RF power amplifier circuit

1. A RF power amplifier circuit having at least one output power transistor and a protection circuit protecting the power transistor against high voltages that lead to a destructive breakdown of the power transistor, the circuit comprising:

(a) a biasing circuit ( 6 ) comprising a voltage-to-current convener and a biasing transistor, said biasing circuit biasing the power transistor ( 2 );

(b) a peak detector ( 8 ) detecting the output voltage of the power transistor, and

(c) a comparator circuit ( 12 ) connected to the peak detector ( 8 ) and designed to reduce the base current of the power transistor ( 2 ) when controlled by the peak detector.

2. The RF power amplifier circuit of claim 1 , wherein the collector of the power transistor ( 2 ) is connected to the peak detector ( 8 ) via a capacitor ( 46 ).

3. The RF power amplifier circuit of claim 1 , wherein the peak detector ( 8 ) comprises a diode ( 52 ) connected to a capacitor ( 46 ) coupling the output signal of the power transistor ( 2 ) to the peak detector ( 8 ).

4. The RF power amplifier circuit of claim 3 , wherein a capacitor ( 53 ) is arranged between the diode ( 52 ) and ground.

5. The RE power amplifier circuit of anyone of the claim 1 , wherein the peak detector ( 8 ) comprises voltage dividing resistors ( 48 , 50 ) connected between the capacitor ( 46 ) and ground.

6. The RF power amplifier circuit of claim 5 , wherein one or several resistors of the voltage dividing resistors arc adjustable.

7. The RF power amplifier circuit of claim 5 , wherein an intermediate node of the voltage dividing resistor arrangement is the output of the peak detector.

8. The RF power amplifier circuit of claim 1 , wherein the comparator circuit ( 12 ) comprises a transistor ( 56 ) and, in series connection, L- 1 diodes 58 ( 1 ) to 58 (L- 1 ).

9. The RF power amplifier circuit of claim 1 , wherein the comparator circuit ( 12 ) is connected via a transistor ( 62 ) to the base of the biasing transistor ( 38 ) of the biasing circuit ( 6 ).

10. The RF power amplifier circuit of claim 1 , wherein the collector terminal of the input transistor ( 56 ) of the comparator circuit ( 12 ) is connected to the supply voltage and the emitter is connected to ground through the series of diodes.

11. The RF power amplifier circuit of claim 1 , having a plurality of power transistors ( 70 , 72 , 74 ) each connected to a biasing circuit ( 82 , 84 , 86 ), wherein the output of the peak detector comprised in a control circuit ( 100 ) is connected through a comparator circuit ( 102 ) and current mirror transistors ( 112 , 114 , 116 ) to the respective biasing circuits ( 82 , 84 , 86 ) of the power transistors ( 70 , 72 , 74 ).

12. The RF power amplifier circuit of claim 1 , comprising a switch ( 10 ) which is connected between Vsupply and the peak detector ( 8 ), the ON and OFF state of the switch being controlled by Vcontrol.

13. The RF power amplifier circuit of claim 1 , comprising a filter comprising resistors ( 40 , 42 ) and a capacitor ( 44 ) connected from a node between the resistors ( 40 , 42 ) to ground, which filter is arranged between the switch ( 10 ) and the peak detector ( 8 ).

14. A wireless communication device comprising an RF power amplifier circuit having at least one output power transistor and a protection circuit protecting the power transistor against high voltages that lead to a destructive breakdown of the power transistor, the circuit comprising:

(a) a biasing circuit ( 6 ) comprising a voltage-to-current converter and a biasing transistor, said biasing the power transistor ( 2 );

(b) a peak detector ( 8 ) detecting the output voltage of the power transistor; and

(c) a comparator circuit ( 12 ) connected to the peak detector ( 8 ) and designed to reduce the base current of the power transistor ( 2 ) when controlled by the peak detector.