Bias circuits for depletion mode field effect transistors
First Claim
1. A bias circuit for providing a gate voltage for a first depletion mode FET (field effect transistor) to be controlled, the bias circuit comprising:
- a second depletion mode FET having similar characteristics to the first FET, the second FET having its gate coupled to its source for conducting a drain-source current via a resistor to produce a first voltage dependent on the drain-source current; and
a comparator for comparing the first voltage with a reference voltage to produce said gate voltage in dependence upon differences between the first voltage and the reference voltage, so that an increase in said drain-source current of the second FET results in a change of said gate voltage of the first FET to reduce drain-source current of the first FET thereby to compensate for process variations of said first and second FETs.
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Abstract
A bias circuit for providing a gate voltage for a first depletion mode FET operating on RF signals comprises a second similar FET in a source-follower configuration with zero gate-source voltage to conduct a drain-source current Idss via a source resistor. A third depletion mode FET has its gate connected to receive a voltage dropped across this source resistor, its source coupled to a diode whose forward voltage drop constitutes a reference voltage, and its drain connected to a second resistor, a voltage drop across which due to the drain-source current of the third FET constitutes a gate-source voltage for the first FET. The bias circuit compensates for process variations in manufacture of the first FET, and also provides temperature compensation.
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Citations
20 Claims
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1. A bias circuit for providing a gate voltage for a first depletion mode FET (field effect transistor) to be controlled, the bias circuit comprising:
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a second depletion mode FET having similar characteristics to the first FET, the second FET having its gate coupled to its source for conducting a drain-source current via a resistor to produce a first voltage dependent on the drain-source current; and
a comparator for comparing the first voltage with a reference voltage to produce said gate voltage in dependence upon differences between the first voltage and the reference voltage, so that an increase in said drain-source current of the second FET results in a change of said gate voltage of the first FET to reduce drain-source current of the first FET thereby to compensate for process variations of said first and second FETs. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A bias circuit for providing a gate voltage for a first depletion mode FET (field effect transistor) to be controlled, the bias circuit comprising:
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a second depletion mode FET having similar characteristics to the first FET, the second FET having a source-follower configuration with its gate coupled to its source for conducting a drain-source current via a first resistor to produce a first voltage dependent on the drain-source current across the first resistor;
a third depletion mode FET having its gate coupled to the first resistor to receive the first voltage, its source arranged to receive a reference voltage, and its drain connected to a second resistor for conducting drain-source current of the third FET via said second resistor to produce said gate voltage for the first FET at the drain of the third FET. - View Dependent Claims (14, 15, 16, 17, 18)
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19. A method of compensating for process variations to which a first depletion mode FET is subject by controlling a gate voltage of the first FET, comprising the steps of:
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biassing a second depletion mode FET, subject to substantially the same process variations as the first FET, with an approximately zero gate-source voltage so that it conducts a drain-source current via a first resistor to produce a first voltage;
supplying the first voltage and a reference voltage to the gate and source respectively of a third depletion mode FET whereby the third FET conducts a drain-source current via a second resistor coupled to the drain of the third FET; and
deriving the gate voltage of the first FET from a junction between the drain of the third FET and the second resistor. - View Dependent Claims (20)
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Specification