Patent ID: 8103226

Claim:
A power amplifier circuit for a radio frequency (RF) transmitter, the power amplifier circuit comprising: a power amplifier having at least one output transistor having an output transistor terminal coupled to a supply voltage, the power amplifier including a high-band output transistor and a low-band output transistor selectably activatable in response to a band-select signal, the high-band output transistor having a high-band output transistor terminal coupled to a supply voltage, the low-band output transistor having a low-band output transistor terminal coupled to a supply voltage; a duty cycle detector having an input coupled to the output transistor terminal, the duty cycle detector including: a limiter section coupled to the output transistor terminal, the limiter section blocking positive voltage excursions and passing negative voltage excursions, the limiter section including a first limiter and a second limiter, the first limiter being coupled to the high-band output transistor terminal, the first limiter blocking positive voltage excursions and passing negative voltage excursions, the second limiter being coupled to the low-band output transistor terminal, the second limiter blocking positive voltage excursions and passing negative voltage excursions; and an averaging filter section coupled to an output of the limiter section, the averaging filter section including a first averaging filter coupled to an output of the first limiter and a second averaging filter coupled to an output of the second limiter; and a comparator section having first and second comparator inputs and producing a saturation detection output signal, one of the first and second comparator inputs being coupled to an output of the duty cycle detector, and the other of the first and second comparator inputs being coupled to a reference voltage circuit, the comparator section including a switching section responsive to the band-select signal, the switching section coupling the first comparator input to an output of the first averaging filter and coupling the second comparator input to a reference voltage circuit when the band-select signal indicates high-band operation, the switching section further coupling the first comparator input to an output of the second averaging filter and coupling the second comparator input to a reference voltage circuit when the band-select signal indicates low-band operation.