Low-noise, low residual offset, chopped amplifiers for high-end applications

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6.1. Introduction

In the previous chapter a chopped transconductance amplifier has been presented. This amplifier is capable of reducing 1/f noise and offset by chopping up to 1MHz but the residual offset can be as high as 370m V. As explained in section 5.6.6. from Chapter 5, charge injection and parasitic coupling in the input modulator will cause spikes which after amplification and demodulation generate residual offset. In order to minimize the effect, a low output impedance voltage signal source is required. The large output impedance of the OTA, in a follower configuration, driving the input stage, generates spikes at the output which are responsible for the large residual offsets at frequencies higher than 1MHz. In this chapter we are investigated further the possibility of reducing the charge injection residual offset and the increase of chopper frequency up to 10MHz.

In some applications, an amplifier has to drive a low-ohmic load with high efficiency. Therefore, a class AB output stage is needed. The output stage introduces its own offset which is added to the total offset. If low offset is a desired constraint, the contribution of the output stage to the total offset should be minimized. For low-voltage applications large swing is a requirement (Chapter 3). This chapter focuses on the design and the realization of low voltage amplifiers with rail to rail class AB output stages capable of chopping up to 10MHZ, with low noise, high linearity and low residual offset. The generality of the method makes them suited for a large class of designs.

A chopper stabilized opamp can be used wherever offset and noise specifications are important. The chopped amplifiers presented in this chapter are primarily meant as amplifiers capable of driving headphones in portable digital audio. In those applications, extra offsets give extra dissipation in the load. Different headphones have different impedance. The impedance of headphones varies between few tens of W up to few kW . In a class AB output stage with rail-to-rail output, the low frequency gain of the amplifier depends on the load. The linearity and offset are also variables dependent on the low frequency gain. It is desired to have high linearity and low noise for all possible loads. Accuracy requires high gain which might be difficult to be accomplished in a low voltage design. We are considering also gain enhancement techniques to boost the gain and to improve the accuracy. In portable digital audio we need high dynamic ranges and high accuracy with minimum power. By adding a class AB output stage with strong requirements on linearity, the power consumption will increase. Again the application specific constraints dictate the amount of power needed to fit the design within specifications.

In conventional chopper stabilized opamps [1], [2], [3] for 1/f noise and offset reduction, differential amplifiers are being used and bandwidth is limited to few tens of kHz. Switching at the differential output will introduce most of the switching noise and residual offset. Other solutions for offset reduction like ping-pong techniques have the disadvantage of high power consumption and linearity problems [4]. Rail to rail input stages have offset and linearity problems [5]. This chapter presents two chopped amplifiers which circumvent the above mentioned drawbacks. They are designed in sub-micron technologies and are capable of reducing the 1/f noise up to 10MHz without excessively increase of the switching offset. They can drive low ohmic loads (32W ) with high linearity in high-end audio applications.

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