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Maker Pro

20MHz Wideband Amp

T

Tim Williams

Jan 1, 1970
0
Schematic:
http://webpages.charter.net/dawill/Images/20MHz_Amp.gif
Pictures:
http://webpages.charter.net/dawill/Images/20MHz_Amp1.jpg
http://webpages.charter.net/dawill/Images/20MHz_Amp2.jpg

With the 100 + 10pF on the +in transistor and 10pF on the tail, I've
eliminated the UHF/microwave oscillation as best I can tell. It no longer
changes gain or bias when I wave my hand around and touch parts. My
laptop's wifi probably stays connected, too (I didn't have it with me when
I did this).

Now I can concentrate on the characteristics. The squarewave response is
good, but it has a strong bounce on it. In fact, hooking it to an RF
generator (Eico 322, not much output into 50 ohms, but frequency appears to
be accurate) I see a substantial rise of perhaps 20dB at 26MHz! It is of
equal and opposite amplitude on both outputs (viewing with 10x probes, one
with somewhat more capacitance), so I suspect it comes from the LTP.
Feeling around the circuit produces no change (aside from loading down the
2N5179 collectors, which predictably reduces HF gain overall, not
specifically the 26MHz peak), implying it might be an LC sort of thing.

Ideas?

Tim
 
J

john jardine

Jan 1, 1970
0
Tim Williams said:
Schematic:
http://webpages.charter.net/dawill/Images/20MHz_Amp.gif
Pictures:
http://webpages.charter.net/dawill/Images/20MHz_Amp1.jpg
http://webpages.charter.net/dawill/Images/20MHz_Amp2.jpg

With the 100 + 10pF on the +in transistor and 10pF on the tail, I've
eliminated the UHF/microwave oscillation as best I can tell. It no longer
changes gain or bias when I wave my hand around and touch parts. My
laptop's wifi probably stays connected, too (I didn't have it with me when
I did this).

Now I can concentrate on the characteristics. The squarewave response is
good, but it has a strong bounce on it. In fact, hooking it to an RF
generator (Eico 322, not much output into 50 ohms, but frequency appears to
be accurate) I see a substantial rise of perhaps 20dB at 26MHz! It is of
equal and opposite amplitude on both outputs (viewing with 10x probes, one
with somewhat more capacitance), so I suspect it comes from the LTP.
Feeling around the circuit produces no change (aside from loading down the
2N5179 collectors, which predictably reduces HF gain overall, not
specifically the 26MHz peak), implying it might be an LC sort of thing.

Ideas?

Tim

I'd lose the 220p on the output stage.
The rising response it causes can transmogrify into a gyrated inductance
that resonates with the output Ccbs and strays, forming a nasty hi Q peak at
the top end. Sounds like you've already spotted it. (probably then no need
for the 10p 100ohm)
There's no overall feedback at the moment and the enormous X300? gain should
run out of steam about 6-8MHz. This value set by the 1k3 loads and scope
capacitance break.
No real alternative to reach the 20Mhz other than inductive peaking (yuck)
or buffering the output loads.
Due to the 3GHz GBW, fitting (much) higher ft transistors would be the next
step.
(BTW neat video)
 
T

Tim Williams

Jan 1, 1970
0
john jardine said:
I'd lose the 220p on the output stage.

Eww, no way! Bandwidth drops to like 5MHz and square waves are SLOOOW!
Plus, everyone else does it. (And yes I'll jump off a cliff when they do
too! ;-) )

Besides, it's present even while puttering around with said capacitor. I'm
pretty sure it's in the LTP, but where, or from what, I know not.
(probably then no need for the 10p 100ohm)

No, that's for the UHF+ parasite that I can't see on my mere 200MHz scope.
The 26MHz hump is there in all cases.
There's no overall feedback at the moment and the enormous X300? gain

Come to think of it, I never did bother to measure it, but yeah, something
like that. Gotta go from 0.5V or less up to full screen 100Vp-p, dontcha
know :)
Due to the 3GHz GBW, fitting (much) higher ft transistors would be the next
step.

Eh? The '5179's (or are they 6304's in circuit, I forget) are 1.2GHz or
something, and already well in excess of my needs. I don't even need to
add emitter compensation. (Though I did find some benefit while
breadboarding, but I didn't get any UHF parasites then, either!)
(BTW neat video)

Where, you mean on YouTube?

Tim
 
M

MooseFET

Jan 1, 1970
0
Schematic:http://webpages.charter.net/dawill/Images/20MHz_Amp.gif
Pictures:http://webpages.charter.net/dawill/...ages.charter.net/dawill/Images/20MHz_Amp2.jpg

With the 100 + 10pF on the +in transistor and 10pF on the tail, I've
eliminated the UHF/microwave oscillation as best I can tell. It no longer
changes gain or bias when I wave my hand around and touch parts. My
laptop's wifi probably stays connected, too (I didn't have it with me when
I did this).

