Current-Mode Log-Domain Filter Design Based On Bipolar Junction Transistor

Log-domain companding technique, which has received great research attention attributed to its low voltage supply, low power consumption, wide tune-ability, high operating frequency and large dynamic range potentials, is used to design the log-domain continuous-time fully integrated filters.Log-domain filter, which is a significant class of structures classified as externally linear internally nonlinear networks, makes explicit use of the large-signal exponential current versus voltage characteristics of the active devices such as diode, bipolar junction transistor (BJT), and the MOS transistor operating in the subthreshold region. The input signal is compressed logarithmically before being processed then expanded exponentially at the output. For the advantages of simple structure, excellent bandwidth, high accuracy and so on, current-mode log-domain filter obtains rapid development. The realization and frequency characteristic analysis of log-domain filter is the main topic of this dissertation. The main works are discussed as follows:First, a new design method of current-mode lowpass log-domain filter is proposed. The high order differential equation from the transfer function is decomposed into a set of first order differential equations by setting state variables, the corresponding circuit of each item which is designed using the BJT translinear unit, is combined to realize lowpass log-domain filter. All the first-order, the second-order, the third-order and the fifth-order lowpass log-domain filters are implemented. Based on the first-order lowpass log-domain filter, a first-order log-domain filter with multifunction is designed according to the structure relative among first-order lowpass, highpass, and allpass filter. PSPICE simulations show that they have good frequency response, and can be operated at low voltage and electronically tuned by changing an external current.Second, based on RLC passive network, a second-order log-domain filter which has bandpass and lowpass outputs is implemented by using E cell. Every linear integrator in the signal flow graph of RLC ladder prototype is replaced with the exponential transconductor cell; a logarithmical compressing cell is connected to the input node and exponential expanding cells are connected to the output nodes. The proposed filter has a simple structure and can be electronically tuned widely.