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IEICE Transactions on Electronics 2008 E91-C(6):871-878; doi:10.1093/ietele/e91-c.6.871
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Copyright © 2008 The Institute of Electronics, Information and Communication Engineers

Special Section on Analog Circuits and Related SoC Integration Technologies - Papers

Analysis of CMOS Transconductance Amplifiers for Sampling Mixers

Ning LI1, Win CHAIVIPAS1, Kenichi OKADA1 and Akira MATSUZAWA1

1 The authors are with the Department of Physical Electronics, Graduate School of Science and Engineering, Tokyo Institute of Technology, Tokyo, 152-8552 Japan. E-mail: lining{at}ssc.pe.titech.ac.jp


  Abstract

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.

Key Words: Windowed integration, moving average, transconductance amplifier, sampling mixer

Manuscript received October 12, 2007. Manuscript revised December 21, 2007.


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