Many single-supply operational amplifiers are capable of rail-to-rail input operation, if they have the proper input circuitry to support this function. This article is an expansion of the blog post “What does ‘rail to rail’ input operation really mean?
” By examining an additional input-stage topology and implementing these op amps into some common op-amp circuits, you will see what I mean.
The article discusses a composite input-stage topology for single-supply op amps. You may recall that this topology exhibited an offset-voltage, crossover distortion as the input common-mode voltage traveled across the full rail-to-rail input voltage range. Switching from one differential input stage to the other causes this distortion.
Revisiting single-supply input topologies
The CMRR (common-mode rejection ratio) specification describes changes in the amplifier’s offset-voltage versus common-mode input changes. The specification conditions can subtly describe the amplifier’s rail-to-rail input topology. A typical common-mode rejection specification for an amplifier with a composite input stage has two or more CMRR specifications (Table 1
Table 1 The single-supply op-amp data sheet describes the CMRR test conditions as well as the actual specified values. These specification tables provide evidence of the characteristics of the input-stage topology, which in this case is an amplifier with a composite input stage.
In Table 1
, the CMRR is equal to 20*log (ΔVCM
). Line 1 verifies that the amplifier has true rail-to-rail input capability. Line 2 specifies the CMRR from ground to 1.8V below the positive power-supply rail as 92 dB (typ). Note that this specification does not describe the range specified in Line 1. Line 3 specifies the CMRR across the entire input range as 70 dB (typ, VS
=5V). This disparity in specification only points out that the amplifier’s input stage has a composite input topology.
There are more ways to tackle this input topology problem with the IC design. The composite input stage is one example. As a second IC-design strategy, a unique zero-crossover input topology provides superior common-mode performance over the entire input range.
In Figure 1
, a regulated charge pump lifts the top of the differential input as well as its biasing current source to 1.8V above the power-supply voltage, VS
. This increased headroom allows for a single differential PMOS input stage to replace the composite differential input stage. A typical common-mode rejection specification for an amplifier with a charge-pump input stage has one CMRR specification. This specification encompasses the full amplifier input range.
Figure 1 This single PMOS differential input stage in conjunction with a high-side charge-pump eliminates the common-mode crossover distortion found in composite input stages.Figure 2
compares the response of the charge pump to composite amplifier inputs. The difference between these two topologies in this figure
Figure 2 The op-amp offset voltage of an op amp with a charge-pump input remains constant across changes in the common-mode voltage. The offset voltage of an op amp with a composite input varies across changes in the common-mode voltage.
The op-amp topology in Figure 1
provides superior common-mode performance over the entire input range. In fact, the input range extends at least 100 mV beyond both power-supply rails. You may think that the charge pump’s output ripple voltage will become a noise problem. However, the charge-pump design can provide an output ripple voltage that is low enough so as to not produce undesirable noise or distortion at the output of the op amp.Application solutions
Circuit designers can use rail-to-rail input op amps in virtually any op-amp configuration. To achieve optimum performance, however, circuit designers need to consider the behavior of the single-supply op-amp input stage. Let’s consider the behavior of the composite and charge-pump amplifier inputs in a single-supply inverting amplifier, noninverting amplifier, and buffer circuits.
In many applications, the amplifier’s common-mode input voltage can remain at a static voltage. This is true for the inverting-amplifier circuit found in Figure 3
. The inverting-amplifier circuit gains the input signal as well as changes the signal polarity.
Figure 3 The inverting-amplifier circuit keeps the amplifier’s common-mode voltage at a constant voltage, VB.
The circuit requires a bias voltage, VB
, to keep the output range at VO
between the power-supply rails. When VB
establishes the common-mode voltage of the amplifier, the transfer function for this circuit is expressed as
voltage can be anywhere between ground and VS
, as long as the combination of the elements in this circuit (VB
, and VIN
) keeps the output (VO
) between VS
and ground. Since the VB
voltage is static, the amplifier’s common-mode voltage remains constant. When using composite input amplifiers, choose the VB
voltage to be below or above the PMOS/NMOS transition region.