Low-voltage low-power high dB-linear CMOS exponential function generator using highly-linear V-I converter

Abstract

A CMOS voltage-to-current converter with exponential characteristic is presented in this paper. The concept of Taylor series expansion is used for realizing the exponential characteristic. The proposed exponential V-I converter is composed of a current-to-current squarer and a linear V-I converter with the use of linearization technique. Based on a 0.25 µm CMOS process, simulations show a 23 dB of linear-output current range and the linearity within 20 dB with error less than ± 0.5dB is achieved. The total power consumption is below 0.2 mW with 1.25 V supply voltage. The proposed circuit can be used for the design of an extremely low-voltage low-power variable gain amplifier (VGA).

A 35 dB-linear exponential function generator for VGA and AGC applications

“…(6), the exponential characteristic is easily achieved by setting the multiplying factors K 1 and K 2 . Also, Typically, the output parameter of the dB-linear V-I and I-V converters is a direct function of the input parameter [4,6,7]. Differently, this paper considers the output current of the EVIC is an indirect function of input voltage, and a direct function of input current such that the exp 2 (ax) can be done in the current domain easily to get rather higher dB-linear range as depicted in Fig.…”

“…Alternatively, the Taylor series expansion can also be used for implementation of the exponential [4][5][6][7].…”

“…By applying the Taylor concept, the dB-linear V-I converter (EVIC) can be implemented by using the composition of a V-I squarer circuit, a linear V-I converter and a constant bias current [4][5][6], or by using composite NMOS transistors [7]. However, these previously reported EVICs tend to show very small dB-linear variation of the output current (less than 15 dB with a linearity error less than ± 0.5 dB) [6,7].…”

“…Moreover, EVICs in [6,7] are not power efficient (≅ 0.9 mW) and operate at high supply voltage (≅ 3 V) [6,7]. Although [5] reported higher dB-linear range, the differential input dynamic range is severely limited (≅ 0.27 V) and the output range is restricted to less than 20 dB.…”

“…The EVIC in this paper uses the current-to-current squarer [8] instead of voltage-to-current squarer [6] such that the power consumption of the overall circuit is reduced extremely (≅ 0.3 mW), and the V-I linear with new linearization technique so that the circuit can operate at very low voltage application (≤ 1.25 V) [5]. The EVIC is based on current mode, by inserting the current squarer at the output of the core EVIC, a rather high dB-linear range can be obtained…”