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ADAPTIVE CLASS-BIAS ADJUSTMENT LOOP FOR BIPOLAR POWER MODULES

IP.com Disclosure Number: IPCOM000006421D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2002-Jan-02
Document File: 3 page(s) / 161K

Publishing Venue

Motorola

Related People

Donald Wong: AUTHOR [+3]

Abstract

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.

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MOTOROLA INC. Technical Developments Volume 15 May 1992

ADAPTIVE CLASS-BIAS ADJUSTMENT LOOP

FOR BIPOLAR POWER MODULES pi

by Donald Wong, Y.K. Chan and William Lee ~

  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.

  This disclosure details an adaptive arrangement such that a commercially available power amplifier module can be cotigured to dynamically vary its Class-Bias (from Class C to Class AB and A) under an empirically delined relationship with its programmable output RF level. Accordingly, a typical commercial power amplifier

module may consist of fou: (4) cascaded bipolar stages. The first stage is a futed S&XII signal gain stage, while the second stage is gain cdntrollable through base. and collector biasing. The final t&o output stages are pseudo Class C biased. Relative pc$ver envelope at the module output can be determined by using a directional coupler and Schottky barrier diodks for level detection. Both forward and reverse power'ipropagation are monitored. The resultant detection voltages are compared with a reference, which is provided through a Digital-to-...