A few comments:
1. My preferred 'spice' program is LTSpice (now being supported by Analog Devices). However it only runs (apparently) on Windows or Apple devices. Oh, well.
2. Your 'load' (I assume that is a ferrite transformer with 5:1 turns ratio) sort of indicates the RF load is about 220 ohms * 25 or about 5K ohm

That seems high - the device is set for about 5 mA quiescent (i.e. Vb - Vbe or about 4 v - 0.7v or about 3.3 V (quite close to what you see) divided by 620'ish ohms) so theoretically you have a collector voltage swing downwards of about 12V - Ve or about 7V and then you would clip.  

And your results indicate that you are clipping some place - the base will swing up but it won't swing down or maybe it runs out of steam when it swings down - the swing down on the base also coincides with collector going up.  At this point, I am going to go off and ponder this a bit while the turkey is cooking. 

73
Bob Groh, WA2CKY

On Thu, Nov 24, 2022 at 12:43 AM Leslie Smith <lnsmith99@gmail.com> wrote:
Hello all!

Some weeks ago, on this forum, the topic of signal wave-form to a HF mixer was 'live'.  If the LO signal was a square wave, would harmonics from the oscillator result in birdies (etc).

I read the postings.  I decided to build a basic JFET mixer and feed it with both square and sine waves.  This posting describes a problem in generating a distortion free sine-wave for the experiment.

In the circuit sent with this posting a 5.8MHz sine-signal is link-coupled to a buffer amplifier.  The buffer is a conventional (or so I thought) class A bipolar signal amplifier with degenerative emitter feedback.  My difficulty can be described easily.  At low signal levels the circuit performs as expected.   When the signal is increased the signal out is rather like (almost exactly like) the signal from a 1/2 wave rectifier. 

In the diagram attached, a CRO placed across the 56 ohm emitter resistor shows that the emitter (or collector) current flows for 1/2 the AC cycle.  My description of "1/2 AC cycle" is merely chance.  I happened to probe the circuit when the signal level drove the emitter voltage below the working bias of the transistor at the mid-point of the signal.  This is point "B" on the sketch and approximately 3.11V.   As the signal level (seen at the emitter or point "A" on the sketch) is increased, the clipping also increases.  The AC signal at "A" has the same form as the signal at "B".  This shows the current in the emitter/collector circuit.

As you can imagine, the inductive load in the collector circuit 'plays' with the signal somewhat; monitoring the current/voltage across the 56 ohm emitter resistor is more informative. 

Anyone here got any ideas what's going on in the circuit?  Whence the distortion.  I need a circuit that will give me 6-10V AC peak to peak in the low HF range.

Finally:  I have no experience in using SPICE.  I would like to use it.  Anyone here willing to give some pointers on getting/installing/running SPICE on a Linux box?

73 etc

Leslie.

PS.  Built this cct on a strip-board.  First use of lead-free solder.  I hate the stuff!  It doesn't melt like 60/40 solder.  It won't wick into de-soldering braid.  I made three (yes 3!) fine solder splashes between tracks on the strip-board.  Rarely get them using 60/40 solder.  Thinking seriously about taking my life in my hands and sticking with 60/40 conventional solder.




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