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I attempted to build a single transistor amplifier to try and increase the voltage signal coming from a Si5351 module, set to 144 Megahertz.

My primary goal is to increase the voltage about 3 to 4 times. The default output of the Si5351 with 85ohm load is around 1.3volts or so. I want to get up to about 4 volts. Mind you, not to match with any load as of yet - just to get the voltage increased first. I seem to be having problems at every turn (even LTSpice simulations don't work like I expect..sigh)

I naively thought I could design a simple Class A common emitter to try to do this, but it's just not working and I don't know why. The bias voltages all make sense.

I am using a BF240 transistor data sheet here, which according to the datasheet, has an Ft of 1.1Ghz. So in theory I should be able to get at least a gain of 7-ish. I bought the BF240's from Jameco electronics.

Here is a picture of my current prototype construction: enter image description here

It's an arduino which programs the Si5351 module, which outputs 144 Megahertz. I then measure the input and output of my transistor amp.

Here is the schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

I put on a 10K or so load just to keep out any possible strangeness.

When I measure the DC bias voltages, everything seems good. I have about 1.75volts on the base, and .9 volts at the emitter, this emitter resistor is about 46 ohms so it should be about 20ma. The BF240 is supposed to be able to handle up to 50ma, so it should be safe. I measure Dc voltage of about 6.8 volts on the collector. So the DC bias looks good. I double checked the transistor by removing it and measuring the base/emitter junction and it appears OK, not shorted, etc.

I'm pretty sure I have the pinout soldered correctly as well - I even used a transistor tester I own and it verified the pinout matches with the datasheet, with flat side facing you , is collector, emitter, base. I tried two different transistors and get similar results in amplification , though..

I should be getting a gain of around 4 or 4.5, but as you can see I'm getting a NEGATIVE! gain, it's cut in half..wtf.

This is the input signal, measured just before the input capacitor: enter image description here

And here is what I'm getting on the collector of the resistor:

enter image description here

I know I have a pretty long wire running from the Si5351 but I'm not looking for perfection yet, and the signal makes it across OK - it's literally the transistor doesn't appear to be amplifying at all, even though it's supposed to be a higher frequency transistor.

I can't imagine it's my layout that is causing it to simply NOt amplify at all.

My oscilloscope probe is set to 10x and I'm using the super short probe spring ground. The scope is a 200Mhz scope with 200mhz probes.

Scope Probe: enter image description here

I clearly have to be doing something WAY wrong because it's not making sense to me. I know RF is "magic" but this is a bit weird.

I simulated the circuit in LTSpice and it looks like it gives a gain of around 4, which is what I wanted..

enter image description here

What the heck do I have wired wrong - or these transistors are not correct or something..

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Voltage Spike ♦
    Dec 30, 2020 at 7:10

2 Answers 2

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Your scope probe loads the circuit with about 16 pF or 70 Ohms at 144 MHz. The output impedance is higher, close to the value of the collector resistor. Thus you have very significant voltage attenuation at the output. The impedance at the input is lower because you are probing it directly across the output of the signal source. The amplifier may have gain close to what you expect.

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  • \$\begingroup\$ Thanks, I managed to measure it properly by adding a "high impedance" DIY little measuring circuit, the measured the signal through a resistor divider. I believe I was able to get an accurate measurement now, and it was around 8dB power gain through this amplifier, I also changed the collector resistor to be a impedance matching transformer instead (wound on a toroid). Seems to work well now. \$\endgroup\$
    – niko20
    Apr 12, 2021 at 23:00
  • \$\begingroup\$ That’s great, and you are welcome. The built-in way to say thanks here is to up-vote an answer. You can also select an answer as best if you wish. \$\endgroup\$
    – 10ppb
    Apr 16, 2021 at 18:52
  • \$\begingroup\$ If anyone tells you he/she never did this mistake they are either lying or never tried anything like this. \$\endgroup\$
    – Uncle Dino
    May 12, 2021 at 23:46
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Try backing off on the collector current say to 5 mA. The Gain-Bandwidth product is sagging at 20 mA.see graph from datasheet At 5 mA looks like the maximum achieveable gain of the BF240 is about 900/144 = approx 6.

Also consider using a cascode configuration instead if this does not yield the gain you are after. This may be helpful:

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/cascode-amplifier/

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  • \$\begingroup\$ I changed my values, to use 120ohms for emitter and 470 ohms in collector, and still doesn't want to work, I have 1.65volts input, and 1.54 output. (Less than input). I may have to just assume these transistors are garbage and won't work at VHF after all \$\endgroup\$
    – niko20
    Dec 29, 2020 at 4:01
  • \$\begingroup\$ It’s extremely unlikely that these transistors don’t do what the data sheet says. You are probably not accounting for all the parasitics, especially output loading. \$\endgroup\$
    – 10ppb
    Mar 28, 2021 at 14:02
  • \$\begingroup\$ I managed to get the circuit working by adding a impedance increasing circuit on to my scope. I also changed to using transformer couples output. With those, measuring was correct and I now see an output of about 8db gain. So it is working correctly now! I definitely learned about measuring rf circuits correctly \$\endgroup\$
    – niko20
    Mar 29, 2021 at 1:22
  • \$\begingroup\$ Yep. R's make TERRIBLE collector impedances at RF frequencies. Just think: a "stray" capacitance of 5 pF is very small (easy to get with wiring/cable assemblies). Now at 220Ω, guess what, with 5 pF in parallel with 220Ω gives a 3dB corner of 1/(2*piRC) = 1/(2*pi*220*5E-12) = 144.6 MHz! So it only takes 5 pF to kill your output by 1/2. Conclusions: 1) always use L's not R's for this application; 2) If you can, attach C across the L to resonate it at the output frequency, unless L is "large", and 3) use transformer if you can. ALSO--bypass that 50Ω emitter resistor for more gain! \$\endgroup\$
    – Atomique
    Jul 13, 2022 at 19:48

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