News Column

Patent Issued for Compensation Technique for Feedback Amplifiers

July 23, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- Analog Devices, Inc. (Norwood, MA) has been issued patent number 8773199, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventor is Wan, Quan (Belmont, MA).

This patent was filed on September 26, 2012 and was published online on July 8, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "The dynamic performance of a voltage feedback amplifier (i.e., its ability to accurately amplify time-varying signals) is limited by its open-loop unity gain bandwidth or gain-bandwidth product ('GBW') and slew rate. Because the GBW of an amplifier is constant, the closed-loop bandwidth is inversely proportional to its gain; if a given amplifier is configured for a high gain, for example, its bandwidth correspondingly drops (sometimes dramatically). This drop in bandwidth may degrade the amplifier's performance, especially with high-frequency inputs. Some amplifiers (such as general-purpose instrumentation amplifiers) include an input ports (e.g., pins on a chip package) to which a customizable gain-setting resistor ('R.sub.G') may be connected. A user may therefore select a desired gain value by varying the size of R.sub.G but, in doing so, deleteriously change the amplifier's bandwidth. FIG. 1 illustrates an exemplary three operational-amplifier ('op-amp') instrumentation amplifier 100 that includes input amplifiers 102 (divided into first-stage A.sub.1 and second-stage A.sub.2 amplifiers) and an output amplifier 104. Because the output amplifier 104 is configured to have a gain of one, its closed-loop bandwidth is fixed at approximately half of its unit-gain bandwidth, regardless of the gain settings of the entire amplifier 100. Therefore, the bandwidth of the amplifier 100 is usually limited by the bandwidth of the input amplifier 102. For example, assuming the input amplifier 102 has a unit-gain bandwidth of 100 kHz, at a gain of 1000, the closed-loop 3 dB bandwidth (i.e., cutoff frequency) of the amplifier 100 is reduced to 100 Hz (because, as noted above, its GBW is fixed, and 100 kHz/1000=100 Hz).

"If the gain is set to a high value, the resulting low bandwidth of the amplifier 100 may be increased by configuring other components in the circuit. For example, part of the amplifier's compensation capacitance 106 may be switched out (i.e., electrically disconnected from the circuit) to increase its open loop unity gain bandwidth (and, as a result, its closed loop bandwidth as well) thereby improving the high-frequency performance of the circuit. This adjustment, however, requires additional input ports for the control signals necessary to change the compensation capacitance 106; in the simplest case, one pin may be used to switch part of the capacitance 106 in or out, but more pins are required for finer-grained control. Many amplifiers, such as commercial general-purpose resistor-programmable instrumentation amplifiers, cannot provide these additional ports because they would increase the cost of the amplifier, the complexity of the control circuitry, and/or the size of the amplifier package. A current-feedback amplifier may be used for high-speed operation, because its dynamic performance is not limited by GBW and slew rate, but these amplifiers have lower DC gain and are thus not suitable for high-precision applications. A need therefore exists for a way of boosting the bandwidth of a voltage-feedback amplifier at high gains without requiring additional input ports."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventor's summary information for this patent: "In general, a closed-loop gain of a voltage feedback circuit may be set by a resistor network composed of two resistors, R.sub.1 and R.sub.2. In a non-inverting configuration, R.sub.1 may be connected between an inverting input node of the amplifier and a ground or virtual ground node. In an inverting configuration, R.sub.1 may be connected between the inverting input node and a signal source. In both cases, R.sub.2 is a feedback resistor connected between the output of the amplifier and the inverting input node of the amplifier, therefore forming a first feedback path between the output node and the inverting input node. In existing circuits, a second feedback path is formed using a capacitor connected between an intermediate node and output node of the amplifier, thereby providing fixed Miller compensation, as discussed above. This configuration sets the GBW of the amplifier for stability consideration, but it may also limit the slew rate and closed loop bandwidth of the amplifier.

