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Active Low Pass Filter with Gain

IP.com Disclosure Number: IPCOM000093349D
Original Publication Date: 1967-Aug-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Kengla, CA: AUTHOR [+2]

Abstract

The circuit is a temperature-compensated integrator or low-pass filter capable of a long time constant, e.g., 1 minute, and high DC stability. It can be used for smoothing signals with both AC and DC components while providing a stable, fixed value of steady-state DC gain. The circuit can be implemented to operate over a temperature range of -20 Degrees C to +70 Degrees C with a DC output voltage drift of not more than 500 microvolts per degree centigrade.

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Active Low Pass Filter with Gain

The circuit is a temperature-compensated integrator or low-pass filter capable of a long time constant, e.g., 1 minute, and high DC stability. It can be used for smoothing signals with both AC and DC components while providing a stable, fixed value of steady-state DC gain. The circuit can be implemented to operate over a temperature range of -20 Degrees C to +70 Degrees C with a DC output voltage drift of not more than 500 microvolts per degree centigrade.

The low-pass filter or integrator is basically a temperature compensated high- gain amplifier with two independent negative feedback loops. The input signal to be filtered is applied at the input through resistor Ri to the noninverting input of the high-gain amplifier A1. The output signal is derived from amplifier A1 at the output and is also fed back through both the AC and the DC feedback loops.

The DC feedback loop establishes the steady-state DC gain. It includes feedback resistor Rf which connects, through the parallel combination of a temperature compensating circuit and an offset circuit, to the inverting input of amplifier A1. The temperature compensating circuit includes temperature sensitive diode D which compensates for temperature-caused variations in the operation of amplifier A1. Diode D compensates for those variations in the operation of amplifier A1 by establishing a temperature-variant voltage, regulated by Rt as biased through R4, at the amplifier's inverting input. The offset circuit includes bias resistors R5 and R6 and variable resistor R7 which together establish, through Ro, an offset voltage equal to the shift in DC level inherently produced by amplifier A1.

Tem...