Now I can concentrate on the characteristics. The squarewave response is
good, but it has a strong bounce on it. In fact, hooking it to an RF
generator (Eico 322, not much output into 50 ohms, but frequency appears to
be accurate) I see a substantial rise of perhaps 20dB at 26MHz! It is of
equal and opposite amplitude on both outputs (viewing with 10x probes, one
with somewhat more capacitance), so I suspect it comes from the LTP.
Feeling around the circuit produces no change (aside from loading down the
2N5179 collectors, which predictably reduces HF gain overall, not
specifically the 26MHz peak), implying it might be an LC sort of thing.

Sort of working left to right on the design and pointing out each
thing I would do differently:

The 22R on the emitters of the 2N5179s lead to a node with a fairly
large capacitance. I would have used a higher resistance, lets say
100R to the common point and a 44R between the emitter with very short
legs. You may want to put a series RC in parallel with this to boost
the high frequency gain a bit when you make a change I suggest below.

You show only one 0.1u capacitor for the collectors of the 2N3904s. I
would use two at least, if I'm following the layout correctly. You
want the 680R to connect directly to the capacitor and not to the
collector first.

The 220pF between the emitters of the 2SC1569s looks to have too long
of leads. You are stuck with this for mechanical reasons but it
doesn't take much to make the 220pF into a series tuned circuit. This
would cause it to give more boost than you want as it comes into
resonance. I suggest making one of its leads into a 22R resistor to
spoil out its Q.
 
B

Bob

Jan 1, 1970
0
Schematic:http://webpages.charter.net/dawill/Images/20MHz_Amp.gif
Pictures:http://webpages.charter.net/dawill/...ages.charter.net/dawill/Images/20MHz_Amp2.jpg

With the 100 + 10pF on the +in transistor and 10pF on the tail, I've
eliminated the UHF/microwave oscillation as best I can tell. It no longer
changes gain or bias when I wave my hand around and touch parts. My
laptop's wifi probably stays connected, too (I didn't have it with me when
I did this).

Now I can concentrate on the characteristics. The squarewave response is
good, but it has a strong bounce on it. In fact, hooking it to an RF
generator (Eico 322, not much output into 50 ohms, but frequency appears to
be accurate) I see a substantial rise of perhaps 20dB at 26MHz! It is of
equal and opposite amplitude on both outputs (viewing with 10x probes, one
with somewhat more capacitance), so I suspect it comes from the LTP.
Feeling around the circuit produces no change (aside from loading down the
2N5179 collectors, which predictably reduces HF gain overall, not
specifically the 26MHz peak), implying it might be an LC sort of thing.

Ideas?


Analog stuff with discrete transistors beyond basic switch is not my
area
of expertise so I may be totally wrong here but I think the 10pF
capacitor is
far too small for circuit operation below about 10MHz.

The current through both the 2n5179 has to flow to ground through that
capacitor. At 1MHz it has a reactance of 15000ohms.
At 10MHz it is 1500ohms.

If you have a high impedance in the path to ground little current
will flow through either of those transistors.

Is that cap supposed to have negligable impedance at intended
operating frequencys?

I had to redraw the circuit with the rails at the top and bottom of
the
diagram to make sense of it.

It looks to me that the path through the two transistor at the
bottom of your diagram is a feedback pathto set the DC bias
point of the output?

Bob
I take it the path through the two tran
 
T

Tim Williams

Jan 1, 1970
0
MooseFET said:
Sort of working left to right on the design and pointing out each
thing I would do differently:

The 22R on the emitters of the 2N5179s lead to a node with a fairly
large capacitance.

- The bulky 1/2W resistors do go over a good spot of ground plane (in the
picture, below the green 3pF that isn't there anymore). Capacitance would
lead to increased HF response, though my intuition says that's a
contributing factor to the UHF parasite.
I would have used a higher resistance, lets say
100R to the common point and a 44R between the emitter with very short
legs. You may want to put a series RC in parallel with this to boost
the high frequency gain a bit when you make a change I suggest below.

Ah, that's a possibility.
You show only one 0.1u capacitor for the collectors of the 2N3904s. I
would use two at least, if I'm following the layout correctly. You
want the 680R to connect directly to the capacitor and not to the
collector first.

I have one ceramic for the +9V (the red jumper on the underside) and one
for the -9V. I tried holding one on the underside (of both nodes), with no
effect.
The 220pF between the emitters of the 2SC1569s looks to have too long
of leads. You are stuck with this for mechanical reasons but it
doesn't take much to make the 220pF into a series tuned circuit.

Hmm, it's about 1" is about 30nH, so Fo ~ 62MHz. It's in the same order of
magnitude, but that means estimated L is up by a factor of 4 or more.
Playing with hundreds of pF around each emitter-to-ground, individually,
has little effect, aside from a local change in compensation.

