An apparatus comprising an amplifier, wherein the amplifier comprises: an output stage comprising a positive output terminal (Vout+) providing a positive output voltage and a negative output terminal (Vout-) providing a negative output voltage; a load tank coupled in parallel with the output stage and configured to filter signals received at the amplifier; and a negative resistance block coupled in parallel with the output stage and the load tank; and wherein the negative resistance block comprises: a first transconductance device comprising first, second and third terminals, of which the first terminal is a drain, the second terminal is a gate and the third terminal is a source, with the first terminal coupled to the positive output terminal of the output stage and the second terminal coupled to the negative output terminal of the output stage; and a second transconductance device comprising first, second and third terminals, of which the first terminal is a drain, the second terminal is a gate and the third terminal is a source, with the first terminal coupled to the negative output terminal of the output stage and the second terminal coupled to the positive output terminal of the output stage; characterised in that the second terminals of the first and second transconductance devices are capacitively coupled (712, 714) to the negative output terminal of the output stage and the positive output terminal of the output stage, respectively, and the second terminals of the first and second transconductance devices are each resistively coupled (702, 704) to a tuning current source. an output stage comprising a positive output terminal (Vout+) providing a positive output voltage and a negative output terminal (Vout-) providing a negative output voltage; a load tank coupled in parallel with the output stage and configured to filter signals received at the amplifier; and a negative resistance block coupled in parallel with the output stage and the load tank; and wherein the negative resistance block comprises: a first transconductance device comprising first, second and third terminals, of which the first terminal is a drain, the second terminal is a gate and the third terminal is a source, with the first terminal coupled to the positive output terminal of the output stage and the second terminal coupled to the negative output terminal of the output stage; and a second transconductance device comprising first, second and third terminals, of which the first terminal is a drain, the second terminal is a gate and the third terminal is a source, with the first terminal coupled to the negative output terminal of the output stage and the second terminal coupled to the positive output terminal of the output stage; characterised in that a first transconductance device comprising first, second and third terminals, of which the first terminal is a drain, the second terminal is a gate and the third terminal is a source, with the first terminal coupled to the positive output terminal of the output stage and the second terminal coupled to the negative output terminal of the output stage; and a second transconductance device comprising first, second and third terminals, of which the first terminal is a drain, the second terminal is a gate and the third terminal is a source, with the first terminal coupled to the negative output terminal of the output stage and the second terminal coupled to the positive output terminal of the output stage; characterised in that the second terminals of the first and second transconductance devices are capacitively coupled (712, 714) to the negative output terminal of the output stage and the positive output terminal of the output stage, respectively, and the second terminals of the first and second transconductance devices are each resistively coupled (702, 704) to a tuning current source.