Operational Transconductance Amplifiers:Noise Behavior of the OTA.

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Introduction

In many analog or mixed analog–digital VLSI applications, an operational amplifier may not be appropriate to use for an active element. For example, when designing integrated high-frequency active filter circuitry, a much simpler building block, called an operational transconductance amplifier (OTA), is often used [1]. This type of amplifier is characterized as a voltage-driven current source and in its simplest form is a combination of a differential input pair with a current mirror as shown in Figure 25.1. It is a simple circuit with a relatively small chip area. Further, it has a high bandwidth and also a good common- mode rejection ratio up to very high frequencies. The small signal transconductance, gm = Iout / Vin, can be controlled by the tail current. This chapter discusses CMOS OTA design for modern VLSI applications. We begin the chapter with a brief study of noise in OTAs, followed by OTA design techniques.

Noise Behavior of the OTA

The noise behavior of the OTA is discussed here. Attention will be paid to thermal and flicker noise and to the fact that, for minimal noise, some voltage gain, from the input of the differential pair to the input of the current mirror, is required. Then, only the noise of the input pair becomes dominant and the other noise sources can be neglected to first order. The noise behavior of a single MOS transistor is modeled by a single noise voltage source. This noise voltage source is placed in series with the input (gate) of a “noiseless” transistor. Figure 25.2(a) shows the simple OTA, including the noise sources, while Figure 25.2(b) shows the same circuit with all the noise referred to the input of the stage.

Operational Transconductance Amplifiers-0312

Operational Transconductance Amplifiers-0313

where I0 represents the tail current of the differential pair. Note that the term between brackets represents the relative noise contribution of the current mirror. This term can be neglected if M3 and M4 are chosen relatively long and narrow in comparison to M1 and M2.

It should be mentioned that the thermal noise of an N-MOS transistor and a P-MOS transistor with equal transconductance is the same. In most standard IC processes, a 3–10 times lower 1/f noise is observed for P-MOS transistors in comparison to N-MOS transistors of the same size. However, in modern processes, the 1/f noise contribution of N- and P-MOS transistors tends to be equal.

For the 1/f noise, it is usually assumed for standard IC processes that

Operational Transconductance Amplifiers-0314

Here, the noise contributions of the current mirror (M3, M4) will be negligible if L3 is chosen much larger than L1.

The offset voltage of a differential pair is lowest when the transistors are in the weak-inversion mode; but on the contrary, the mismatch in the current transfer of a current mirror is lowest when the transistors are deep in strong inversion. Hence, the conditions that have to be fulfilled for both minimal equivalent input noise and minimal offset are easy to combine.

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