Isolation amp transistors
Gerhard wrote:
"I have made a new isolation amplifier but I'm absolutely not happy with
the available transistors. Anything in sot-89 is either to slow (
Zetex/Diodes Inc, the 2N3904-alikes)
or is much too hot.
I want at least 200 MHz to have no phase shift at 100. BFQ19s gave me 1
GHz of BW. The version in the plot is already heavily sandbaged but
still has quite an S21 overshoot on the high frequency end. The
input-voltage to cascode current converter is especially problematic in
that the smallest capacitive load on the emitter tends to make it more
unstable. That spoils S11, of course. I even took the feedback from a
tap of the emitter resistor. Backward isolation is 120 dB over most of
the useful range but changes depending on the damping methods.
Any ideas of more friendly transistors? BFQ31 were quite well-behaved
but are extinct now. I still have a reel, but stuff from the secret
drawer is unfair. And it's PNP."
Gerhard, what sort of damping are you using? I think the simplest is a small series base resistor on any common-base stage like the cascode upper Q. The same thing should help on the lower transconductance converter Q. If added base R degrades LF/MF performance too much, maybe lossy ferrite beads would do instead.
My favorite VHF Q is the good old 2N5179 or similar, but it appears you want something in surface mount, and not obsolete. I'm not familiar with the modern SMT stuff. If your present transistors are working, but just need a bit more stability, it seems it should be OK with the right scheme, and not the transistors' fault.
Ed
The 2N5179 has high base spreading resistance (decreases isolation).
Rick Karlquist
N6RK
On 2022-07-06 12:18, ed breya via time-nuts wrote:
My favorite VHF Q is the good old 2N5179 or similar, but it appears you want something in surface mount, and not obsolete. I'm not familiar with the modern SMT stuff. If your present transistors are working, but just need a bit more stability, it seems it should be OK with the right scheme, and not the transistors' fault.
Ed
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Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nuts time-nuts@lists.febo.com wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base …. Yes, inductance
is even worse.
For “best isolation” in a cascode you very much want the base of the common base
stage nailed to ground. Typically “lower” Ft transistors with a decent base structure
are the best choice for the common base stage. Both stages benefit from low 1/F noise
in the audio range if this is for a phase noise test set. This is why people use what would
normally be considered “audio” transistors ….
Bob
Rick Karlquist
N6RK
On 2022-07-06 12:18, ed breya via time-nuts wrote:
My favorite VHF Q is the good old 2N5179 or similar, but it appears you want something in surface mount, and not obsolete. I'm not familiar with the modern SMT stuff. If your present transistors are working, but just need a bit more stability, it seems it should be OK with the right scheme, and not the transistors' fault.
Ed
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how about grounded grid ?
Bob can you get better isolation with a vaccuum tube cascode than a
solid state cascode ?
-glen
On 07/07/2022 15:22, Bob kb8tq via time-nuts wrote:
Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nutstime-nuts@lists.febo.com wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base …. Yes, inductance
is even worse.
For “best isolation” in a cascode you very much want the base of the common base
stage nailed to ground. Typically “lower” Ft transistors with a decent base structure
are the best choice for the common base stage. Both stages benefit from low 1/F noise
in the audio range if this is for a phase noise test se
Am 2022-07-07 7:22, schrieb Bob kb8tq via time-nuts:
Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nuts
time-nuts@lists.febo.com wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base
…. Yes, inductance is even worse.
and at frequencies where beads work, they also generate thermal noise,
like any other dissipative thingie. See the sim of a random ferrite bead
from the LTspice library. V1 is only there as a compiler pleaser to
enable the proper syntax. It really has no influence.
For “best isolation” in a cascode you very much want the base of
the common base stage nailed to ground. Typically “lower” Ft
transistors
with a decent base structure are the best choice for the common base
stage.
Both stages benefit from low 1/F noise > in the audio range if this is
for
a phase noise test set. This is why people use what would normally be
considered “audio” transistors ….
Cascodes do not add much noise when they have a decent beta. Zin at the
Emitter is a few Ohms and Zout of the driving stage is maybe KOhm. That
makes the driving stage DICTATE the collector current. Also, 1/f noise
is not THAT bad since the load resistance is near 0 in the 1/f region.
Thus, at least no gain at 1/f frequencies. In a linear amplifier, it
would not get mixed up anyway. Makes me like the Driscoll oscillator.
I could not find a transformer (Macom, Pulse Eng.) that provided an
acceptable S22. The resistive 50 Ohm in par with (4.7u + bead) worked
best.
At least the momentary collector voltage can exceed the supply.
Appreciated in the light of 12V operation. But without the transformer,
one pays with a lot of bias current, therefore sot-89.
The circuit is not exact, it was in the middle of what/if experiments.
Thus, some funny values.
The cascode is mostly harmless (TM). What hurts, that is Q2 stability.
@Florian: 150 mA, not too much Vce required, 1-1.5 GHz ft.
Cheers, Gerhard
I assume with grounded grid, that you mean a vacuum tube with heated
cathode. The cathode will have a noise P = kTB, where T is the red hot
cathode, and B is your bandwidth. k is as usual Boltzmann constant.
That said, if you could cancel the capacitances inside the tube, then you
could get a decent isolation, but it's much easier with semiconductors.
