An envelope tracking supply modulator arranged to generate a modulated supply voltage (V PA) according to an envelope input (S ENV) of a radio-frequency signal (S RF), and provide the modulated supply voltage (V PA) to a power amplifier for amplifying the radio-frequency signal (S RF), wherein the envelope input is a differential signal consisting of a positive signal and a negative signal and wherein the envelope tracking supply modulator comprises: an amplifier circuit (106, 200) comprising: a voltage-to-current conversion circuit (108, 202), arranged to generate a current signal according to the envelope input, wherein the voltage-to-current conversion circuit (108, 202) comprises: an operational transconductance amplifier, OTA, arranged to output the current signal at an output port of the OTA; and a current-to-voltage conversion circuit (110, 204), arranged to generate an output voltage signal according to the current signal, wherein the current-to-voltage conversion circuit (110, 204) comprises a linear amplifier, LA, wherein an input port of the LA is coupled to the output port of the OTA, and the output voltage signal is involved in setting the modulated supply voltage (V PA) of the power amplifier derived from an output signal at an output port of the LA, wherein the OTA is a source degenerated amplifier, the source degenerated amplifier comprises a first differential amplifier, a second differential amplifier and a differential pair (MN1, MN2) with source degeneration comprising a first transistor and a second transistor, wherein the first differential amplifier has a first input coupled to the negative signal, a second input directly connected to the source of the first transistor and an output directly connected to the gate of the first transistor, wherein the second differential amplifier has a first input coupled to the positive signal, a second input directly connected to the source of the second transistor and an output directly connected to the gate of the second transistor, wherein each of the first and second differential amplifiers (A1, A2) is a two-stage amplifier with at least one compensation capacitor. a voltage-to-current conversion circuit (108, 202), arranged to generate a current signal according to the envelope input, wherein the voltage-to-current conversion circuit (108, 202) comprises: an operational transconductance amplifier, OTA, arranged to output the current signal at an output port of the OTA; and a current-to-voltage conversion circuit (110, 204), arranged to generate an output voltage signal according to the current signal, wherein the current-to-voltage conversion circuit (110, 204) comprises a linear amplifier, LA, wherein an input port of the LA is coupled to the output port of the OTA, and the output voltage signal is involved in setting the modulated supply voltage (V PA) of the power amplifier derived from an output signal at an output port of the LA, an operational transconductance amplifier, OTA, arranged to output the current signal at an output port of the OTA; and a current-to-voltage conversion circuit (110, 204), arranged to generate an output voltage signal according to the current signal, wherein the current-to-voltage conversion circuit (110, 204) comprises a linear amplifier, LA, wherein an input port of the LA is coupled to the output port of the OTA, and the output voltage signal is involved in setting the modulated supply voltage (V PA) of the power amplifier derived from an output signal at an output port of the LA, wherein the OTA is a source degenerated amplifier, the source degenerated amplifier comprises a first differential amplifier, a second differential amplifier and a differential pair (MN1, MN2) with source degeneration comprising a first transistor and a second transistor, wherein the first differential amplifier has a first input coupled to the negative signal, a second input directly connected to the source of the first transistor and an output directly connected to the gate of the first transistor, wherein the second differential amplifier has a first input coupled to the positive signal, a second input directly connected to the source of the second transistor and an output directly connected to the gate of the second transistor, wherein each of the first and second differential amplifiers (A1, A2) is a two-stage amplifier with at least one compensation capacitor.