Fast amplitude detector and automatic gain control

Abstract

An amplitude detector has a phase shifter such as one using an analog differentiator and an adjustable gain stage, or one using a determinable delay, the phase shifter coupled to shift phase of an input signal to the amplitude detection apparatus. The detector also has a first analog multiplier coupled to square the input signal, a second analog multiplier coupled to square output of the phase shifter; and an analog adder coupled to sum outputs of the first and second analog multiplier. An automatic gain control circuit has the amplitude detector coupled to control gain of a controllable amplifier.

What is claimed is: 
     
         1 . An amplitude detection apparatus comprising:
 a phase shifter comprising an adjustable gain stage, the phase shifter coupled to receive an input signal;   a first analog multiplier coupled to square the input signal;   a second analog multiplier coupled to square an output of the phase shifter; and   an analog adder coupled to sum outputs of the first and second analog multiplier.   
     
     
         2 . The amplitude detection apparatus of  claim 1  further comprising an analog square root circuit coupled to receive an output of the analog adder. 
     
     
         3 . The amplitude detection apparatus of  claim 1  wherein the amplitude detection apparatus is fabricated upon a monolithic integrated circuit, and further comprising trim circuitry adapted to compensate for manufacturing variation by adjusting gain of the gain stage of the phase shifter. 
     
     
         4 . An automatic gain control circuit comprising the amplitude detection apparatus of  claim 1  coupled to control gain of a controllable amplifier. 
     
     
         5 . The automatic gain control circuit of  claim 4  wherein the controllable amplifier is digitally controlled, and further comprising: an analog-to-digital converter coupled to convert an analog output of the analog adder into a digital signal, the digital signal coupled to control the controllable amplifier. 
     
     
         6 . The amplitude detection apparatus of  claim 1  wherein the phase shifter comprises an all-pass filter delay unit. 
     
     
         7 . The amplitude detection apparatus of  claim 1  wherein the phase shifter comprises an analog differentiator and an adjustable gain stage, the analog differentiator coupled to differentiate the input signal to the amplitude detection apparatus, the adjustable gain stage being configured to compensate for frequency dependent gain of the analog differentiator. 
     
     
         8 . The amplitude detection apparatus of  claim 7  further comprising an analog square root circuit coupled to receive an output of the analog adder. 
     
     
         9 . The amplitude detection apparatus of  claim 8  wherein the amplitude detection apparatus is fabricated upon a monolithic integrated circuit, and further comprising trim circuitry adapted to compensate for manufacturing variation by adjusting gain of the gain stage of the phase shifter. 
     
     
         10 . An automatic gain control circuit comprising the amplitude detection apparatus of  claim 8  coupled to control gain of a controllable amplifier. 
     
     
         11 . An uninterruptable power supply comprising the amplitude detection apparatus of  claim 7 , wherein the uninterruptable power supply is configured to provide power from a battery to an output of the uninterruptable power supply when the amplitude detection apparatus detects a dropout of an alternating-current power. 
     
     
         12 . An electrical impedance imaging apparatus comprising the automatic gain control circuit of  claim 10  coupled to condition voltage or current signals from electrodes of the electrical impedance imaging apparatus. 
     
     
         13 . A method of providing fast-response automatic gain control of an amplifier coupled to receive an input signal comprising:
 phase-shifting the input signal to generate a phase-shifted input signal;   squaring the phase-shifted input signal to provide a squared phase-shifted input signal;   squaring the input signal to provide a squared input signal;   summing the squared phase-shifted input signal and the squared input signal, and performing a square root to produce a magnitude signal; and   using the magnitude signal to control gain of an amplifier coupled to amplify the input signal.   
     
     
         14 . The method of  claim 13 , further comprising compensating for frequency dependence of gain of the circuitry used to perform the phase shift. 
     
     
         15 . The method of  claim 14 , wherein a frequency detection circuit is used to control the compensating for frequency dependence of gain of the circuitry used to perform the phase shift.