"The compensation technique of the present invention extends the small-signal bandwidth of an amplifier while also eliminating/alleviating its slew limitation under a large-signal step response. In various embodiments, the second feedback path between the output node and intermediate node includes a capacitor and some series resistance R.sub.X incorporated from the resistor network R.sub.1 and R.sub.2. R.sub.X may be composed of part of R.sub.2, entire R.sub.2, or entire R.sub.2 and part of R.sub.1. The second feedback path between the intermediate node and the output node of the amplifier provides adaptive Miller compensation. The second feedback path shares some resistance with the gain-setting network R.sub.1 and R.sub.2, which causes the Miller effect to depend on the closed loop gain setting. The Miller effect of the compensation capacitor is therefore adaptively changed with respect to the closed-loop gain, thus effectively increasing the equivalent open loop unity gain bandwidth or GBW of amplifiers. Thus, the closed-loop bandwidth of feedback amplifiers is also increased accordingly.

"In one aspect, a system for extending a bandwidth and improving slew behavior of a feedback amplifier includes an amplifier having an input node, an output node, and an intermediate node; a first feedback path between the input node and output node; and a second feedback path between the intermediate node and output node. The first feedback path shares at least a portion of the second feedback path. The first path and the second path may share a feedback resistor, and the second path may include a compensation capacitor that is not shared with the first feedback path.

"The first feedback path may include a feedback resistor split into two parts, and the second feedback path may share one of the two parts of the feedback resistor of the first path. A resistor may be connected between an input node of the amplifier and a ground and may be split into two parts; the second feedback path may include one of the two parts. The amplifier may be a multiple-stage amplifier. A closed-loop 3 dB bandwidth of the amplifier may remain approximately constant at its unit-gain bandwidth regardless of a gain setting of the amplifier. A closed-loop 3 dB bandwidth may be extended by a factor of

".times..beta. ##EQU00001## wherein .beta. is the feedback coefficient and A.sub.2 is the gain between the intermediate node and output node.

"A second amplifier for amplifying and/or buffering the voltage on the output node may be included. A gain-setting resistor may be included; the signal derived from the output voltage and the closed loop gain of the amplifier may depend on the ratio of the gain-setting resistor and the feedback resistor. The amplifier and the second amplifier may include an instrumentation amplifier. The second amplifier may be a unity gain amplifier, a differential amplifier, and/or an operational amplifier. The amplifier may include a first stage for amplifying an input voltage on the input node to produce the intermediate voltage on the intermediate node and a second stage for amplifying the intermediate voltage to produce the output voltage.

"In another aspect, a method for extending a bandwidth of a feedback amplifier includes providing a first signal derived from an output voltage of an amplifier to an input node of the amplifier and providing a second signal derived from the output voltage of an amplifier to an intermediate node of the amplifier. The second signal increases a bandwidth of the feedback amplifier as a gain of the amplifier increases.

"Providing the first signal may include sharing an entire feedback resistor with the second signal and/or sharing a portion feedback resistor with the second signal. A high input impedance may be provided in a first amplifier and a unity gain may be provided in a second amplifier. A gain of the feedback amplifier may be adjusted; adjusting the gain may include adjusting a resistance of a gain-setting resistor.

"In another aspect, a system for extending a bandwidth and improving slew behavior of a feedback amplifier includes an amplifier having an input node, an output node, and an intermediate node; a first feedback path for providing, to the input node, a signal derived from an output voltage on the output node; and a second feedback path for providing, to the intermediate node, a signal derived from an output voltage on the output node. The second feedback path may include a compensation capacitor. A closed-loop 3 dB bandwidth of the amplifier may remain approximately constant at its unit-gain bandwidth regardless of a gain setting of the amplifier.

"These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations."

For the URL and additional information on this patent, see: Wan, Quan. Compensation Technique for Feedback Amplifiers. U.S. Patent Number 8773199, filed September 26, 2012, and published online on July 8, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8773199.PN.&OS=PN/8773199RS=PN/8773199

Keywords for this news article include: Analog Devices Inc., Medical Device Companies.

Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC


For more stories covering the world of technology, please see HispanicBusiness' Tech Channel



Source: Journal of Engineering


Story Tools






HispanicBusiness.com Facebook Linkedin Twitter RSS Feed Email Alerts & Newsletters