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Philips Semiconductors
A linear broadband 12 W ampli�er for band IV/V TV transposers based on the BLV58
1 ABSTRACT
Application Note AN98028
A broadband linear amplifier design is presented, suitable for application in TV transposers operating in bands IV and V (470 � 860 MHz). The design is based on a single BLV58 bipolar transistor in push pull configuration. Results at the published class A bias point (25 V/3.2 A) for the BLV58 include 12 W peak sync output power at �52 dB three tone IMD level and 11 dB gain in the (470 � 860) MHz range. Po-sync level improved by 4 W at 26 V/3.8 A. 2 INTRODUCTION
For solid state TV transposers Philips introduced the BLV58, a bipolar linear push-pull power transistor designed to operate in the 470 � 860 MHz range. Narrowband intermodulation distortion level is < �45 dB and powergain >10 dB at 860 MHz. In this application note an amplifier design is to be discussed which demonstrates the broadband capabilities of a BLV58 in push-pull configuration. Special attention is paid to the broadband tuning procedure for good and reproducible linearity. Fullband performance data are presented measured with two different 3-tone systems at two bias levels. 3 GENERAL CONSIDERATIONS
UHF solid state TV transposers for service in band 4 and 5 are commonly realized using broadband amplifiers which are capable to handle both bands. The typical frequency range is 470 � 860 MHz or channel 21 to 69. Some transposers also service additional channel at the high end of band V. But these are quite rare. High powers are obtained by combining power from several lower power modules. The basic low power module consists of two push-pull amplifier combined using 3 dB quadrature hybrids. The push-pull amplifier is designed for flat gain response by allowing input mismatch which gradually improves with frequency. To have good input return loss throughout the band two of these push-pull amplifiers are combined using 3 dB quadrature hybrids. Low and flat intermodulation distortion (IMD) is required throughout the band. As linearity performance is strongly determined by the collector loading, special attention must be paid to the tuning procedure for obtaining minimum and reproducible IMD response. The class-A bias point used for this design is that published in the datasheet. However broadband performance has been evaluated at elevated bias levels within the DC safe operating area. Some means of bias temperature compensation is applied to achieve thermal stability. 4 AMPLIFIER DESCRIPTION
Figure 1 shows the schematic of the total push pull amplifier without the biasing circuitry. It utilizes mixed lumped and distributed low pass/high pass impedance matching sections for maximum bandwidth. The low pass sections consists of shunt capacitors and series transmission lines. The high pass sections consist of series capacitors and short circuited stubs. The stubs also form a part of the balance to unbalance(balun) transformers. The length of the stubs is less than 1/8 wavelength at 470 MHz. Balance is maintained by loading both the outer and inner conductors with identical stubs. The series capacitors also act as DC blocking. The board material used for this amplifier is ULTRALAM 2000 from ROGERS Corp. which has a good price/performance ratio and good mechanical stability. It is a PTFE based substrate with εr = 2.55 and 30 mils (0.76 mm) thickness. The printed-circuit board layout is shown in Figs 2 and 3 shows the component layout diagram. The list of components is given on page 7. 5 LOAD NETWORK DESIGN
The theoretical design was based on load impedance data from the datasheet. The conjugate of the broadband load impedance for one section was modelled as shown in Fig.4. R represents the load resistance to obtain good linearity, C the effective output capacitance of the transistor and L the bonding wires and package lead inductance.