BR.
Thomas.
tor. 7. jul. 2022 kl. 07:57 skrev glenlist via time-nuts <
time-nuts@lists.febo.com>:
how about grounded grid ?
Bob can you get better isolation with a vaccuum tube cascode than a
solid state cascode ?
-glen
On 07/07/2022 15:22, Bob kb8tq via time-nuts wrote:
Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nuts<
time-nuts@lists.febo.com> wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base
…. Yes, inductance
is even worse.
For “best isolation” in a cascode you very much want the base of the
common base
stage nailed to ground. Typically “lower” Ft transistors with a decent
base structure
are the best choice for the common base stage. Both stages benefit from
low 1/F noise
in the audio range if this is for a phase noise test se
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--
With Best regards, Thomas S. Knutsen.
Please avoid sending me Word or PowerPoint attachments.
Hi,
On 2022-07-07 07:22, Bob kb8tq via time-nuts wrote:
Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nuts time-nuts@lists.febo.com wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base …. Yes, inductance
is even worse.
For “best isolation” in a cascode you very much want the base of the common base
stage nailed to ground. Typically “lower” Ft transistors with a decent base structure
are the best choice for the common base stage. Both stages benefit from low 1/F noise
in the audio range if this is for a phase noise test set. This is why people use what would
normally be considered “audio” transistors ….
The NIST isolation amplifiers does exactly this. Looking for Fred Walls
in the NIST T&F archive usually makes me find the article quick.
Cheers,
Magnus
Hi
The tube cascode has it’s own issues. Setting up a tube circuit for
the sort of isolation we are talking about here is very difficult.
Bob
On Jul 6, 2022, at 9:46 PM, glenlist via time-nuts time-nuts@lists.febo.com wrote:
how about grounded grid ?
Bob can you get better isolation with a vaccuum tube cascode than a solid state cascode ?
-glen
On 07/07/2022 15:22, Bob kb8tq via time-nuts wrote:
Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nutstime-nuts@lists.febo.com wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base …. Yes, inductance
is even worse.
For “best isolation” in a cascode you very much want the base of the common base
stage nailed to ground. Typically “lower” Ft transistors with a decent base structure
are the best choice for the common base stage. Both stages benefit from low 1/F noise
in the audio range if this is for a phase noise test se
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Hi
On Jul 7, 2022, at 12:09 AM, Gerhard Hoffmann via time-nuts time-nuts@lists.febo.com wrote:
Am 2022-07-07 7:22, schrieb Bob kb8tq via time-nuts:
Hi
On Jul 6, 2022, at 1:53 PM, Richard Karlquist via time-nuts time-nuts@lists.febo.com wrote:
The 2N5179 has high base spreading resistance (decreases isolation).
As does sticking a resistor (even a small one) in series with the base
…. Yes, inductance is even worse.
and at frequencies where beads work, they also generate thermal noise,
like any other dissipative thingie. See the sim of a random ferrite bead
from the LTspice library. V1 is only there as a compiler pleaser to
enable the proper syntax. It really has no influence.
A cascode buffer is a very common thing in an oscillator. Over the decades
folks have made a lot of them. Having them turn into oscillators at UHF /
microwave frequencies is not at all unusual. Having the loose isolation for
various reasons ( somebody used the wrong bypass cap maybe …) is also
not at all unusual.
Coming up with a model for a spice analysis that will always catch these
things is non-trivial. A transistor intended to be used to < 50 MHz rarely has
a model that includes everything that’s relevant at 1.8 GHz.
For “best isolation” in a cascode you very much want the base of
the common base stage nailed to ground. Typically “lower” Ft transistors
with a decent base structure are the best choice for the common base stage.
Both stages benefit from low 1/F noise > in the audio range if this is for
a phase noise test set. This is why people use what would normally be
considered “audio” transistors ….
Cascodes do not add much noise when they have a decent beta. Zin at the
Emitter is a few Ohms and Zout of the driving stage is maybe KOhm. That
makes the driving stage DICTATE the collector current. Also, 1/f noise
is not THAT bad since the load resistance is near 0 in the 1/f region.
Thus, at least no gain at 1/f frequencies. In a linear amplifier, it
would not get mixed up anyway. Makes me like the Driscoll oscillator.
I could not find a transformer (Macom, Pulse Eng.) that provided an
acceptable S22.
In this case, the amplifier is driving into a double balanced mixer that has
a very similar transformer. Whatever you are seeing with the part you
buy off the shelf is already there in the next stage.
If you are trying for S22 from DC to light then yes, transformers have issues.
They also have issues if your “acceptable” number something past 40 db.
Mixers (used as phase detectors) need to see termination at fairly specific
frequencies. This helps quite a bit.
Bob
The resistive 50 Ohm in par with (4.7u + bead) worked best.
At least the momentary collector voltage can exceed the supply.
Appreciated in the light of 12V operation. But without the transformer,
one pays with a lot of bias current, therefore sot-89.
The circuit is not exact, it was in the middle of what/if experiments.
Thus, some funny values.
The cascode is mostly harmless (TM). What hurts, that is Q2 stability.
@Florian: 150 mA, not too much Vce required, 1-1.5 GHz ft.
Cheers, Gerhard
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