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Low frequency breadboarding of discrete RF transistors

S

Spehro Pefhany

Jan 1, 1970
0
I'm looking at playing with some SiGe parts for a close-to-DC
application (< 1MHz). These things have scary high fT like 30-80 GHz.

Any tips for avoiding oscillation that I might not even be able to
see? Ground plane, run at low current?
 
J

Joerg

Jan 1, 1970
0
Spehro said:
I'm looking at playing with some SiGe parts for a close-to-DC
application (< 1MHz). These things have scary high fT like 30-80 GHz.

Any tips for avoiding oscillation that I might not even be able to
see? Ground plane, run at low current?

As Tim wrote, resistors. Usually one 0402 or smaller resistor around
50ohms (whatever you have there) right at the base and in series with it
suffices. A 0402 bypass cap from base to emitter also helps but you
could get away without one.

I don't know what you want to do with those. Not having the emitter on
ground can get very spooky. The collector can be separated by a snippet
of copper foil as a little "wall".

Most important is to breadboard on copperclad. I prefer living-bug style
with the devices glued to plastic spacer pieces. This snippets of FR4,
ice cram stick, wood shim pieces.
 
S

Spehro Pefhany

Jan 1, 1970
0
As Tim wrote, resistors. Usually one 0402 or smaller resistor around
50ohms (whatever you have there) right at the base and in series with it
suffices.

Argh. Can't do that, 50R would ruin everything.
A 0402 bypass cap from base to emitter also helps but you
could get away without one.

Might have to go with that. There are very low values available.
I don't know what you want to do with those. Not having the emitter on
ground can get very spooky. The collector can be separated by a snippet
of copper foil as a little "wall".

Most important is to breadboard on copperclad. I prefer living-bug style
with the devices glued to plastic spacer pieces. This snippets of FR4,
ice cram stick, wood shim pieces.

Thanks, Joerg, Tim.
 
J

Joerg

Jan 1, 1970
0
Spehro said:
Argh. Can't do that, 50R would ruin everything.

Ok, how about a very small SMT ferrite bead? Not sure if a real bead
with a wire through it will work. It might if you drill a hole right in
front of the base and sort of sink the bead 1/3rd into the board,
lengthwise. Make sure the ferrite doesn't touch the ground plane anywhere.

Might have to go with that. There are very low values available.

You only need a few hundred pF and it doesn't have to be NP0.

Thanks, Joerg, Tim.

We are here to serve :)

Thanks for the hint about the X7S datasheet in the other thread, made my
life easier. Drops the size of the caps from 2220 to 1812, meaning we
can get more on the board.
 
S

Spehro Pefhany

Jan 1, 1970
0
What do you intend to do with these things?

Amplify some relatively low frequency sinusoidal signals. Those parts
can have some interesting noise characteristics.

The ferrite bead sounds like a good idea... hmm might have to work
with the &*&$&*$ microscopic 0201 parts to get much Z at a few GHz.
I breadboarded some of the Infineon 45 GHz SiGe things, soldered down
to copperclad. I was exploring how fast they would turn on as fairly
low current switches. They seemed stable but were surprisingly slow.

PHEMTS are faster in real life. Their feedback capacitance is absurdly
low so they tend to be stable. I have breadboarded phemt circuits on
copperclad and got good, fast, clean switching.

EL07 driving an NE3508:

ftp://jjlarkin.lmi.net/BB_fast.JPG

This gave me a pretty good 5-volt p-p square wave at 1 GHz.

John

Square wave @ 1Ghz. Ha.


Best regards,
Spehro Pefhany
 
O

Okkim Atnarivik

Jan 1, 1970
0
: As Tim wrote, resistors. Usually one 0402 or smaller resistor around
: 50ohms (whatever you have there) right at the base and in series with it

56ohm 0402 piggybag'd with Murata LQW15 22nH at base and collector works
nicely.

: I don't know what you want to do with those. Not having the emitter on
: ground can get very spooky. The collector can be separated by a snippet
: of copper foil as a little "wall".

Although it feels spooky, it is possible to make a long-tailed pair
out of those in a stable manner, without spoiling the noise. I didn't
need the fence, although I had to use one in the older BF862 FET-based
amplifier.

Regards
Mikko
 
J

Joerg

Jan 1, 1970
0
Okkim said:
: As Tim wrote, resistors. Usually one 0402 or smaller resistor around
: 50ohms (whatever you have there) right at the base and in series with it

56ohm 0402 piggybag'd with Murata LQW15 22nH at base and collector works
nicely.

: I don't know what you want to do with those. Not having the emitter on
: ground can get very spooky. The collector can be separated by a snippet
: of copper foil as a little "wall".

Although it feels spooky, it is possible to make a long-tailed pair
out of those in a stable manner, without spoiling the noise. I didn't
need the fence, although I had to use one in the older BF862 FET-based
amplifier.

The BF862 is a moped against some of the new BJTs such as a BFP620, it
has an ft about 100 times higher :)
 
J

Joerg

Jan 1, 1970
0
Phil said:
But it has a 1 kHz 1/f corner, starting at a flatband value of 0.8
nV/sqrt(Hz), whereas the SiGe parts' corners are 100 kHz or higher.

I meant WRT the tendency to oscillate. They sure aren't so great to
regulate and current-steer laser diodes. For that I'd use devices such
as the BCX70K.

Some of Mikko's noise data on those SiGe things are amazing--200
pV/sqrt(Hz)--but they kind of fall apart in the audio range.

Yup, not that great for audio. Most devices aren't. I want my tubes back :)
 
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