Variable-gain amplifier having error amplifier with constant loop gain
AssigneeAnalog Devices, Inc.
InventorBarrie Gilbert
Life of this patent
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Abstract
A translinear amplifier is disclosed. A loop amplifier drives the bases of the input and output transistor pairs from the differential collector voltage of the input pair. The loop amplifier contains a third differential pair (a gain pair). The tail current of the gain pair is inversely related to the tail current of the input pair, such that loop amplifier gain remains stable when the transconductance of the input pair changes (due, e.g., to input gain changes). In one embodiment, a linear-in-dB interface is provided that adjusts input pair tail current exponentially (and gain pair tail current exponentially and inversely) to linear voltage changes at a gain input.
Claims26
A variable-gain amplifier (VGA) comprising:
- four first matched bipolar transistors arranged as an input pair and an output pair, each transistor connected at its base to the base of a corresponding transistor in the other pair;
- an input-current source connected to the emitters of the input pair;
- an output-current source connected to the emitters of the output pair;
- a loop amplifier coupled between the collectors of the input pair and the connected bases of the input and output pairs, the loop amplifier comprising a gain transistor pair, each transistor in the gain pair having a base coupled to a corresponding input pair transistor collector;
- a controllable gain-current source connected to the emitters of the gain pair;
- and
- a gain-control circuit coupled to the gain-current source, the gain-control circuit adjusting the gain-current-source current such that the product of the input pair transconductance and the gain pair transconductance remains approximately constant.
A method of amplifying an input signal, the method comprising:
- applying the signal as a differential current to the collectors of a first transistor pair having a first tail current;
- mirroring the current densities in the first transistor pair to a second transistor pair having a second tail current by connecting the base of each transistor in the first pair to the base of a corresponding transistor in the second pair;
- driving the bases of the transistor pairs from a loop amplifier taking its input at the collectors of the first transistor pair;
- adjusting the drive amplifier gain inversely proportional to changes in the transconductance of the first transistor pair, such that a loop gain for the drive amplifier remains substantially constant;
- and
- detecting an amplified signal as a differential current at the collectors of the second transistor pair.
Dependencies are inferred from the claim text.