Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1987-01-23
1989-04-18
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
330258, 330300, 330311, H03F 345
Patent
active
048230928
DESCRIPTION:
BRIEF SUMMARY
IMPROVEMENTS IN OR RELATING TO TRANSCONDUCTORS
The present invention relates to transconductance amplifier stages employing field effect transistors and particularly though not exclusively to active filters including MOST transconductance stages and capacitors, fabricated as monolithic integrated circuits.
Conventional types of RC active filter have the potential to be constructed using integrated circuits technology. This requires that the time constants or RC products of these circuits must be accurately defined, implying that the absolute values of resistance and capacitance should be closely controllable. This is not possible however with the manufacturing tolerances and temperature coefficients typically associated with monolithically integrated components.
Switched capacitor circuits offer one solution to this problem. These give a frequency domain transfer function with shape defined by ratios of capacitors and with the frequency axis scaled by an accurate externally generated clock frequency. However such filters are sampled data in nature and so in general require an anti-aliasing prefilter and a smoothing post-filter to interface satisfactorily with continuous time or asynchronously sampled circuitry. High frequency components of power supply noise and operational amplifier noise can be coupled into the signal path and aliased into the baseband filter output. The multi-phase non-overlapping clock waveforms generally necessary require carefully designed clock generators and buffers. The high frequency components of these waveforms can radiate to interfere with other circuitry. The synthesis of such filters requires special techniques and special purpose circuit simulators.
An alternative circuit concept involves the use of integrated transconductance amplifiers or "transconductors". In combination with on-chip capacitors in appropriate circuit topologies, these have the potential to realise filters with a wide range of frequency responses. The time-constants of such circuitry are defined by the values of transconductance and capacitance. The shape of the frequency response is determined by the ratios of capacitors and by the ratios of transconductances and is thus insensitive to correlated manufacturing variability in the absolute values or to uniform temperature variations. The frequency axis is scaled according to the absolute values of the transconductances and capacitances and so will vary widely with manufacturing tolerances and temperature variations. This variation can be removed however by using a known "master-slave" approach. In this, the transconductance of all the transconductors is controlled to be equal to that of an extra identical transconductor, which is part of a control loop such as a phase-locked loop locked onto an external reference frequency or a feedback loop based on an external reference resistor.
Several such circuits are known, using bipolar technology. However, MOS technology and CMOS in particular has general advantages of a higher packing density, operation at lower current densities, and higher circuit impedances, leading to compact and low-power circuitry. Digital circuitry can also be readily included on the same chip. These considerations have led to improved circuits for MOS transconductors, particularly in CMOS, though the invention is also thought to be applicable to other similar technologies, including but not exclusively single channel MOS, JFET or MESFETS.
MOS long-tailed pair circuits have been suggested for use as transconductance stages, but even when extra circuitry is added these suffer from limited signal range and non-linear distortion, particularly when the variations of bias point with manufacturing tolerances and temperature are taken into account. Fullydifferential circuitry using a pair of common-source MOSTs, biased to operate in the saturation region of operation, has been suggested, but the transconductance is then dependent on common mode input signals and even a differential input signal component gives rise to a large common-mode output
REFERENCES:
patent: 4010425 (1977-03-01), Dingwall et al.
patent: 4406990 (1983-09-01), Noro
patent: 4427903 (1984-01-01), Sugimoto
patent: 4520324 (1985-05-01), Jett, Jr. et al.
Matsui, Matsuura, and Iwasaki; "1982 International Symposium on Circuits and Systems, vol. 2 of 3"; pp. 241-244; May 10-12.
Mullins James B.
Wolfson Microelectronics Limited
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