VHF Amplifier Circuit Description

Introduction

  This article describes the circuit of high frequency pre-amplifier working from 88 MHz to 108 MHz in the VHF band, the amplifier uses a single BFS17 transistor in a grounded base configuration. This configuration is used in order to avoid loss of high frequency gain, caused by the Miller effect. An interesting feature of this amplifier is that it can be used as a VHF pre-amplifier with DC power available through a local input or as a remotely powered VHF pre-amplifier with a remote DC source connected via the coaxial cable centre using a FLEXI-BOX Bias Tee.

Circuit

  The schematic diagram of the locally powered version is shown in Figure 1. The input to TR1 emitter is very low impedance ( ≈ Z_b/β||R1), and the 50 Ohm input signal is matched to this impedance by L1, C3 and C4. The matching also provides band-pass filtering hence the very narrow frequency response as shown in Figure 2. This filtering is an advantage when the pre-amp is used for FM radio as it functions as a pre-selector.

Figure 1

Figure 2

  The input matching circuit provides an input signal to TR1 emitter that has a much lower voltage than the original input voltage and a much higher current. This higher current is passed by TR1 and the collector current is varied by the same amount as the change of emitter current. This change of current across the collector load L2/L3 produces a considerable voltage gain but now at a higher impedance. The high impedance is matched to the 50 Ohm load by C8,C9 and L4. The variable capacitor (C6) tunes the output circuit and is set to give the flattest frequency response. Biasing of TR1 base is via R3 and R2 with decoupling providing by C7. De-coupling capacitors C1, C2 and C5 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 within the voltage ratings of the BFS17 transistor. With a supply current of less than 10 mA, the circuit could powered by batteries. The pre-amplifier gives a gain of approximately 11 dB and a maximum output of power of approximately +4 dBm with 1 dB compression, the output level ripple is less than + 0.6 dB from 88 MHz to 108 MHz. The noise figure of the BFS17 is given as 4.5 dB at 500 MHz typical. A lower noise transistor could be substituted in noise critical applications.

Remotely Powered Version

  With one very simple modification, the pre-amplifier can be powered via the centre conductor of the coaxial output connector. The coupling capacitor C8 (1nF) is changed to a zero Ohm link. This leaves L2 unchanged and so it's effect on tuning and output matching also remains unchanged. L2 now just passes DC to capacitors C1, C2 and C5 with any ac present decoupled to ground. The DC on these capacitors still biases TR1 base via R3 and R2 as before with C7 still providing decoupling.

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