Analysis of CMOS Transconductance Amplifiers for Sampling Mixers

Ning LI  Win CHAIVIPAS  Kenichi OKADA  Akira MATSUZAWA  

Publication
IEICE TRANSACTIONS on Electronics   Vol.E91-C   No.6   pp.871-878
Publication Date: 2008/06/01
Online ISSN: 1745-1353
DOI: 10.1093/ietele/e91-c.6.871
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Analog Circuits and Related SoC Integration Technologies)
Category: 
Keyword: 
Windowed integration,  moving average,  transconductance amplifier,  sampling mixer,  

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Summary: 
In this paper the transfer function of a system with windowed current integration is discussed. This kind of integration is usually used in a sampling mixer and the current is generated by a transconductance amplifier (TA). The parasitic capacitance (Cp) and the output resistance of the TA (Ro,TA) before the sampling mixer heavily affect the performance. Calculations based on a model including the parasitic capacitance and the output resistance of the TA is carried out. Calculation results show that due to the parasitic capacitance, a notch at the sampling frequency appears, which is very harmful because it causes the gain near the sampling frequency to decrease greatly. The output resistance of the TA makes the depth of the notches shallow and decreases the gain near the sampling frequency. To suppress the effect of Cp and Ro,TA, an operational amplifier is introduced in parallel with the sampling capacitance (Cs). Simulation results show that there is a 17 dB gain increase while Cs is 1,pF, gm is 9,mS, N is 8 with a clock rate of 800,MHz.