A Differential Op-Amp Circuit Collection

A Differential Op-Amp Circuit Collection

All op-amps are differential input devices. Designers are accustomed to working with
these inputs and connecting each to the proper potential. What happens when there are
two outputs? How does a designer connect the second output? How are gain stages
and filters developed? This application note will answer these questions and give a
jumpstart to apprehensive designers.

The idea of fully-differential op-amps is not new. The first commercial op-amp, the K2-W,
utilized two dual section tubes (4 active circuit elements) to implement an op-amp with
differential inputs and outputs. It required a ±300 Vdc power supply, dissipating 4.5 W of power,
had a corner frequency of 1 Hz, and a gain bandwidth product of 1 MHz(1).
In an era of discrete tube or transistor op-amp modules, any potential advantage to be gained
from fully-differential circuitry was masked by primitive op-amp module performance. Fullydifferential
output op-amps were abandoned in favor of single ended op-amps. Fully-differential
op-amps were all but forgotten, even when IC technology was developed. The main reason
appears to be the simplicity of using single ended op-amps. The number of passive components
required to support a fully-differential circuit is approximately double that of a single-ended
circuit. The thinking may have been "Why double the number of passive components when
there is nothing to be gained?"
Almost 50 years later, IC processing has matured to the point that fully-differential op-amps are
possible that offer significant advantage over their single-ended cousins. The advantages of
differential logic have been exploited for 2 decades. More recently, advanced high-speed A/D
converters have adopted differential inputs. Single-ended op-amps require a problematic
transformer to interface to these differential input A/D converters. This is the application that
spurred the development of fully-differential op-amps. An op-amp with differential outputs,
however, has far more uses than one application.

BASIC CIRCUITS
The easiest way to construct fully-differential circuits is to think of the inverting op-amp feedback
topology. In fully-differential op-amp circuits, there are two inverting feedback paths:
. Inverting input to noninverting output
. Noninverting input to inverting output
Both feedback paths must be closed in order for the fully-differential op-amp to operate properly.
When a gain is specified in the following sections, it is a differential gain - that is the gain at
VOUT+ with a return of VOUT-. Another way of thinking of differential outputs is that each signal is
the return path for the other.

A New Pin
Fully-differential op-amps have an extra input pin (VOCM). The purpose of this pin is to provide a
place to input a potentially noisy signal that will appear simultaneously on both inputs - i.e.
common mode noise. The fully-differential op-amp can then reject the common mode noise.
The VOCM pin can be connected to a data converter reference voltage pin to achieve tight tracking
between the op-amp common mode voltage and the data converter common mode voltage. In
this application, the data converter also provides a free dc level conversion for single supply
circuits. The common mode voltage of the data converter is also the dc operating point of the
single-supply circuit. The designer should take care, however, that the dc operating point of the
circuit is within the common mode range of the op-amp + and - inputs. This can most easily be
achieved by summing a dc level into the inputs equal or close to the common mode voltage.

Gain
A gain stage is a basic op-amp circuit. Nothing has really changed from the single-ended
design, except that two feedback pathways have been closed. The differential gain is still Rf /Rin
a familiar concept to analog designers.
Gain = Rf/Rin
This circuit can be converted to a single-ended input by connecting either of the signal inputs to
ground. The gain equation remains unchanged, because the gain is the differential gain.

Instrumentation
An instrumentation amplifier can be constructed from two single-ended amplifiers and a fullydifferential
amplifier as shown in Figure 2. Both polarities of the output signal are available, of
course, and there is no ground dependence.

FILTER CIRCUITS
Filtering is done to eliminate unwanted content in audio, among other things. Differential filters
that do the same job to differential signals as their single-ended cousins do to single-ended
signals can be applied.
For differential filter implementations, the components are simply mirror imaged for each
feedback loop. The components in the top feedback loop are designated A, and those in the
bottom feedback loop are designated B.
For clarity decoupling components are not shown in the following schematics. Proper operation
of high-speed op-amps requires proper decoupling techniques. That does not mean a shotgun
approach of using inexpensive 0.1-μF capacitors. Decoupling component selection should be
based on the frequencies that need to be rejected, and the characteristics of the capacitors used
at those frequencies.

Single Pole Filters
Single pole filters are the simplest filters to implement with single-ended op-amps, and the same
holds true with fully-differential amplifiers.
Double Pole Filters
Many double pole filter topologies incorporate positive and negative feedback, and therefore
have no differential implementation. Others employ only negative feedback, but use the
noninverting input for signal input, and also have no differential implementation. This limits the
number of options for designers, because both feedback paths must return to an input.
The good news, however, is that there are topologies available to form differential low pass, high
pass, bandpass, and notch filters. However, the designer might have to use an unfamiliar
topology or more op-amps than would have been required for a single-ended circuit.
Multiple Feedback Filters
MFB filter topology is the simplest topology that will support fully-differential filters.
Unfortunately, the MFB topology is a bit hard to work with, but component ratios are shown for
common unity gain filters.
Akerberg Mossberg Filter
Akerberg Mossberg filter topology is a double pole topology that is available in low pass, high
pass, band pass, and notch. The single ended implementation of this filter topology has an
additional op-amp to invert the output of the first op-amp. That inversion in inherent in the fullydifferential
op-amp, and therefore is taken directly off the first stage.
Biquad Filter
Biquad filter topology is a double pole topology that is available in low pass, high pass, band
pass, and notch. The highpass and notch versions, however, require additional op-amps, and
therefore this topology is not optimum for them. The single-ended implementation of this filter
topology has an additional op-amp to invert the output of the first op-amp. That inversion is
inherent in the fully-differential op-amp, and therefore is taken directly off the first stage.


Danny Camperos  CRF

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