A converting circuit for converting a differential signal (IN,INX) to a single-ended signal, the differential signal being input to the converting circuit and the single-ended signal being outputted to the converting circuit, comprising: a cascode amplifier comprising a first transistor and a second transistor, wherein: the first transistor comprises a control terminal, a first terminal, and a second terminal, a signal which is part of the differential signal (IN, INX) being input to the control terminal, and the control terminal being connected to receive a biasing voltage; and the second transistor comprises a first terminal and a second terminal, the first terminal of the second transistor being connected to the first terminal of the first transistor, the output signal being outputted from the second terminal of the second transistor; characterised by: a variable capacitor for adjusting the frequency characteristic of the converting circuit, the capacitor being connected to the second terminal of the first transistor; and a current source connected to the second terminal of the first transistor, which current source comprises a third transistor, comprising a control terminal, and a grounded capacitor connected to the control terminal of the third transistor. a cascode amplifier comprising a first transistor and a second transistor, wherein: the first transistor comprises a control terminal, a first terminal, and a second terminal, a signal which is part of the differential signal (IN, INX) being input to the control terminal, and the control terminal being connected to receive a biasing voltage; and the second transistor comprises a first terminal and a second terminal, the first terminal of the second transistor being connected to the first terminal of the first transistor, the output signal being outputted from the second terminal of the second transistor; the first transistor comprises a control terminal, a first terminal, and a second terminal, a signal which is part of the differential signal (IN, INX) being input to the control terminal, and the control terminal being connected to receive a biasing voltage; and the second transistor comprises a first terminal and a second terminal, the first terminal of the second transistor being connected to the first terminal of the first transistor, the output signal being outputted from the second terminal of the second transistor; characterised by: a variable capacitor for adjusting the frequency characteristic of the converting circuit, the capacitor being connected to the second terminal of the first transistor; and a current source connected to the second terminal of the first transistor, which current source comprises a third transistor, comprising a control terminal, and a grounded capacitor connected to the control terminal of the third transistor. a variable capacitor for adjusting the frequency characteristic of the converting circuit, the capacitor being connected to the second terminal of the first transistor; and a current source connected to the second terminal of the first transistor, which current source comprises a third transistor, comprising a control terminal, and a grounded capacitor connected to the control terminal of the third transistor.