Patent ID: 6137366
Filing Date: 2000-10-24
Classification: H03F

Abstract:
An output stage suitable for use in a radio frequency (RF) power amplifier, the output stage comprising:a first transistor having a base, a collector and an emitter, wherein a base-emitter voltage V.sub.be applied across the base and the emitter of the first transistor can control a collector current I.sub.cQ1 flowing between the collector and the emitter of the first transistor, the collector current I.sub.cQ1 being a function of a base current I.sub.bQ1 flowing into the base of the first transistor; andcontrol circuitry for electrically coupling the base of the first transistor to a common ground reference, the control circuitry including a variable impedance device, the control circuitry operable to sense the base-emitter voltage V.sub.be and adjust the impedance of the variable impedance device such that a portion of the base current I.sub.bQ1 is adsorbed by the variable impedance device when the base-emitter voltage V.sub.be exceed a certain threshold voltage,whereby the control circuitry tends to prevent secondary breakdown of the first transistor by adsorbing the portion of the base current thereby limiting I.sub.cQ1 ;wherein the variable impedance device includes a second transistor having a collector, as base, and an emitter;wherein the collector of the second transistor is coupled to the base of the first transistor, the base of the second transistor is driven by a signal related to the base-emitter voltage V.sub.be, and the emitter of the second transistor is coupled to the common ground reference;wherein the control circuitry further includes a pair of resistors R.sub.1 and R.sub.2 coupled in series between the base of the first transistor and the common around reference, and a node at which the resistors R.sub.1 and R.sub.2 are coupled providing at the base of the second transistor the signal having a voltage (V.sub.be *R.sub.2 (R.sub.1 +R.sub.2))).