Differential amplifiers ideally amplify only the voltage difference across their input terminals - the differential input voltage, vI - and not the average voltage across them to ground, the common-mode (CM) input voltage, vCM. However, the output voltage, vO, to ground of a real voltage diff-amp will be affected to some extent by vCM. This article explains causes for CM amplification and how it can be measured with an instrument designed for this purpose, the Floating Differential Source.
The basic one-op-amp diff-amp is shown below.
Its input voltage is vI, the difference between the voltage to ground of the + input, vI+, and the – input, vI-, which is the differential input voltage,
The common-mode input voltage is defined as the average of the two input voltages to ground;
Ground is the – terminal of the output port of the amplifier. As vCM changes, no change should occur in the output of a true differential amplifier. It should amplify only the difference voltage across its input - hence the name differential amplifier.
The one-op-amp diff-amp has two voltage dividers, each associated with one of the op-amp inputs. Their voltage transfer functions are
From them, the circuit equations are
These equations can be expressed as a simplified block diagram of the amplifier, shown below, for infinite op-amp open-loop gain: K →
For matched resistors: Ri+ = Ri- = Ri and Rf+ = Rf- = Rf , then the familiar gain formulas result from the cascaded blocks:
Real op-amps not only have a finite K; they also have a non-zero CM gain, KCM. The op-amp itself has a differential (open-loop) voltage gain of
where its differential input voltage is v+ – v- , and a non-zero common-mode voltage gain of
When the input circuit equations are substituted into the equation for vO ,
Solving for the output voltage,
The voltage gain of the diff-amp is defined as
and its CM voltage gain is
Then the output voltage can be expressed in them as
When the coefficients of v+ and v- are equated to the corresponding coefficients in the circuit equation for vO above and solved for the differential and CM gains, they are
For an ideal diff-amp with ACM = 0, the conditions from the above equation are that KCM = 0 and that the dividers are matched: Ti+ = Ti-. Resistor mismatch in the dividers can increase ACM which decreases a performance parameter defined as the common-mode rejection ratio,
The CMRR is the ratio of the differential to CM gains of the diff-amp and it expresses numerically how much more of the desired gain (Av) the amplifier has than undesired gain (ACM).
Diff-Amp Gain Errors
In 1991, a classic paper was published in the IEEE Transactions on Instrumentation and Measurement (Vol. 40, No. 4, AUG 1991), titled “Common Mode Rejection Ratio in Differential Amplifiers” by Ramón Pallás-Areny and John G. Webster. It includes the previous derivation of one-op-amp diff-amp differential and CM gains. They went further, to include in these gains divider mismatch as expressed by their resistor tolerances. The voltage gains can be expressed from each of the inputs as
The output voltage can be expressed in these gains as
The new quantities,
are the node resistances at the inputs of the op-amp. They simplify the voltage divider expressions. For instance,
which is the non-inverting diff-amp voltage gain. Similarly,
It is more convenient to use R+ and R– to simplify the divider expressions.
The diff-amp gains can be expressed in the input gains as
Solving for the input gains,
The gains that include the resistors for an op-amp with K →
with balanced inputs
Resistor error tolerance is assumed the same for all the resistors. In other words, all have the same tolerance rating of, for instance,
= +/-1 % or
= +/-0.1 %. With tolerance included, each resistor value has the form, R(1 +/-
) and whether ε is + or – depends on which sign will maximize error for a worst-case analysis. With tolerances included, the gains are
where the ideal diff-amp gain is
The CM gain is
The boxed equations are the design formulas.
With the above expressions for the gains, substituting and simplifying,
The CMRR can include resistor mismatch by separating the effects of op-amp and resistor-mismatch. Given that
and assuming that the op-amp KCM = 0, then the diff-amp CMRR is only a result of resistor mismatch:
Then dividing the CMRR expression through by (Ti+ – Ti- ), and with op-amp KCM included,
The diff-amp CMRR with an op-amp having a CMRR as given is approximately the parallel combination of the resistor-mismatch and op-amp CMRRs. For resistor mismatch alone,
A diff-amp with an ideal gain of 1 and resistor tolerance of 1 % can be expected to have a CMRR of no less than
if the op-amp CMRR, K/KCM >> 50 (≈ 34 dB). Typical op-amp CMRR ranges from 1000 (60 dB) to 105 (100 dB). Consequently, resistor mismatch can easily dominate CMRR performance. The use of 0.1 % resistors increases CMRR to about 500 (54 dB). In many cases, a trimpot is used for Rf+ to cause the Ti to become matched, thereby increasing the diff-amp CMRR immensely so that the op-amp CMRR becomes a factor.