MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 16

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Working Your Amplifier Inside The Single-Supply Voltage “Box”
By Bonnie C. Baker, Microchip Technology Inc.
It may seem easy enough to transfer classical dual-supply,
operational-amplifier circuits directly into a single-supply
environment. This is true for a few circuits, but now your amplifier
output is swinging much closer to the supply rails than before,
and your ground reference has disappeared. In this article, we
are going to examine standard voltage-feedback, amplifier circuits
(such as non-inverting gain) inverting gain, difference amplifiers,
instrumentation amplifiers and a photo-sensing configuration.
Within these discussions, we will explore the advantages and
limitations of working inside the single-supply environment.
Issues to Consider When You Convert from
Dual Supply to a Single Supply
You could say that an operational amplifier is an operational
amplifier, regardless of the supply voltage that is used.
The general characteristics from one device to another are
consistent. All voltage-feedback amplifiers have two high-
impedance inputs and a low-impedance output. When in a
closed-loop circuit, the voltage of the two inputs of the amplifier
track each other. The amplifier open-loop gain from input to
output is usually above 80 dB or 10,000 V/V. These general
characteristics are not the issues that get you into trouble when
you convert from dual-supplies to a single-supply. In this article,
the voltage of the dual supplies is ±15V and the voltage of the
single supply is 0-5V.
You should focus on two performance characteristics when
performing this conversion. These two issues are the input-
voltage range and output-voltage swing. The input and output
characteristics are fully specified in single-supply operational
amplifier data sheets. Generally, if you are converting to a single-
supply amplifier circuit these specifications, from amplifier to
amplifier, are very close. However, if you don’t account for these
performance limitations, you will drive the input or output of your
amplifier well outside the operating ranges. Outside these ranges
the “good” and the “best” single-supply amplifiers act the same.
In this discussion, it is presumed that you understand the circuit
design topologies of single-supply amplifiers. “Operational
Amplifiers Part 1 of 6: What Does “Rail-to-Rail” Input Operation
Really Mean?”
single-supply amplifier input stages. If you read this article,
you will find that when common-mode voltage goes beyond the
capabilities of the input transistors the output of the amplifier will
latch to either rail. “Operational Amplifiers Part 2 of 6: Working
with Single-Supply Operational-Amplifier Output Characteristics”
discusses the details of single-supply amplifier output-stage
performance. This article defines two areas of the amplifier
output-stage operation. If you drive your amplifier “hard” into
either rail (within a few 10s of millivolts), the amplifier will leave
its linear region. If you only drive your amplifier a few hundred
millivolts from the rails (as defined in the open-loop gain
specification), the amplifier will perform in its linear region.
We will concentrate on these two performance characteristics.
You will find that this gives you enough guidance to successfully
convert all of your amplifier dual power-supply circuits to single
supply. And, as stated before, besides these two areas, an
amplifier is an amplifier.
14
Analog and Interface Guide – Volume 2
Operational Amplifiers
(1)
discusses the details of the topology of the
(2)
The Buffer Amplifier Configuration
The buffer is the easiest amplifier circuit to transfer from a
dual-supply to a single-supply environment. Figure 1 shows an
amplifier configured as a buffer.
Figure 1: The amplifier buffer operates with the same circuit
topology for a dual or single-supply application.
When an amplifier is configured using a single-supply source,
there are a few unexpected limitations of the input stage and
output stage. The input stage of single-supply amplifiers can stop
the signal from entering the amplifier. If the data sheet of the
amplifier does not claim rail-to-rail operation, it most likely will
not be a rail-to-rail input amplifier. This limits your input swing. If
the input to the amplifier, V
transistor, the output travels to the positive rail.
The output stage also limits the performance of this circuit in a
single-supply configuration. With dual-supplies, the output stage
is able to perform across its full output range without distortion.
For instance, if the amplifier in Figure 1 had a ±15V supply and
the input, V
offset errors exist). This conclusion is obvious. However, if you
use 0-5V supplies on the same circuit, with an input voltage of
zero volts the output will not produce the same voltage. The
output will ride 10s of millivolts above ground. The actual limit for
this value is the low-level, output-voltage swing and dependant on
the particular amplifier you are using.
If you are counting on sending a negative voltage through this
circuit, it will not work. Again, this may seem obvious. However,
you are converting from a dual-supply to a single-supply
environment. If the electronics before the amplifier still use a
dual supply, this can be a problem.
The phenomena will also happen at the positive, output-voltage
rail. If you are using a rail-to-rail input amplifier and you drive
the output high, the output will fall short of reaching the supply
rail, or 5V. It will actually fall short by 10s of millivolts below the
supply. The amplifier’s data sheet calls out this actual limit as
the high-level output-voltage swing.
This may not seem like a significant problem unless you are
counting on using these extreme voltages in your system. For
instance, if your amplifier is driving an A/D converter that has an
input range of 0 to 5V, several codes on the bottom and top of
your digital output word will never appear.
V
IN
IN
, was 10V, V
OUT
+
IN
–
, goes above the limits of the input
would be 10V also (assuming no
V
OUT
= V
IN
V
OUT