blast from past

That's odd. John L replied to your post and stated:

I looked for his post but couldn't find it. The problem I see with this approach is the outputs have to be combined somehow. This would make the gain 6dB higher in the overlap region.

There's certainly more to things than that. But the topic I started was about how to make a super-wideband pullup current source.

John

Well then what can do my germanium transistors have, if not TO-5? They sure as heck have an epoxy underbelly (and not much power dissipation!).

Tim

Not sure about the case names, but my memory says that the 2905 is in a larger metal can than the 2907 (something like 8mm diameter vs. about 5).

Didi

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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Original message:

Could be a news server issue. This is what John wrote:

"It's easy to have the bandpass of a fast ac-coupled forward gain path, and the bandwidth of an opamp-based DC servo loop, overlap by a few decades of frequency, 10KHz to 1 MHz or something like that. So there's no transition problem."

My experience is pretty much the same. However, you have to watch out for phase margin and stability, like you would have to on a capacitively loaded opamp.

That's the reason for the servo loop ;-)

The tricky part is if the wideband amp has a 50 ohm output impedance, and you want the overall output impedance to be 50 ohms from DC on up.

--------- ---------- | splitter| cap cap | | in----------| ac|--||--- ac amp---||----| combiner |---->--out | dc | | | --------- ---------- | | | | +-------------dc amp----->--------+

The node you want to servo isn't accessable.

The easiest thing to do now is not a servo, but an R-C || R-L combiner, which does need a matched crossover, namely the tau's have to be equal. But that's not real hard to do.

John

Or try to give the servo loop a 50ohm behavior by including a small current sense resistor.

Doesn't it still have to behave like an inductor? You don't want a frequency range where the ac amp's 50 ohms overlaps with the DC amp's

One thing to do is to make a programmable pseudo-battery out of the final coupling cap; a smart current source can do that. That simulates well, but has about as many practical problems as all the other methods. Overall, the RC-RL combiner looks pretty good.

John

You'd have to monitor output current and voltage at the BNC (not at the DC amp output) and servo to 50 ohms. At some point this bridge and the LF amp will roll off and that point has to be where the RF amp has already taken over.