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VFO buffer/amplifier

This two stage buffer/amplifier configuration appears in many designs, usually with fixed gain, ie resistor 'Rf' and it's associated dc blocking capacitor not fitted. The voltage gain when driven by a high impedance source is determined by the shunt resistance of the 620 ohm resistor in parallel with the BFS17 base to emitter equivalent signal impedance which together form the lower half of a series pot down network, where the upper part is that of the 2400 ohm resistor.

Driving the circuit from a 50 ohm source will lower further the equivalent impedance of the lower part of the pot down circuit, so the voltage gain will increase as a result.

With current projects in mind, I thought it would be interesting and useful to plot the frequency response of this circuit with source and load impedance's set at 50 ohms. The first graphs are taken using a trusty Marconi TF2370 analyser and tracking generator, so have an upper frequency limit of 100 MHz whenever the lower limit is set to the origin. Initially, resistor 'Rf' is not fitted. Further graphs will be added to show the reduced gain but increased bandwidth of fitting Rf .

Rf not fitted
Rf
not fitted

Rf = 407 ohms
20db
buffer

Rf = 105 ohms
10dB
buffer

Interestingly, the -3dB gain frequency for the 10 and 20dB gain amplifiers were similar, ie, 291 Hz for the 10dB gain amplifier and 265 MHz for the 20dB gain amplifier. For the projects currently in mind, the highest frequency used will be 230.4 MHz and in it's case a 10 dB gain is required. Generally though, the working frequency will be less than 100 MHz, so the TF2370 can be used for all the remaining tests.

For buffer use, the amplifiers reverse isolation figure is worth knowing. Since Zin and Zout are both 50 ohms,
this is the same as S21.

Using this buffer in the current GB3CAM 10 GHz baacon would (should) have been beneficial. At the time of writing, a modified 14.4 MHz tcxo is used and this directly feeds the multipliers. Although the modified 'tcxo' was housed in an ovened enclosure, non of the multipliers were, so they are subject to varying ambient temperature. This inevitably will show up as a slight loading variation with temperature on the ovened oscillator - a bit of a silly oversight really. So the next beacon (and/or CAM's eventual replacement) will include one of these buffers within the ovened enclosure.