US 5,920,808 A · Method and apparatus for reducing key-up distortion by pre-heating transistors

Abstract

A biasing circuit for a power amplifier, for use in a transmitter, that substantially reduces distortion during key-up and thereby reduces key-up time. The power amplifier includes an output transistor and a bias circuit. The bias circuit is applied to all the class AB stages of the power amplifier. The bias circuit provides to the output transistor a first bias level during the preheat period and a second bias level during the transmit period. This first bias level is predetermined to cause the output transistor to reach the steady-state junction temperature achieved by the output transistor during the transmit period (i.e., when transmitting output signals biased with the second bias level). The preheat period ends when this steady-state temperature is reached. Thus, the power amplifier can then transition to a transmit period having already reached the steady-state junction temperature. Because the output transistor is already heated to the steady-state junction temperature, "thermal" distortion (i.e., the distortion incurred when the output transistor's junction temperature changes while transmitting) is avoided and the key-up time is reduced.

Claims23

Dependent claims

A power amplifier for use in a transmitter, the power amplifier comprising:

an output transistor having an input lead and an output lead;
and
a bias circuit coupled to said input lead of said output transistor, wherein said bias circuit is configured to provide a first bias level to said output transistor when said transmitter is operating in a preheat period and to provide a second bias level when said transmitter is transmitting in a transmit period, wherein said output transistor is operative to generate substantially equal heat in said preheat period and said transmit period.

A transmitter comprising:

a modulator coupled to receive an input signal, said modulator configured to output a modulated carrier signal dependent on said input signal;
a predistorter coupled to said modulator, said predistorter configured to output a predistorted signal dependent on said modulated carrier signal from said modulator;
a power amplifier coupled to said predistorter, said power amplifier configured to output an amplified signal dependent on said predistorted signal from said predistorter, wherein said power amplifier comprises:
an output transistor having an input lead and an output lead, and
a bias circuit coupled to said input lead of said output transistor, wherein said bias circuit is configured to provide a first bias level to said output transistor when said transmitter is operating in a preheat period and to provide a second bias level when said transmitter is transmitting in a transmit period;
and
a trainer coupled to power amplifier and said predistorter, said trainer configured to provide to said predistorter a training signal dependent on said amplified signal from said power amplifier,
wherein said predistorter is operative to introduce distortion to said modulated signal from said modulator substantially canceling distortion introduced by said power amplifier when said transmitter is transmitting.

5 more elements

Dependencies are inferred from the claim text.

Method and apparatus for reducing key-up distortion by pre-heating transistors

Life of this patent

Method and apparatus for reducing key-up distortion by pre-heating transistors

Life of this patent

Abstract

Claims23