Transconductance current source

A transconductance circuit comprising:

• an input terminal and an output terminal;
• a first current source having a gate connected to said input terminal;
• and
• a second current source, in parallel with said first current source, and having a higher transconductance and a wider dynamic range than the first current source, and wherein the current sources are configured so that at a low input voltage only the first current source is on, and wherein:
• the transconductance circuit comprises a voltage drop circuit providing a lower bias voltage for the second current source than for the first current source,
• the voltage drop circuit comprises a source follower and a current source for the source follower, in which the input terminal is linked with the gate of the source follower and the first current source, and
• the source follower and the current sources are configured so that a gate-source voltage drop across the source follower ensures that the second current source is off until a point during which the input voltage increases at which the second current source is switched on.