Circuit arrangement of an amplifier with current controlled gain and corresponding method

An ultrasonic obstacle detector having a receiver comprising:

• a variable gain amplifier comprising:
• a first differential transistor pair coupled to a signal input,
• a first current source configured to provide a first bias current to the first differential transistor pair,
• a pair of diodes coupled to an output of the first differential transistor pair,
• a second differential transistor pair having an input coupled to the pair of diodes,
• a second current source configured to provide a second bias current to the second differential transistor pair, and
• a current control circuit coupled to a first reference current input, a second reference current input, the first current source and the second current source, the current control circuit configured to cause a common-mode output current of the pair of diodes to be substantially equal to a first reference current provided at the first reference current input, and a common-mode output current of the second differential transistor pair to be substantially equal to a sum of the first reference current and a second reference current provided at the second reference current input;
• and
• a reference current generator coupled to the first reference current input and the second reference current input and configured to provide the first reference current and the second reference current to the current control circuit, to control a gain of the variable gain amplifier by modifying the first reference current and the second reference current, and to keep the sum of the first reference current and the second reference current constant over a plurality of gain settings.

A method for operating a variable gain amplifier comprising a first differential transistor pair coupled to a signal input, a first current source configured to provide a first bias current to the first differential transistor pair, a pair of diodes coupled to an output of the first differential transistor pair, a second differential transistor pair having an input coupled to the pair of diodes, and a second current source configured to provide a second bias current to the second differential transistor pair, the method comprising:

• causing a common-mode output current of the pair of diodes to be substantially equal to a first reference current by adjusting a current of the first current source;
• causing a common-mode output current of the second differential transistor pair to be substantially equal to a sum of the first reference current and a second reference current by adjusting a current of the second current source;
• and
• adjusting a gain of the variable gain amplifier by adjusting the first reference current and the second reference current.