Building a HF Amplifier Project

Introduction

  A high frequency amplifier working across the HF bandwidth was required that has good return loss across the whole frequency band. It was decided to design an amplifier using the shunt series pair configuration and a reference to this can be found on page 872 in the excellent electronics reference book "The Art of Electronics by Paul Horowitz and Winfield Hill, Cambridge University Press, ISBN 0-521-37095-7". The circuit as shown in the book would have a relatively high input and output impedance and would not function very well in a 50 Ohm system. A termination resistor was added with a coupling capacitor at the input and an emitter follower circuit was also added followed by a resistor in the output to lower the output impedance and bring it to 50 Ohms. The resulting circuit is shown below in Figure1.

Figure 1

Circuit Description

  The HF amplifier circuit is shown in the schematic diagram in fig 1 above. The input signal is terminated by R1 and the overall input impedance is set by R1 in parallel with R2 plus the input impedance at TR1. The gain of the TR1 stage is set by R5/R2, although R5 comes from the output of TR2, there is no voltage gain from TR2 because the output is taken from TR2 emitter. The gain of the TR2 stage is then set by the ratio of R6/R7. Finally the overall gain of 14 dB is set by R8. TR3 buffers the output of TR2 and sources 52 mA into R9||R11 which are paralleled in order to dissipate 200mW. This gives a maximum output of +11 dBm with 1 dB compression at 30 MHz. The output impedance is set by the very low emitter impedance of TR3 in series with R10. De-coupling capacitors C1, C2 and C3 are added to the + 7.5 Volt supply rail to ensure that the amplifier does not oscillate via the supply. The supply rail voltage is set low, at 7.5 Volts, in order to keep the dissipation in R9||R11 to a minimum.

Construction

  The prototype circuit was built on a Transmission Line PCB, which comes with the Transmission Line FLEXI-BOX kit and is designed for the express purpose of this type of circuit development. The FLEXI-BOX is a screened project box especially designed for this sort of project. The circuit boards are available as a single item and can also be used separately from the box with SMA connectors soldered directly onto the board. The return loss of the CPWG track is normally better than -20 dB from 0.3 MHz to 3 GHz. A blank PCB is shown in Figure 2 and it can be seen that there are no ground links fitted at this stage.

Figure 2

  Grounding is decided after the PCB is fitted into the enclosure.

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