The present invention relates generally to power amplifier circuits, and more particularly to power amplifier circuits that are controlled to maintain a desired constant output power level in burst mode operations. Of special interest is the use of the present invention in controlling the output power of digital cellular telephones.
The prior art techniques used in continuously transmitting power amplifiers found in present FM cellular systems are inadequate for burst mode digital transmissions found in the new standard for cellular telephone units. The prior art techniques may cause overshoots of RF power to appear at the output of the power amplifier at the start of a transmit burst.
In a conventional power amplifier used in cellular telephone service, there is included a control loop typically comprising a power amplifier under control, a power detector with an associated conditioning circuit that generate a DC voltage proportional to the output power of the amplifier, and a voltage comparator that compares the feedback voltage from the power detector to that of an input control voltage from a controller. A loop filter tailors the response of the control loop to assure loop stability as well as other loop characteristics, such as loop damping and responsiveness.
The difference between the control input voltage and the feedback voltage is an error voltage or signal. The error signal is used to drive the power amplifier biasing circuit or an attenuator placed at the input or output of the amplifier. This closed-loop control circuit acts in such a way as to force a null condition to exist in the comparator, so that the input control voltage equals the feedback voltage. The overall purpose of the power amplifier output control loop is to reference the output power to a known power detector in order to avoid output power variations due to changes in temperatures and supply voltages. The controller typically contains a lookup table for the power detector voltages as a function of the true output level of the power amplifier.
Operationally, as the cellular telephone unit enters a cell site to place a phone call, the base station directs the telephone unit to output a specified RF level. This level must be held to within some predetermined tolerance regardless of disturbances such as changes in battery voltages and temperature, and the like. The above-described conventional circuitry is inadequate for cellular telephone service made to meet newer, burst mode, standards.
The problem with the prior art control technique lies in the fact that under the newer standard, the RF transmission is not continuous as in an analog FM system. Under the new standard, transmissions are time division multiplexed, in that the amplifier is controlled to output RF energy only a portion of the time, and the rest of the time period is dedicated to receive functions. During the receive portion of the period, the amplifier is normally shut off.
This transmit-receive-transmit pattern repeats for the duration of the phone conversation. If the conventional technique is used for controlling the power amplifier operating in a burst mode scenario as above-described, there will be no output power during the receive period and the power detector would indicate zero power, while the input control voltage is still set for the desired output power.
Therefore, during the transmitter off period, the loop is open and the comparator sees two unequal input voltages so it will try to swing towards the high voltage end. Then, as the amplifier is switched on for the transmit burst, the high voltage condition that exists in the comparator/loop filter will cause the amplifier to output maximum power momentarily before the loop can correct itself back to equilibrium, causing the power overshoots mentioned above. From this condition it should be obvious that a new controlled power amplifier system that prevents the transient problem would constitute an important advancement in the art.