Operational Trans conductance Amplifiers:Common-Mode Feedback

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Common-Mode Feedback

A fully differential OTA circuit, as in Figure 25.19, has many advantages compared with its single- ended counterpart. It is a basic building block in filter design. A fully differential approach, in general, leads to a more efficient current use, doubling of the maximum output-voltage swing, and an improvement of the power-supply rejection ratio (PSRR). It also leads to a significant reduction of the total harmonic distortion, since all even harmonics are canceled out due to the symmetrical structure. Even when there is a small imperfection in the symmetry, the reduction in distortion will be significant.

However, this type of symmetrical circuit needs an extra feedback loop. The feedback around a single- ended OTA usually only provides a differential-mode feedback and is ineffective for common-mode signals.

So, in the case of the fully differential OTA, a common-mode feedback (CMFB) circuit is needed to control the common output voltage. Without a CMFB, the common-mode output voltage of the OTA is not defined and it may drift out of its high-gain region. The general structure of a simple OTA circuit with a differential output and a CMFB circuit is shown in Figure 25.21. The need for a CMFB circuit is a drawback since it counters many of the advantages of the fully differential approach. The CMFB circuit requires chip area and power, introduces noise, and limits the output-voltage swing.

Figure 25.22(b) shows a simple implementation of a CMFB circuit. A differential pair (M1, M2) is used to sense the common-mode output voltage. So, the voltage at the common source of this differential pair (Vs) is used. Its voltage provides, with a level shift of one VGS, the common-mode output voltage of the OTA. The voltage at this node is the first-order insensitive to the differential input voltage. The relationship between the differential input voltage Vin of the differential pair, superimposed on a common- mode input voltage VCM, and its common-source voltage Vs is shown in Figure 25.22(a). The common- mode output voltage of the OTA is determined by the VGS of M1/M2 and M9/M10 and can be controlled by the voltage source V0. There might be an offset in the DC value of the two output voltages due to a mismatch in transistors M9 and M10.

If the amplitude of the differential output voltage increases, the common-mode voltage will not remain constant, but will be slightly modulated by the differential output voltage, with a modulation frequency

Operational Transconductance Amplifiers-0329

Operational Transconductance Amplifiers-0330

that is twice the differential input signal frequency. This modulation is caused by the “nonflat” charac- teristic of the Vs versus Vin characteristic of the differential pair (M1, M2) (see Figure 25.22(a)).

Another commonly used CMFB circuit is shown in the fully differential folded cascode OTA in Figure 25.23 [13]. In this circuit, a similar high-output resistance and high unloaded voltage gain can be achieved as in the normal cascode circuits. An advantage of the folded cascode technique, however, is a higher accuracy in the signal-current transfer because current mirrors are avoided.

In Figure 25.23, all transistors are in saturation, with the exception of M1, M11, and M12, which are in the triode region. The CMFB is provided with the help of M11 and M12. These two transistors sense the output voltages VP and VQ. Since they operate in the triode region, their sum-current is insensitive to the differential output voltage (VP – VQ) and depends only on the common output voltage ((VP+VQ)/2). Because the current that flows through M17 and M18 forces the value of the above- mentioned sum-current, they also determine, together with Vbias4, the common-mode output voltage. By choosing Vbias1 in such a way that IM19 is twice IM17, and making the width of transistor M1 twice that of M11 (=M12), the nominal common-mode output voltage will be equal to the gate voltage of M1.

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