ADAPTIVE CLASS-BIAS ADJUSTMENT LOOP FOR BIPOLAR POWER MODULES
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2002-Jan-02
Strong market demands for wide output power level adjustment range, microcontroller based, FM wireless communication network terminals have created a unique challenge for transceiver designers. Conventional use of tixed level, Class C, power ampliier modules in FM transceiver for higher power-added effGency is fast becoming inadequate since they usually have limited input power range for automatic output power level adjust- ment. This limitation is caused by the Class-Bias (Class C) condition of the power stages in the module since a significant amount of ac level is required to overcome the reverse bias voltage and maintain the conduction angle for the power transistors to deliver the ac power to the load. Since the Automatic Level Control (ALC) circuit should best be used in the driver stage, its inclu- sion would not levitate the aforementioned limitation. In fact, experiment has demonstrated that the conven- tional control port has a limited (6 to 8 dB) RF spurious free adjustment range. The problem of spurs generation is due to the impedance mismatch of the I/O circuits at low drive level, which are optimized principally for Class C parameters of the power BJT (i.e. drive level depend- ent). In addition, the reduced input drive level will cause the majority of the conduction angles (Class C) of the ac signal to fall into the maximum hre region. The result- ant excessive gain and termination mismatches, in turn, will render the BJT to become rather unstable to oper- ate. Another alternative may be to switch in a high power programmable step attenuator at the. output of the power module. This arrangement is clearly not an economic solution in terms of cost, size, energy consumption, and thermal considerations. Use of other Class-Bias (e.g. Class A) for both high and medium power in FM trans- mission application is deemed battery power in&icient. This, however, may not be a prohibitive concern at low RF levels.