20MHz Wideband Amp

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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)

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.

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.

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 :-)

next

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!)

Where, you mean on YouTube?

Tim

-- Deep Fryer: A very philosophical monk. Website @

Pictures:

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

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.

Pictures:

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

- 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.

Ah, that's a possibility.

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.

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.

I can try that.

Tim

-- Deep Fryer: A very philosophical monk. Website @

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

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.

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.

I'm

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.