Test Equipment Tip for Initial Testing of Transmitter Amplifiers

While many of us start out building transmitter stages and tuning for maximum output, we should quickly learn that over-driving a transmitter stage must be avoided. Even a slightly over driven amplifier stage generates significant harmonics and distortion products. So here is a quick tip on using the oscilloscope to check for the presence of a transmitter stage being over-driven. 

A 10MHz analogue oscilloscope will be fine for looking at HF signals since we are going to measure relative voltages. I am unsure if a low bandwidth digital CRO will work.

Connect your oscilloscope to the collector or drain of the active device. With no drive and the oscilloscope set to AC coupling ensure the horizontal trace is on the centre graticule. The time-base setting is not critical so I use something that gives me a band across the screen with drive applied rather than the actual waveform. Try 1ms per division and adjust to suit your preference.

Now apply perhaps 10% of the drive you expect to use when the amplifier is in operation. Adjust the trace so it sits between say the second graticule above the centre line and the second graticule below the centre line. As you increase the drive you should see the trace touch the third graticule above and below the centre line at the same time.

Adjust the vertical amplifier gain to reduce the signal size on the screen then increases the drive again. Again, you should see the trace touch the third graticule above and below the centre line at the same time. Keep repeating this until you notice that one of the third graticule's above or below the centre is being touched before the other is being touched.

At this point the amplifier is no longer linear. If you have a fast oscilloscope you can increase the time-base speed to observe a few cycles of the waveform. You will notice that when non-linear the waveform peaks that were last to hit the third graticule will be distorted.

You can back off the drive until both the positive and negative peaks are moving in unison, which I find easier to discern with a slow timebase setting rather than a few cycles being displayed on the screen. That is the limit for linear operation of the stage.

Spectrum analysers and other nice test gear allow you to measure how non-linear your transmitter is. But isn't it easier just to avoid over-driving the transmitter to begin with? Hopefully this tip helps you avoid over-driving your transmitter stage giving you a nice clean signal when you finally go on air.

Regards
Richard VK6TT

250mW Class A Amplifier with 2SC5707

Further to my recent posts on using the 2SC5707 of unknown origin I finally got around to some more testing. I calculated the component values for a 250mW Class A amp and after etching a circuit board built the amplifier for testing. Overall, the results were pleasing and justify further experimentation.

Referring to my bible, "Solid State Design" by Hayward and DeMaw, I expected a gain of 17dB from the circuit below:

I achieved this 17dB of gain at 10MHz. However, the gain had fallen by 4dB at 20MHz so at present I will limit my use of this transistor to 14MHz and lower.

My clip on heatsink  was a 15mm x 15mm piece of circuit board, held on with a wooden clothes peg. Worked extremely well for testing but I have a more permanent solution in mind which I will post about shortly.

Overall a nice little amplifier and I will box it up for use on the workbench to increase the drive from my signal generator as required. But ironical that it will be a 15c transistor between two expensive BNC connectors.

With 8 transistors left I will really push them in the next experiment to see if there is any smoke in them!