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  Auto gain controllerThe Patent Description & Claims data below is from USPTO Patent Application 20070222511. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a division of U.S. application Ser. No. 10/956,057 filed Oct. 4, 2004, which is incorporated herein by reference. FIELD OF THE INVENTION [0002] This invention relates in general to automatic gain control, and more particularly, to a circuit and method for providing automatic gain control for a transimpedance amplifier. BACKGROUND OF THE INVENTION [0003] A transimpedance amplifier ("TIA") is commonly used to convert an input current signal into an output voltage signal proportional to the input current signal. The input current signal is generally provided by a photo detector such as a photo diode, which converts optical energy transmitted from an optical source into electrical energy in the form of an electrical current. A TIA is often implemented by providing a feedback resistor coupled between an input and an output of an operational amplifier. However, since the optical source may operate in an unstable light environment, the electrical current provided by a photo detector is subject to change as the optical energy changes. As an example, the electrical current may range from 1 microampere to 1 milliampere. Given a resistor of 100 kilo-ohms used in the TIA, the output voltage may range from 1 millivolt to 0.1 volt to 100 volts, which is not acceptable. Moreover, the resistance of a feedback resistor is related to the sensitivity of a TIA. It is generally desirable to use a feedback resistor having a resistance value as great as possible if the bandwidth of the TIA permits. A gain-controlled TIA has been therefore developed to allow a normal gain in response to a normal input current, and a controlled gain in response to an input current that will result in an unacceptable gain region. [0004] FIG. 1 is a schematic diagram of a gain-controlled TIA circuit 10. Referring to FIG. 1, gain-controlled circuit 10 includes an operational amplifier 12, a resistor 14 and an n-channel metal-oxide-semiconductor ("NMOS") transistor 16. Gain-controlled TIA circuit 10 includes an input terminal (not numbered) to receive an input current I.sub.IN, and an output terminal (not numbered) to provide an output voltage V.sub.OUT. In operation, in response to a normal input current I.sub.IN, NMOS transistor 16 is turned off, gain-controlled TIA circuit 10 provides V.sub.OUT equal to -I.sub.IN.times.R, where R is the resistance value of resistor 14. When input current I.sub.IN exceeds an acceptable region, NMOS transistor 16 turns on, which functions to serve as a variable resistor. Gain-controlled TIA circuit 10 provides V.sub.OUT equal to -I.sub.IN.times.(R//R'), where R' is the resistance value of NMOS transistor 16. Gain-controlled TIA circuit 10 therefore provides a gain of R in response to a normal input current, and a gain of R//R' equal to (R.times.R')/(R+R') in response to a relatively large input current. [0005] In gain-controlled TIA circuit 10, the variable resistance R' decreases as a drain-to-source current IDS of NMOS transistor 16 increases, which in turn results from an increase in a gate-to-source voltage V.sub.GS of NMOS transistor 16. Accordingly, the variable resistance R' decreases as a voltage V.sub.G applied to a gate of NMOS transistor 16 increases. It is important to control the bias voltage V.sub.G in order to provide a controlled gain for TIA circuit 10. An example of a gain-controlled TIA circuit is disclosed in U.S. Pat. No. 6,593,810 (hereinafter the '810 patent) to Yoon, entitled "2.5 Gigabits-per-Second Transimpedance Amplifier." In the '810 patent, a resistor divider network 47 including resistors R.sub.1 and R.sub.2 is coupled between an output 90 of a replica biasing stage 40 and ground. Resistor divider network 47 provides a scaled output voltage V at a terminal 90 disposed between resistors R.sub.1 and R.sub.2, which is proportional to a voltage level V.sub.O at output 90 of replica biasing stage 40. The scaled output voltage V is compared with a voltage level at an output 80 of a gain stage 30 to determine whether to turn on a degenerative feedback element 15. However, the voltage level V.sub.O may be deviated due to the loading resistors R.sub.1 and R.sub.2. As a result, the '810 patent may not provide a fixed scaled output voltage V, which adversely affects the performance of a TIA system. BRIEF SUMMARY OF THE INVENTION [0006] The present invention is directed to a circuit and a method that obviate one or more problems resulting from the limitations and disadvantages of the prior art. [0007] In accordance with an embodiment of the invention, there is provided a gain-controlled transimpedance amplifier circuit that comprises a first gain unit including an input for receiving a first current and an output, a current source for providing a second current, a second gain unit including an input and an output, a first impedance unit of a first impedance coupled in parallel with the second gain unit, and a comparator including an output, a first input coupled to the output of the first gain unit, and a second input coupled to the output of the second gain unit. [0008] Also in accordance with the present invention, there is provided a gain-controlled transimpedance amplifier circuit that comprises a first gain unit for converting a first current into a first voltage, a first impedance unit providing a first impedance path coupled in parallel with the first gain unit, a second impedance unit providing a second impedance path coupled in parallel with the first gain unit and the first impedance path, a voltage generator for providing a second voltage, and a comparator for activating the second impedance unit when the first voltage is smaller than the second voltage, and deactivating the second impedance unit when the first voltage is greater than the second voltage. [0009] Further in accordance with the present invention, there is provided a method for providing gain control for a transimpedance amplifier that comprises providing a first current, providing a first gain unit to receive the first current, coupling a first impedance unit in parallel with the first gain unit, measuring a first voltage level at an output of the first gain unit when the first current is zero, coupling a second impedance unit in parallel with the first gain unit and the first impedance unit, providing a second current, providing a second unit to receive the second current, coupling a third impedance unit in parallel with the second gain unit, generating a second voltage level at an output of the second gain unit, and activating the second impedance unit when a voltage level at the output of the first gain unit is smaller than the second voltage level. [0010] Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. [0011] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. [0012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the present invention and together with the description, serves to explain the principles of the invention. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS [0013] The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. [0014] In the drawings: [0015] FIG. 1 is a schematic diagram of a gain-controlled transimpedance amplifier ("TIA"); [0016] FIG. 2 is a circuit diagram of a gain-controlled TIA circuit in accordance with an embodiment of the present invention; and [0017] FIG. 3 is a circuit diagram of a gain-controlled TIA circuit in accordance with another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0018] FIG. 2 is a circuit diagram of a gain-controlled transimpedance amplifier ("TIA") circuit 100 in accordance with an embodiment of the present invention. Referring to FIG. 2, gain-controlled TIA circuit 100 includes a first gain unit 102, a voltage generator 104, a first impedance unit 106, a second impedance unit 108 and a comparator 110. First gain unit 102 receives a current signal I.sub.IN, which is provided by, for example, a photo detector, and converts the current signal I.sub.IN into a voltage signal in conjunction with first impedance unit 106 and second impedance unit 108. In one embodiment according to the invention, first gain unit 102 includes odd-numbered inverter(s). In one aspect, as shown in FIG. 2, three inverters (not numbered) are cascaded to ensure an enough gain output. First impedance unit 106 provides a first impedance path coupled in parallel with first gain unit 102. Second impedance unit 108 provides a second impedance path coupled in parallel with first gain unit 102 and first impedance unit 106. In one embodiment, first impedance unit 106 includes a resistor. Second impedance unit 108 includes odd-numbered transistors. In one aspect, three transistors are gate-connected to each other. Continue reading... Full patent description for Auto gain controller Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Auto gain controller patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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