Operational Transconductance Amplifiers:An OTA with an Improved Output Swing.

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An OTA with an Improved Output Swing

A CMOS OTA with an output swing much higher than that in Figure 25.1(a) is shown in Figure 25.3. This configuration needs two extra current mirrors and consumes more current, but the output voltage “window” is, in the case when common-mode input voltage is zero, about doubled. The rules discussed earlier for sizing the input transistors and current-mirror transistors to reduce noise and offset still apply. However, there is still a tradeoff. On the one hand, a high voltage gain from the input nodes to the current mirror is good for reducing noise and mismatch effects; in contrast, too much gain also reduces the upper limit of the common-mode input voltage range and the phase margin needed to ensure stability (this will be discussed later) [2]. A voltage gain on the order of 3–10 is advised. The frequency behavior of the OTA in Figure 25.3 is rather complex since there are two different signal paths in parallel, as shown in Figure 25.4. In this scheme, rp represents the parallel value of the output resistance of the stage (ro6 || ro8) and the load resistance (RL); therefore,

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The capacitor Cp represents the sum of the parasitic output capacitance and the load capacitance Cp = Co + CL. Using the half-circuit principle for the differential pair, a fast signal path can be seen from M2 via current mirror M7, M8 to the output. This signal path contributes an extra high- frequency pole. The other signal path leads from transistor M1 via both current mirrors M3, M4 and M5, M6 to the output. In this path, two extra poles are added. The transfer of both signal paths and their combination are shown in the plots in Figure 25.5, assuming equal pole positions of all

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When the OTA is used for high-frequency filter design, an integrator behavior is required, that is, a constant 90° phase at least at frequencies around wT. Therefore, a high value of the ratio w2/wT is needed to have as little influence as possible from the second pole. It is obvious from Eq. (25.12) that the low-frequency voltage gain from the input nodes of the circuit to the input of the current mirrors (=gm1/gm3) must not be chosen too high. As mentioned, this is in contrast to the requirements for minimum noise and offset.

Sometimes, OTAs are used as unity-gain voltage buffers; for example, in switched capacitor filters. In this case, the emphasis is put more on obtaining high open-loop voltage gain, improved output window, and good capability to drive capacitive loads efficiently (or small resistors); its integrator behavior is of less importance.

To increase the unloaded voltage gain, cascode transistors can be added in the output stage. This greatly increases the output impedance of the OTA and hardly decreases the phase margin. The penalty that has to be paid is an additional pole in the signal path and some reduction of the maximum possible output swing. This reduction can be very small if the cascode transistors are biased on the weak-inversion mode. The open-loop voltage gain can be on the order of 40–60 dB. A possible realization of such a configuration is shown in Figure 25.6 [3].

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