A Linear CMOS OTA and Its Application to a 3.3V 20MHz High-Q gm-C Bandpass Filter

A design of a linear and fully-balanced operational transconductance amplifier (OTA) with improved high DC gain and wide bandwidth is presented.Derivative from a single common-source field effect transistor (FET) cascade and its DC I-V characteristics,the third-order coefficient g3 has been well compensated with a parallel FET operated in the triode region,which has even-odd symmetries between the boundary of the saturation and triode region.Therefore,for high linearity,a simple solution is obtained to increase input signal amplitude in saturation for the application of OTA continuous-time filters.A negative resistance load (NRL) technique is used for the compensation of parasitic output resistance and an achievement of a high DC-gain of the OTA circuits without extra internal nodes.Additionally,derivations from the ideal -90. phase of the gm-C integrator mainly due to a finite DC gain and parasitic poles will be avoided in the frequency range of interest.HSPICE simulation shows that the total harmonic distortion at 1Vp-p is less than 1% from a single 3.3V supply.As an application of the VHF CMOS OTA,a second-order OTA-C bandpass filter is fabricated using a 0.18μm CMOS process with two kinds of gate-oxide layers,which has achieved a center frequency of 20MHz,a 3dB-bandwidth of 180kHz,and a quality factor of 110.

A Linear CMOS OTA and Its Application to a 3. 3V 20MHz High-Q gm-C Bandpass Filter