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

[Bob Groh]

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 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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