[ARC5] Solid state small-signal RF amplifier cct - what's going on here

[J Mcvey]

 should read "running class A" ...

    On Thursday, November 24, 2022 at 08:44:59 PM EST, J Mcvey <ac2eu at yahoo.com> wrote:  
 
  Also, the impedance looking into the base assuming a beta of 100, would be at least1500 0hms, so your input transformer seems rather extreme. (15:1)Try nixing it altogether. You might be clipping your generator trying to drive the circuit!
If that doesn't help , there may be saturation problems with the core. Running class with a core reduces it's AC throughput , but it's not all that much current.

    On Thursday, November 24, 2022 at 01:43:39 AM EST, Leslie Smith <lnsmith99 at 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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