[time-nuts] Re: Bruce's common emitter transformer feedback amplifier: oscillations with capacitive load

[ghf at hoffmann-hochfrequenz.de]

I would not call these things CE amplifiers at all. CE implies that the
emitter is somewhat grounded, and not that it follows the base voltage
by virtue of feedback through the transformer.

If anything, the transistor feels like a follower, even if the
voltage there is ignored. The output signal is the collector
CURRENT, and if you would use a cascode stage, that would be
clear at first sight. A cascode or even two would be a good
idea wrt backward isolation. Cascodes do not add noise
worth speaking of.
Backward isolation is not helped if you do transformer feedback
around a cascode.

Followers have the habit to oscillate when there is the slightest
capacitive load at the emitter, that is the mechanism of nearly
all microwave VCOs. The high RL impedance at the emitter is
nothing more than an invitation, and a load capacitance that
comes through the transformer just completes that.

I would not wonder if there was a negative real part of input
impedance over some frequencies.

And Norton amplifiers ( CB btw ) are not known for excellent
backward isolation; there is not much more to win than the
reverse of the forward gain.

Chris Trask has written interesting things about "augmented"
amplifiers, which seems to be Norton++, but it's well after
midnight here. Should be easy to google. No idea if it
would help.

regards, Gerhard



Am 2023-04-23 15:42, schrieb Bob Camp via time-nuts:
> Hi
> 
> A schematic would be a big help here …..
> 
> Flying blind, best guess is that your output stage has an issue. If
> the two output bufferers
> are identical to the input stage, that’s probably the issue. A
> different configuration for the
> buffers would be the most obvious fix.
> 
> Bob
> 
>> On Apr 22, 2023, at 9:21 PM, Matt Huszagh via time-nuts 
>> <time-nuts at lists.febo.com> wrote:
>> 
>> Hi,
>> 
>> I built Bruce Griffith's common emitter transformer feedback amplifier
>> (http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
>> mostly identical to the first schematic in the previous link. But, I
>> added a second stage for increased reverse isolation. I placed 65R
>> across the first stage output, which gave me about the right gain in
>> simulations. I also added a second 2-stage buffer in parallel. I
>> custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
>> use this to get buffered/isolated front and rear 10 MHz outputs from 
>> the
>> internal 10811A in my HP 5065A.
>> 
>> Overall, the performance seems excellent. I've attached a residual 
>> phase
>> noise measurement, showing the combined 2 cascaded stages. These 
>> results
>> seem only a little worse than what John Miles measured for his single
>> stage (http://www.ke5fx.com/norton.htm).
>> 
>> I measured the reverse isolation to be 76 dB and 77 dB and the 
>> isolation
>> between outputs to be 49 dB or 59 dB, depending on which output I 
>> inject
>> the signal into. This is all at 10 MHz of course.
>> 
>> However, I am experiencing one issue, which is that if I add a
>> capacitive load to the output (such as from a coaxial cable), I get
>> large oscillations (10+Vrms) at the output. The oscillations are at
>> about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
>> when the output is unloaded, nor when it's loaded by the typical
>> 50ohms. Since this only appeared when I added a capacitive load to the
>> output, I figured it was due to the parallel LC between the output
>> winding and coaxial capacitance. My output impedance came out a bit
>> lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
>> between the output winding and coaxial connector. This cut the
>> oscillations nearly in half (now about 6.5Vrms), but didn't kill
>> it. I've also tried loading down the output with a 1k parallel 
>> resistor,
>> but though this helps too, the oscillations are still pretty
>> large. Going lower helps further, but I don't want to load down the
>> output too much.
>> 
>> Any thoughts? Has anyone else seen this behavior? I can post more
>> details and images.
>> 
>> Thanks,
>> Matt