Pade yaklaşıklığı kullanan akım modlu CMOS eksponansiyel fonksiyon üretici devresi

Over the past few years, as a result of the high demand to more powerful mobile devices, the importance of the high speed wireless communication becomes greater and greater. Automatic gain control (AGC) is necessary for controlling the signal amplitude and maximizing the dynamic range of the communication systems. The most important element of AGC loop is Variable Gain Amplifier (VGA). VGAs are widely used in biomedical applications, in imaging and signal processing circuits as well as the communication systems. VGAs can be classified according to their control function. The control function can be an analogue (continuous) signal or a discrete signal. The VGAs that are controlled with continuous signal are either controlled with a linear signal or an exponential signal. Exponential control signals are preferred because of their higher dynamic range characteristics. To realize exponential signals, devices with exponential DC characteristics can be used. Whilst CMOS technology lacks devices that have exponential characteristics in linear or saturation regions, exponential function approximations are used as the functions that will be realized with MOSFET devices working in saturation region. In the scope of this thesis work, first the exponential function approximations that are used for designing the control blocks of VGAs are classified. It is found out that Taylor series approximation, pseudo exponential approximation and their combinations are mostly used in literature. Recently Pade approximation is also reported in a work. Mathematical verifications are also presented for these approximations using MATLAB. Second, the Exponential Function Generator (EXPFG) circuit designs in literature are examined and a performance comparison of these circuits are provided. Moreover, a new exponential function approximation, using Pade approximationis proposed. Pade approximation is preferred because it provides a better result compared to most common approximation method, Taylor series approximation with the same order. Finally, three exponential function generator circuits are designed in CMOS 0.35µm technology using the proposed approximation methodology. Their performance is measured to verify the mathematical analysis using SPICE tool. The results verified that Pade approximation equations can be used in design of EXPFGs and a proper order of numerator and denominator can be selected according the specifications of the VGA.