An operational amplifier comprises: a first amplifier stage 4 comprising a first differential pair of transistors 8, 10 arranged to receive and amplify a differential input signal 18, 20 thereby providing a first differential output signal 22, 24; and a second amplifier stage 6 comprising a second differential pair of transistors 26, 28 arranged to receive and amplify the first differential output signal 22, 24 thereby providing a second differential output signal 38, 40.

A shaper circuit includes a first amplifier including an input and an output, the input being configured to receive an input signal, which includes one or more current pulses, a feedback component coupled to the output and to the input of the first amplifier thereby forming a feedback loop of the first amplifier, and an RC component coupled to the output of the first amplifier and to a reference potential terminal. Therein the shaper circuit is configured to provide an output signal as a function of the input signal, the output signal including one or more voltage pulses, and the RC component is configured to largely cancel a low frequency pole of the feedback loop of the first amplifier.

An apparatus includes an amplifier. The amplifier has two inputs, and an output. The amplifier has a pole in its transfer function. The frequency of the pole depends on the output current of the amplifier. The amplifier further includes a pole frequency tracking (PFT) circuit. The PFT circuit includes a source follower circuit.

A tunable filter is provided. The tunable filter includes: a filter input; a filter output; at least one feedback loop coupled between the filter output and the filter input, where the at least one feedback loop includes at least one tunable feedback capacitance which is configured to tune a cut-off frequency of the tunable filter; and an active element, coupled between the filter input and the filter output and configured to drive the at least one tunable feedback capacitance, the active element having a transfer function with a primary pole and at least one secondary pole, where the active element includes a first stabilization element that is coupled to a first internal node of the active element.

Variable gain amplifiers (VGA) with output phase invariance are provided herein. In certain embodiments, a VGA is operable in a selected gain setting chosen from multiple gain settings that provide different amounts of amplification to a radio frequency (RF) input signal. The VGA includes a gain transistor that has a substantially constant bias current across the gain settings, such that the VGA's output phase, input impedance matching, and/or input return loss are substantially constant. The gain setting of the VGA is selected by controlling relative biasing of a pair of cascode transistors each connected to the gain transistor by a corresponding degeneration resistor. The degeneration resistors provide compensation that reduces or eliminates a difference in output phase of the VGA across gain settings, for instance, by introducing a zero in a transfer function of the VGA that cancels a pole arising from the cascode transistors.

A semiconductor integrated circuit including a differential amplifier circuit, a first output circuit, a second output circuit, a selection circuit, and a feedback circuit. The differential amplifier circuit is configured to operate at a first source voltage. The first output circuit is configured to receive an output of the differential amplifier circuit, output a first output, and operate at the first source voltage. The second output circuit is configured to receive an output of the differential amplifier circuit, output a second output, and operate at a second source voltage lower than the first source voltage. The selection circuit is configured to select one of the first output from the first output circuit and the second output from the second output circuit according to an operating phase determined by an external control signal. The feedback circuit is connected between the differential amplifier circuit and the selection circuit. The feedback circuit is configured to feed the selected output back to the differential amplifier circuit.

Variable gain amplifiers (VGA) with output phase invariance are provided herein. In certain embodiments, a VGA is operable in a selected gain setting chosen from multiple gain settings that provide different amounts of amplification to a radio frequency (RF) input signal. The VGA includes a gain transistor that has a substantially constant bias current across the gain settings, such that the VGA's output phase, input impedance matching, and/or input return loss are substantially constant. The gain setting of the VGA is selected by controlling relative biasing of a pair of cascode transistors each connected to the gain transistor by a corresponding degeneration resistor. The degeneration resistors provide compensation that reduces or eliminates a difference in output phase of the VGA across gain settings, for instance, by introducing a zero in a transfer function of the VGA that cancels a pole arising from the cascode transistors.

In some examples, an amplifier comprises a first integrator to receive a differential input signal, a second integrator coupled to the first integrator, a third integrator coupled to the second integrator, and a comparator to receive outputs of the second and third integrators, to compare each of the outputs to a reference signal that is below a power supply rail voltage supplied to the amplifier, and to produce an error current based on the comparison. The amplifier also comprises a feedback connection between the comparator and inputs to the second integrator. The feedback connection injects the inputs to the second integrator with a current that is determined at least in part by the error current.

A tunable filter is provided. The tunable filter includes: a filter input; a filter output; at least one feedback loop coupled between the filter output and the filter input, where the at least one feedback loop includes at least one tunable feedback capacitance which is configured to tune a cut-off frequency of the tunable filter; and an active element, coupled between the filter input and the filter output and configured to drive the at least one tunable feedback capacitance, the active element having a transfer function with a primary pole and at least one secondary pole, where the active element includes a first stabilization element that is coupled to a first internal node of the active element.

The disclosure relates to a tunable filter. The tunable filter includes: a filter input; a filter output; at least one feedback loop coupled between the filter output and the filter input, where the at least one feedback loop includes at least one tunable feedback capacitance which is configured to tune a cut-off frequency of the tunable filter; and an active element, coupled between the filter input and the filter output and configured to drive the at least one tunable feedback capacitance, the active element having a transfer function with a primary pole and at least one secondary pole, where the active element includes a first stabilization element that is coupled to a first internal node of the active element.