10:
Getting to grips with low-power operational amplifiers
Audio
compressor
Fig.10.9 shows a simple audio compressor. For small signal
inputs the circuit exhibits a gain of approximately 18
1
0.159 action starts when the input voltage
andf 1the
= compression
=
exceeds
2pabout
C1R1 100mV
C1R1 RMS. Beyond this point the voltage
gain falls dramatically as the anti-parallel diodes, D1 and
D2 begin to conduct. The maximum output
1 of the circuit
is approximately 0.7VRMS and its transfer characteristic is
shown in 1Fig.10.10.
0.159
=
f1=
2p C 2 R 2 C 2 R 2
Sinewave oscillator
A sinewave oscillator is shown in Fig.10.11. The frequency
of operation (f) is determined by C1, R3 and C2, R4, given by:
f1=
Fig.10.9. Audio compressor.
1
0.159
=
2p CR
CR
Here, C = C1 = C2 (expressed in farads) and R = R3 = R4
(expressed in ohms).
Fig.10.6. High-gain non-inverting audio amplifier.
60
Fig.10.10. Audio compressor response.
Practical Electronics | October | 2022
Fig.10.11. Sinewave oscillator.
Fig.10.12. Precision AC-to-DC converter.
Using the values shown, the output
frequency is approximately 1kHz and
the output amplitude is 4.5V pk-pk .
RV1 should be adjusted for minimum
distortion. Note that although the
output is reasonably sinusoidal there
will be some distortion present. More
sophisticated arrangements dispense
with the anti-parallel diodes (D1 and D2)
and employ a thermistor to regulate the
gain and reduce distortion.
Precision AC-to-DC converter
Precision AC-to-DC conversion
(rectification) is another useful
application for a low-power op amp.
The circuit of Fig.10.12 uses a dual
operational-amplifier (MCP6002) and
will convert a 0 to 1VRMS AC signal
to a corresponding 0 to 1V DC output.
The circuit operates well over the entire
audio frequency range and its response
is extremely linear.
Fig.10.13. Peak level meter.
Peak-level meter
Another interesting application of a
dual low-power operational amplifier
is the audio peak-level meter shown in
Fig.10.13. This circuit is ideal for use
in a wide range of audio and recording
applications. C2 and R2 set the time
constant (2.2s) of the circuit. If a faster
decay is required, the value of either
C2 or R2 (or both) should be reduced.
Alternatively, the peak hold time can
be increased by increasing the value of
one or both of these components.
Audio mixer
Our final application is a simple fourchannel mixer shown in Fig.10.14. IC1
is configured as a unity-gain inverting
amplifier and the four input voltages
inputs are summed together at the
inverting input of IC1. The input
impedance is nominally 10kΩ for each
of the four channels. If necessary, the
value of R5 can be increased to provide
some gain. For example, values of
220kΩ or 470kΩ will result in gains of
2.2 or 4.7 respectively.
Practical Electronics | October | 2022
Fig.10.14. Audio mixer.
61
The MCP600x family
The MCP600x family is supplied in
a variety of different package styles,
including SOT, SOIC and PDIP packs, as
shown in Fig.10.15. The plastic packaged
dual-in-line versions of the MCP6002 and
Fig.10.15. MCP6001/2/4 pinouts.
Fig.10.16. Typical power supply arrangements.
MCP6004 devices will be adequate for
most purposes and easiest to use.
Power supply
arrangements
We’ve described a variety of lowvoltage applications (and bearing
in mind the MCP600x maximum
voltage of 5.5V) the problem
remains of how to obtain a 5V
supply. There are several ways in
which this can be done.
Where no other source is
available, three series-connected
1.5V alkaline dry cells can be
used, as shown in Fig.10.16(a).
Alternatively, four seriesconnected 1.2V NiMh batteries
could be employed, as depicted
in Fig.10.16(b). Another handy
power source could simply be a
USB power adapter that derives
its input from a standard AC mains
outlet and provides a regulated 5V
output, as shown in Fig.10.16(c).
Finally, if you already have a DC
supply of between +7V and +12V
available, the required 5V supply
could be derived from a simple
Fig.10.17. Improving stability.
Fig.10.18. Using MPLAB Mindi.
62
Practical Electronics | October | 2022
voltage regulator like that shown in
Fig.10.16(d). This circuit can provide an
output of up to several hundred mA, so
there’s plenty of current available for any
additional 5V circuitry.
Improving stability
Problems can sometimes arise when
op amps are used in conjunction with
capacitive loads. This problem can be
alleviated by adding some additional
stabilising resistance (Rstab) in series
with the output, as shown in Fig.10.17.
Depending on the reactive nature of the
load (determined by the values of Cload
and Rload) the value of Rstab should be
typically between 2Ω and 10Ω.
MPLAB Mindi
Microchip, the manufacturer of the
MCP600x family of chips provides
Mindi, a handy design tool that will
allow you to simulate and test a huge
variety of op amp circuits. Mindi
uses a SIMetrix/SIMPLIS simulation
environment, with options to use SPICE
or piecewise linear modelling to cover a
wide range of simulation requirements.
In addition to generic circuit devices,
the simulation interface is paired with
Microchip’s proprietary model files.
Mindi installs and runs locally on
the user’s own PC. Once downloaded,
no further Internet connection is
required, and the simulation run time
is independent of a remotely located
server. As an example, Fig.10.18
shows Mindi being used to plot
the frequency and phase response
of the communication microphone
preamplifier that we met earlier. The red
and blue plots respectively show how
phase shift and gain vary with frequency.
Going further
This section (below) details a variety
of sources that will help you locate the
component parts and further information
that will enable you to get the best out
of today’s low-power op amps. It also
provides links to relevant underpinning
knowledge and manufacturers’ data sheets.
Table 10.3. Going Further with Getting to grips with low-power operational amplifiers
Topic
Source
Notes
MCP600x
datasheet
The MCP600x datasheet can be downloaded from:
https://bit.ly/pe-oct22-mc1
Datasheets can also be found by following the links
provided on various component supplier’s websites
MCP6001,
MCP6002,
MCP6004
ICs can be obtained from several good component suppliers,
including Farnell (https://uk.franell.com), RS Components
(https://uk.rs-online.com), and Mouser (www.mouser.com)
Op amp circuit
theory
Part 5 of Electronics Teach-in 4 provides a general introduction to
op amps (from Electron Publishing: https://bit.ly/pe-oct22-eti4).
The PE Direct Book Service at electronpublishing.com has
several titles suitable for background reading on op amps
and their uses.
MPLAB Mindi
A useful PDF introduction to Mindi is available from Microchip at:
https://bit.ly/pe-oct22-mc2
MPLAB Mindi can be downloaded from:
https://bit.ly/pe-oct22-mc3
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