Gain control circuit

A gain controlled differential amplifier including an input differential stage adapted to receive differential input signals and having first and second outputs, comprising in combination:

• a first pair of electron control means each having first, second and control electrodes, said first electrodes of said first pair being commonly connected to the first output of the input differential stage, said second electrode of a first one of the electron control means of said first pair being directly connected to a source of operating potential, said second electrode of the second electron control means of said first pair being coupled to said source of operating potential and being a first output of the amplifier;
• a second pair of electron control means each having first, second and control electrodes, said first electrodes of said second pair being commonly connected to the second output of the input differential stage, said second electrode of a first one of the electron control means of said second pair being directly connected to said source of operating potential, said second electrode of the second electron control means of said second pair being coupled to said source of operating potential and being a second output of the amplifier;
• reference circuit means having an output and being coupled between said source of operating potential and a ground reference potential for providing a reference potential at said output having a predetermined temperature characteristic, said output being connected to said control electrodes of each of said first electron control means of said first and second pairs of electron control means;
• and
• circuit means having a control terminal and being coupled to said source of operating potential which is responsive to an applied gain control signal for producing an output signal to vary the conductivity of said second electron control means of said first and second pairs of electron control means to vary the gain of the amplifier, said control terminal being coupled to said control electrodes of each of said second electron control means of each pair of electron control means, said output signal having a predetermined temperature characteristic which is substantially matched to said temperature characteristic of said reference potential such that the gain control characteristic of the amplifier circuit is independent to temperature and operating potential variations.

A gain control circuit suitable to be fabricated in monolithic integrated circuit form for linearly varying the gain of a differential amplifier, the amplifier including a differential input stage, first and second differential transistor pairs coupled to the differential input stage and which include outputs of the amplifier and having respective ones of the base electrodes of each transistor pair connected at first and second common nodes, the gain control circuit comprising:

• reference circuit means having an output terminal connected to the first one of the common nodes between said first and second differential transistor pairs including at least one semiconductor diode means for providing a reference potential at said output terminal thereof, said reference potential having a predetermined temperature characteristic;
• and
• circuit means adapted to receive a gain control signal and being coupled to the second common node including at least one semiconductor diode means for varying the gain of the amplifier in accordance to said gain control signal, said circuit means producing a voltage signal at the second common node having a predetermined temperature characteristic being matched to said temperature characteristics of said reference potential such that the charcteristics of the gain control circuit are substantially independent to temperature and operating potential variations, said at least one semiconductor diode means causing the gain of the amplifier to be linearly varied in response to the gain control signal being within a predetermined range.

In a differential amplifier including a differential input stage for receiving input signals, a first differential transistor pair coupled to an output of the input stage and having an output terminal, a second differential transistor pair coupled to another output of the input stage and having an output terminal the base electrodes of a first transistor of each transistor pair being coupled respectively to a first common node, the base electrodes of the second transistor of each transistor pair being coupled to a second common node, a method for varying the gain of the amplifier in response to a gain control signal, comprising the steps of:

• establishing a current through a first plurality of serially connected electron control means to provide a reference potential having a predetermined temperature characteristic to the first common node;
• and
• varying the current through a second plurality of serially connected electron control means coupled to the second common node in response to the gain control signal for producing a voltage thereat having a substantially identical temperature characteristic as said reference potential to vary the conductivity of the transistors of the first and second transistor pairs such that the magnitude of the output signal appearing at the output terminals of the transistor pairs is varied, the gain control characteristic of the amplifier being independent to temperature variations.