A low dropout regulator includes an output circuit and an amplifier. The output circuit includes a signal input end configured to receive an input voltage and a signal output end configured to output an output voltage. The amplifier includes a first stage amplifier circuit, a second stage amplifier circuit, a first feedback circuit and a second feedback circuit. The first stage amplifier circuit includes a positive output end and a negative output end. The second stage amplifier circuit includes an input end and an output end, wherein the input end and the positive output end are coupled at a first node, and the output end is coupled to the output circuit. The first feedback circuit is coupled between the negative output end and the output end. The second feedback circuit is coupled between the first node and the output end.

A circuit comprises: a circuit input; a circuit output; at least one passive feedback loop coupled between the circuit output and the circuit input; an active element, coupled in a feed-forward path of the circuit between the circuit input and the circuit output and configured to drive the at least one feedback loop in order to establish a function of the circuit, wherein the feed-forward path of the circuit comprises a second node (Vx) and a first node which are internal nodes of the active element and which are coupled between the circuit input and the circuit output, wherein the first node is configured to have a first voltage, the first voltage being a function of the circuit output, wherein the active element comprises a first voltage drop element coupled between the second node (Vx) and the first node.

A voltage-to-current converter includes a first differential pair of transistors, a second differential pair of transistors, and a first resistor. The first differential pair of transistors includes a first transistor and a second transistor. An emitter of the first transistor is directly connected to an emitter of the second transistor. The second differential pair of transistors includes a third transistor and a fourth transistor. An emitter of the third transistor is directly connected to an emitter of the fourth transistor. The first resistor is connected to the emitter of the first transistor, the emitter of the second transistor, the emitter of the third transistor, and the emitter of the fourth transistor.

An apparatus includes a photodiode configured to detect an optical signal; a common-base amplifier configured to input a current signal converted from the optical signal by the photodiode; an common-emitter amplifier configured to couple to an output of the common-base amplifier; a first circuit configured to feed back the output of the common-emitter amplifier to an output of the common-base amplifier; and a second circuit configured to, when power of the optical signal exceeds a predetermined level, reduce a load resistance value of the common-base amplifier and increase an emitter current of the common-emitter amplifier.

An apparatus includes a photodiode configured to detect an optical signal; a common-base amplifier configured to input a current signal converted from the optical signal by the photodiode; an common-emitter amplifier configured to couple to an output of the common-base amplifier; a first circuit configured to feed back the output of the common-emitter amplifier to an output of the common-base amplifier; and a second circuit configured to, when power of the optical signal exceeds a predetermined level, reduce a load resistance value of the common-base amplifier and increase an emitter current of the common-emitter amplifier.

A voltage-to-current converter includes a first differential pair of transistors, a second differential pair of transistors, and a first resistor. The first differential pair of transistors includes a first transistor and a second transistor. An emitter of the first transistor is directly connected to an emitter of the second transistor. The second differential pair of transistors includes a third transistor and a fourth transistor. An emitter of the third transistor is directly connected to an emitter of the fourth transistor. The first resistor is connected to the emitter of the first transistor, the emitter of the second transistor, the emitter of the third transistor, and the emitter of the fourth transistor.

A high-speed, high-voltage gallium nitride based (GaN-based) operational amplifier (op amp) is disclosed. The combined high-speed, high-voltage capability allows the GaN-based op amp to serve as a dynamic power supply (DPS) for a radio frequency power amplifier (RFPA). When serving as a DPS for an RFPA, the GaN-based op amp is capable of supplying ampere-scale currents that accurately track rapidly-varying envelopes of non-constant envelope RF signals, thereby allowing the RFPA to convert high-bandwidth non-constant envelope RF signals to high RF output powers with high signal envelope accuracy.

An amplifier circuit includes a first cascode transistor and a second cascade transistor, the first cascade transistor being electrically connected between a first transistor and a first load circuit, the second cascode transistor being electrically connected between a second transistor and a second load circuit. The amplifier circuit includes a first shunt transistor and a second shunt transistor, the first shunt transistor being electrically connected between the first transistor and a first emitter-follower circuit, the second shunt transistor being electrically connected between the second transistor and a second emitter-follower circuit. A differential current signal includes a first differential current and a second differential current, the first differential current flowing through the first cascode transistor and the second cascode transistor, and a second differential current flowing through the first shunt transistor and the second shunt transistor.