I do recall getting it to oscillate at some point, and IIRC it was from
playing around there. That could be local or related by a larger feedback
path.

Could the 2N3904's be any trouble? Other circuits (e.g., cascode video
amps) have base resistors; here I have the 3904 emitter followers straight
into the base. On the breadboard I had found an oscillation that required
a ferrite bead on one 3904's base or emitter (I forget which), but
breadboard activities are to be taken with a grain of salt.
This
would cause it to give more boost than you want as it comes into
resonance. I suggest making one of its leads into a 22R resistor to
spoil out its Q.

I can try that.

Tim
 
T

Tim Williams

Jan 1, 1970
0
Bob said:
The current through both the 2n5179 has to flow to ground through that
capacitor. At 1MHz it has a reactance of 15000ohms.
At 10MHz it is 1500ohms.

Actually, that capacitor isn't even supposed to exist.

The input current flows between the inputs, because it is a differential
amplifier. The tail is supposed to be high impedance, but apparently it
oscillates at UHF, and I need that capacitor to anchor it. Or something.
Ditto the 100 + 10pF, which in and of itself makes absolutely no sense in a
balanced circuit at 20MHz. For that matter, with the 100+10pF I might get
away without the 10pF, I haven't tried that yet.

As you've just shown, compared to the emitter impedance (<< 700 ohms), the
10pF has little effect at signal frequencies, so it can be ignored.
If you have a high impedance in the path to ground little current
will flow through either of those transistors.

The current sink takes care of DC operation.
I had to redraw the circuit with the rails at the top and bottom of
the diagram to make sense of it.

Ah, schematic style. Eh well, it's traditional. A vertically drawn LTP is
more familiar, but it has a lot more crossings, too. Oscilloscope amps and
other balanced circuits are often drawn in this way. Just take a close
look and make sure the upside-down transistor isn't PNP. ;)
It looks to me that the path through the two transistor at the
bottom of your diagram is a feedback pathto set the DC bias
point of the output?

Correct. I got tired of adjusting common mode operating point, so I made
the damn thing set itself.

Tim
 
M

MooseFET

Jan 1, 1970
0
- The bulky 1/2W resistors do go over a good spot of ground plane (in the
picture, below the green 3pF that isn't there anymore). Capacitance would
lead to increased HF response, though my intuition says that's a
contributing factor to the UHF parasite.

I wouldn't expect the capacitance from the resistor to ground to do
much at the 20MHz. At 100MHz or so maybe.

I have one ceramic for the +9V (the red jumper on the underside) and one
for the -9V. I tried holding one on the underside (of both nodes), with no
effect.

Was this "underside" far from the existing 0.1 and did you use really
short leads?
I'd expect to see some change if my theory on this part is right. If
there was no change, my theory is toast.

Hmm, it's about 1" is about 30nH, so Fo ~ 62MHz. It's in the same order of
magnitude, but that means estimated L is up by a factor of 4 or more.
Playing with hundreds of pF around each emitter-to-ground, individually,
has little effect, aside from a local change in compensation.

That much isn't likely to be the cause a 20dB peak. Still I'd put in
the deQing resistor just to see what happens.

[....]
Could the 2N3904's be any trouble? Other circuits (e.g., cascode video
amps) have base resistors; here I have the 3904 emitter followers straight
into the base. On the breadboard I had found an oscillation that required
a ferrite bead on one 3904's base or emitter (I forget which), but
breadboard activities are to be taken with a grain of salt.

2N3904s would much rather oscillate at about 100MHz than 20MHz. Their
capacitances are on the low side for getting oscillations that low.
The base sees a resistor on the collect of the stage before. This
will tend to supress the Colpitts common collector type of
oscillations.
 
J

john jardine

Jan 1, 1970
0
Tim Williams said:
Eww, no way! Bandwidth drops to like 5MHz and square waves are SLOOOW!
Plus, everyone else does it. (And yes I'll jump off a cliff when they do
too! ;-) )

Besides, it's present even while puttering around with said capacitor. I'm
pretty sure it's in the LTP, but where, or from what, I know not.
[...]

First stage GBP 520meg, i.e the 1.2GHz trannie is already starting it's
death rattle. Ouput stage GBP 260meg, which the data sheet suggest is
already over the top. Kind of a zero-sum power game is running, the
practical effects are like squeezing dough.
If you're sniffing at a 70ns risetime then try increasing the generator
source resistance beyond 50ohms. :).
For these jobs I gave up on my 100MHz scope and Wavetek and find a spectrum
analyser with tracking generator vastly superior. Problems are visible at a
glance.

Yep. Youtube. Interesting watching people's induction welding approaches.
Somewhat odd to hear a 'mercan accent, as anyone posting English text here
(other than the obvious tossers) gets internalised as 'BBC newsreader', no
matter where they hail from.
 
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