Every now and then as the mood takes me, I allow myself to be distracted and put aside whatever project I'm on with, to have a little homebrew frolic.
When recently trying to check the frequency of a low level signal, my (homebrew) DFM wasn't sensitive enough to provide a reading. Not something I need to do often, but I thought I might have a look around for a circuit for a simple buffer amplifier. Many years back in the mid 90s when I was still interested in amateur radio, I'd built a CM Howes CBA2 Buffer Amplifier from a kit, (to drive a digital display for a homebrew direct conversion receiver), which ended up gathering dust. In 2015, I sold it to a forum member for a nominal sum:
https://www.vintage-radio.net/forum/...d.php?t=118215
That circuit used a BSX960 and a BSX20 transistor, and the spec stated it covered 500 kHz to 30 MHz and the gain was 10dB. (4dB down at 30 MHz).
Rather then emulate Dave Howes CBA2 circuit, I had a poke around on the net and came across a circuit which uses an MPF102 and two 2N2222 transistors, so thought I'd design a little PCB for it to see how it performs. If performed well enough up to 15 MHz then sharply falls off. I thought I'd redesign the PCB to be smaller and instead of having tracks on the underside, would use 'blocks' as I've done with other homebrew designs, and would also create a ground plan on the top surface. This 'MK2' version does have a little more gain, but as with the 'MK1' performance falls off a cliff at 15 MHz.
As I've said before, my practical skills exceed my technical skills by a good measure, so I've no idea whether the frequency limitation stems from the circuit itself or my PCB layouts, which broadly follow the layout of the circuit. I don't think it's down to the choice of the transistors which have a FT well into the VHF region.
I've attached the circuit, some pics of the PCBs and some 'scope traces to illustrate the performance at various frequencies In this post, the pics related to the 'MK1' version. Any thought on the circuit or layout which might up the performance above 15 MHz would be appreciated.
Pic 1: For information - the CM Howes Circuit (1989).
Pic 2: The circuit that I found in internet.
Pic 3: My 'MK1' component layout, tracks and negative UV mask.
Pic 4: The MK1 PCB built and placed in the project box it was designed to fit into.
Pic 5: A 'scope trace of the MK1 PCB at 100kHz showing 200mV in, and 3 V out.
Some more pics to follow.