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.
Claims17
A variable-gain amplifier 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
- means for adjusting the gain-current-source current such that the product of the input pair transconductance and the gain pair transconductance remains approximately constant.
A variable-gain amplifier comprising:
- an input differential pair of transistors;
- an output differential pair of transistors;
- a loop amplifier to drive control terminals of the input and output pairs in response to a signal applied to the input pair;
- and
- a gain control circuit to coordinate the gain of the input differential pair and the gain of the loop amplifier.
A method comprising:
- applying an input signal to an input differential pair of transistors;
- applying the input signal to a loop amplifier;
- driving control terminals of the input pair and an output differential pair of transistors with the loop amplifier;
- and
- coordinating the gain of the input pair and the gain of the loop amplifier.
Dependencies are inferred from the claim text.