![[Circuit diagram of HF Amplifier]](../../images/hf-amp-cct.gif)
A high frequency amplifier working across the HF bandwidth was required that has good return loss and group delay at lower RF frequencies. The 1 GHz Broadband Amplifier in our range does go down to 300 KHz, but the return loss is not too good at the lower frequencies. The reason for this is that the broadband amplifier uses a lump of ferrite in a RF inductor that feeds power into the IC. The lowest usable frequency is dependant upon the size of this ferrite. The lower the frequency, the larger the ferrite and the greater the cost. Large binocular ferrites are not so readily available and become difficult to fit in a small enclosure. It was decided to produce an amplifier with no ferrite or inductors. This means that the broadband amplifier circuit cannot be used and also the common base configuration as used in the VHF Pre-Amplifier, is of no use because this configuration requires inductors for impedance matching on both the input and the output. It was decided to use the shunt series pair circuit 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. An input termination was added with a coupling capacitor and an emitter follower circuit was also added at the output to lower the output impedance to 50 Ohms. The resulting circuit is shown below in Figure1.
Figure 1
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.
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. These 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
![[Photograph of blank PCB]](../../images/hf-amp2.jpg)
Grounding is decided after the PCB is fitted into the enclosure.
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