Electronic circuit with cascode amplifier

1. An electronic circuit comprising an amplifier, the amplifier comprising:

an amplifying transistor comprising a gate, source and drain, the gate being coupled to an input for receiving a radio frequency (RF) input signal, the source being coupled to a power supply reference connection;

a cascode transistor comprising a gate, source and drain, the source of the cascode transistor being coupled to the drain of the amplifying transistor;

a capacitive voltage divider comprising first and second capacitors, the capacitive voltage divider configured to apply a fraction of an RF signal swing from the drain of the cascode transistor to the gate of the cascode transistor, the fraction being determined by a ratio between capacitance values of the first and second capacitors;

a bias voltage supply circuit comprising a voltage output coupled to the gate of the cascode transistor, the bias voltage supply circuit configured to define a relation between an average gate voltage of the cascode transistor and an average drain supply voltage at the drain of the cascode transistor, the relation configured to increase the average gate voltage in proportion to an increased average drain voltage, and the relation configured to provide a non zero average gate voltage when the average gate voltage is extrapolated to zero;

a modulation control circuit; and

a voltage generator circuit comprising a control input coupled to the modulation control circuit and a cascode transistor output coupled to the drain of the cascode transistor configured to supply the average drain supply voltage under control of the modulation control circuit,

wherein the cascode transistor has a higher drain-gate breakdown voltage than the amplifying transistor and wherein the capacitive voltage divider is configured to make the fraction smaller than a ratio defined by the relation, the ratio comprising the average gate voltage of the cascode transistor supplied by the bias voltage supply circuit divided by the average drain supply voltage.

2. An electronic circuit according to claim 1 , wherein the bias voltage supply circuit comprises:

a resistive voltage divider coupled between the drain of the cascode transistor and the power supply reference connection, the resistive voltage divider including a divided voltage output coupled to the gate of the cascode transistor; and

an offset voltage source coupled to the gate of the cascode transistor.

3. An electronic circuit according to claim 1 , wherein the capacitive voltage divider is configured to set the fraction of the RF signal swing so that a ratio between RF drain-gate voltages of the cascode transistor and the amplifying transistor at maximum drain supply voltage is substantially equal to a ratio between drain-gate breakdown voltages of the cascode transistor and the amplifying transistor.

4. An electronic circuit according to claim 1 , wherein the bias voltage supply circuit is configured to set the average gate voltage of the cascode transistor substantially at a value where a maximum power efficiency of the amplifier as a function of average cascode transistor gate voltage occurs.

5. An electronic circuit according to claim 1 , wherein the first capacitor comprises an intrinsic drain-gate capacitance of the cascode transistor and the second capacitor is coupled between the gate of the cascode transistor and the power supply reference connection.

6. An electronic circuit according to claim 1 , further comprising a load circuit coupled to the drain of the cascode transistor.

7. An electronic circuit according to claim 6 , wherein the load circuit comprises a resonant circuit.