IC Voltage to Current Transducers with Very Small Transconductance
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This paper deals with the design of very small ac transconductance voltage to current transducers intended for the design of low frequency continuous-time filters, very large resistors and other applications. The first type of Operational Transconductance Amplifiers (OTA) is based on a triode biased transistor and a current division technique. The second one uses partial positive feedback which allows to reduce transistor dimensions but the sensitivity to transistor mismatches increases. The proposed techniques can be used for the design of high-order low frequency IC filters, ladder or based on biquads, with moderated transistor dimensions while the dynamic range-cutoff frequency performance is comparable to previously reported structures. A 10 Hz third order lowpass ladder filter has been designed with these techniques, and it shows a dynamic range of 62 dB. Besides, a novel biasing technique for capacitive sources coupled preamplifiers is proposed. Experimental results for a prototype, fabricated in a 1.2 m CMOS process, have shown very low distortion components (THD < 1% for input signals up to 20 mV), noise level below 15 VRMSand dynamic range of 63 dB. The power consumption is only 10 watts and the supply voltages are 1.5 volts.
