Integrated self-adjustable continuous time band pass filter based upon Gm cell with bipolar transistors

A differential gain cell transconductor having a transconductor portion, a translinear gain cell portion and a dynamic bias circuit, said transconductor comprising;

• a pair of differentially connected transconductor input transistors connected to receive a differential transconductor input voltage signal Vi at a pair of input terminals thereof and producing a transconductor output voltage, the transconductor portion having a linearizing resistance RG1 operatively associated therewith such that the transconductance of the transconductor is 1/RG1 and also having an internal bias current I1;
• a translinear gain cell portion having input terminals and output terminals and an internal bias current output I2, the gain cell portion having a gain which is proportional to I2,/I1;
• a circuit connected between the output terminals of the pair of transconductor input transistors and the input terminals of the translinear gain cell portion;
• and
• a dynamic bias circuit having a bias resistor having a resistance RG2 associated therewith which is a replicate of RG1, said bias circuit providing bias currents I1 to the pair of transconductor input transistors, the magnitude of current I1 having a magnitude which varies in inverse proportion to variations in the resistance of RG2 thereby compensating the transconductance gain of the differential cell transducer for variations in RG1.

An integrated gain-cell differential transconductor having an overall transconductance, Gm, comprising:

• a fixed transconductor having a pair of internal bias currents I1 and characterized by a transconductance determined by the reciprocal of the magnitude of a first linearizing resistor, RG1;
• a translinear gain cell operatively coupled to the output of the fixed transconductor and having at least one internal bias current I2, the gain multiple of the translinear gain cell being determined by I2,/I1 and the overall transconductance Gm of the integrated gain-cell transconductor is 1/RG1*I2,/I1;
• and
• variable bias current supply means operatively coupled to the fixed transconductor by producing a control signal from a feedback resistor RG2, having resistance characteristics which replicate RG1, the control signal being operative for varying the bias current I1 of the fixed transconductor in inverse proportion to the variation of RG1 thereby compensating Gm for variations in RG1.

In a bandpass filter having a plurality of transconductor gain cells, a plurality of capacitors connected to the outputs of at least some of the plurality of gain cells, and where the gain cells and the capacitors are connected for realizing a second order filter, wherein at least one of the transconductor gain stages is an integrated gain-cell differential transconductor having an overall transconductance, Gm, which is relatively unchanged by process variation, comprising:

• a fixed transconductor having at least one internal load current I1 and characterized by a transconductance determined by the reciprocal of the magnitude of a linearizing resistance RG1;
• a translinear gain cell operatively coupled to the output of the fixed transconductor and having at least one internal load current I2, the gain multiple of the translinear gain cell being determined by I2,/I1 and the overall transconductance Gm of the integrated gain-cell transconductor being 1/RG1*I2,/I1;
• and
• a variable bias circuit operatively coupled to the fixed transconductor to apply a bias current I1 thereto, the magnitude of I1 varying in inverse proportion to variations in the resistance of a resistor RG2 which is a resistor formed to replicate RG1, thereby compensating transconductance Gm for variations in RG1.

A continuous active filter comprised of a plurality of transconductor gain cells, a plurality of capacitors connected to the outputs of at least some of the plurality of gain cells, and where the gain cells and the capacitors are connected for realizing a continuous active filter, wherein at least one of the transconductor gain stages is an integrated gain-cell differential transconductor having an overall transconductance, Gm, which is relatively unchanged by process variation, comprising:

• a fixed transconductor having at least one internal load current I1 and characterized by a transconductance determined by the reciprocal of the magnitude of a linearizing resistance RG1;
• a translinear gain cell operatively coupled to the output of the fixed transconductor and having at least one internal load current I2, the gain multiple of the translinear gain cell being determined by I2,/I1 and the overall transconductance Gm of the integrated gain-cell transconductor being 1/RG1*I2,/I1;
• and
• a variable bias circuit operatively coupled to the fixed transconductor to apply a bias current I1 thereto, the magnitude of I1 varying in inverse proportion to variations in the resistance of a resistor RG2 which is a resistor formed to replicate RG1, thereby compensating transconductance Gm for variations in RG1.

An implantable medical device comprising:

• an excitation source coupled to at least a first electrode and a second electrode, the excitation source producing current pulses of constant current flowing between the first electrode and the second electrode, the pulses sent in bursts at a burst frequency and having a pulse frequency at which the pulses are repeated;
• a first high pass filter filtering voltage signals received by at least a third electrode and a fourth electrode to produce filtered voltage signals;
• a first bandpass filter coupled to the amplifier and having a center frequency of approximately the pulse frequency;
• a rectifier coupled to the first bandpass filter and rectifying the filtered and amplified voltage signals to produce rectified signals;
• an integrator coupled to the rectifier and integrating the rectified signals to produce integrated signals;
• a sample-and-hold coupled to the integrator and sampling and holding the integrated signals to produce a plurality of samples;
• and
• a second bandpass filter coupled to the integrator and including an upper band cutoff frequency which is less than the burst frequency, the second bandpass filter filtering the plurality of samples to produce an output signal related to a time-varying impedance of a load and wherein the second bandpass filter has a plurality of transconductor gain cells, a plurality of capacitors connected to the outputs of at least some of the plurality of gain cells, and where the gain cells and the capacitors are connected for realizing the bandpass filter, wherein at least one of the transconductor gain stages is an integrated gain-cell differential transconductor having an overall transconductance, Gm, which is relatively unchanged by process variation, the differential transconductor comprising:
• a fixed transconductor having at least one internal load current I1 and characterized by a transconductance determined by the reciprocal of the magnitude of a linearizing resistance RG1;
• a translinear gain cell operatively coupled to the output of the fixed transconductor and having at least one internal load current I2, the gain multiple of the translinear gain cell being determined by I2,/I1 and the overall transconductance Gm of the integrated gain-cell transconductor being 1/RG1*I2,/I1;
• and
• a variable bias circuit operatively coupled to the fixed transconductor to apply a bias current I1 thereto, the magnitude of varying in inverse proportion to variations in the resistance of a resistor RG2 which is a resistor formed to replicate RG1, thereby compensating transconductance Gm for variations in RG1.

A method of compensating an integrated gain-cell differential transconductor having an overall transconductance, Gm, comprising:

• amplifying an input signal with a fixed transconductor having at least one internal load current I1 and characterized by a transconductance proportional to the reciprocal of the magnitude of a first internal resistor having a resistance RG1;
• amplifying the output of the transconductor with a translinear gain cell operatively coupled to the output of the fixed transconductor, the transconductor having at least one internal load currents I2, the gain of the translinear gain cell proportional to I2,/1 and the overall transconductance Gm of the integrated gain-cell transconductor being proportional to 1/RG1*I2,/I1;
• and
• varying the output bias current of bias current supply operatively coupled to the fixed transconductor for applying the current I1 thereto by producing a control signal from a second resistor RG2 having resistance characteristics replicating those of RG1, the control signal operatively coupled to the fixed transconductor for varying the load current I1 of the fixed transconductor in inverse proportion to the variation of RG2, thereby compensating Gm for variations in RG1.