Fast buffer idea

On a sunny day (Mon, 15 May 2017 08:56:58 -0700) it happened John Larkin wrote in :

???? TWO transistor ????

I somehow missed the two transistor circuit that Jan posted. Can you please repost the link?

Thanks

On a sunny day (Mon, 15 May 2017 16:24:56 GMT) it happened Steve Wilson wrote in :

From:

I'm not sure what "input current" you're looking at. I'm seeing an average current of around 600uA into the MOSFET gates at 100MHz.

Integrating the source you get that it's sourcing about 100uW, and a simple a 50 ohm load termination is dissipating 10mW.

Assuming 100uW is the max the source could provide, what kind of length of wire has a a transducer gain of 100? That's a neat trick!

If I had a square-wave source capable of providing say 200mA P2P with an infinite slew rate and zero output impedance then indeed, a piece of wire would be the perfect output buffer. Where do I get one of those?

Sorry, I screwed that up. The average current out of the source is 100uA

It's a regular "diamond buffer" topology. Without emitter degeneration resistors in the output emitters it's asking for thermal runaway, and the output waveform won't be particularly linear.

Try adding 10 ohm degeneration resistors, and check the quiescent current on that bad boy with 10/-10 rails. Yikes!

Taking feedback from the tops of the degeneration resistors to the input collectors makes it more linear by reducing Early effect variations. It's an even better idea with a MOSFET input stage, as ideally if the Vds is near constant you wont have the gm compromised due to channel-length modulation.

OK, thanks.

That is a Sziklai Pair with a voltage divider in the feedback path:

It would normally have the same thermal drift as a simple base-emitter junction, but the divide by two may increase it.

The output impedance may be increased due to the low loop gain. The bandwidth is determined by the transistors you use.

But it is difficult to tell what the transistors are due to the poor clarity of the handwritten numbers. I can't read them even when I expand the drawing.

I really wish you could convert some of your critical circuits to LTspice so we can see the operation a bit better. That would have a far greater impact for the value of your work.

I put a 1 mohm resistor in series with the signal generator and probed the current, with a sine wave signal input. Try it and see what you get.

Assume 1 volt peak input at 100 MHz. And 600 uA peak (not average!) current. That works out to a capacitance of 1 pF. That sounds improbable. Ciss of that fet is 3600 pF.

Apply a 1 volt 100 MHz sine input and measure the V+ supply current.

Its output impedance for large negative-going signals is _dogshit_, regardless of what the small-signal analysis feedback-loop-gain etc. calculations indicate. It can source current okay. It definitely can't sink it.

It doesn't really matter for a composite video signal, though, because it's not really that wide a bandwidth signal. The only potentially difficult fast full-level transition I think of would be if you have to go from the "back porch" immediately to "full luminance."

I was referring to this one:

The negative swing depends on the pulldown resistor, same as any single- ended emitter follower. That should be obvious.

These issues would be immediately apparent in a proper LTspice analysis.

Unfortunately, with some users still devoted to ASCII-art or hand-drawn schematics, it is difficult or impossible to do a reasonable preliminary analysis. The author may have made an incredible blunder in the schematic, or it is impossible to determine the component values in a hand-drawn schematic. Often, you really can't tell if it is worth your time to investigate someone's idea.

In most cases, with an ASCII-art or hand-drawn schematic, it is not worth the time to spend any effort on it. The author really doesn't know if it works or not. It could be difficult to detect errors in the posted drawing, since there is no error detection mechanism.

But with a working LTspice model, it could be worth investigating. If it runs, it means it follows the author's intent. It depends on who authored it.

Unfortunately, many beginners post LTspice circuits that are full of blunders. You can immediately tell their thinking is scrambled beyond hope. This is good information. I simply record their names and ignore anything they post so I don't waste any more of my time.

Old idea. I used that in a product in 1979, which won the admiration of engineers at HP, and some orders. The drive capability depends on the pullup resistors in the bases. Quescent current depends on the output emitter resistors, which are not shown in the link.

These issues could be observed in a simple LTspice analysis.

"Diamond buffers are kind of cool, emitter feedback makes them linear and they don't have much phase shift, but they kind of suck balls whenever you want to actually move appreciable current with them which unfortunately is exactly what you'd want to do with a buffer."

The circuit I posted is exactly the same topology, except with the following "refinements"

MOSFET first stage for higher input impedance

Feedback taken to MOSFET drains from the output collectors to keep their Vds approximately constant

Output quiescent current is primarily set by the (bypassed for AC) input stage source resistors, not the output emitter resistors

"Boosting transistors" which slide the bias up in response to signal so it doesn't draw 150mA sitting around doing nothing.

Output emitters, rather

Indeed.... I remember Jim T boasting..err... I mean informing us all, maybe

-- Kevin Aylward

Actually, you missed accuracy.

The key technical limit is the triple of speed, accuracy, and power. You can't chose them independently.

Typical example, to get lower distortion in a feedback system, gain might be increased. This means you have to slow it down with compensation to make it stable. You can increase he BW back up by increasing the current. Increasing current reduces collector output resistance by Va/Ic, but the gain stays the same because gm has gone up. The lower ro.Cl increases the BW of the stage.

-- Kevin Aylward

it...

Well... in SS, you just place a transmission line symbol from the lib, and double-click on it to pop up its setup box. :-)

-- Kevin Aylward

On a sunny day (Mon, 15 May 2017 13:15:52 -0400) it happened bitrex wrote in :

OK, challenge? I have the transistors:

So build it, used a 10K pot (single 10 V supply) to put the input at half way (5 V) 47k in series, poly coupling cap, 1000Hz audio from eeePC:

NO resistors here without flash so you can read the mA meter:

Sorry for the crowded place, lots of things multitasking here.

I did not load it, but it sure wanted to beep on my FM radio as I did not decouple the supply lines... Old Trio Kenwood scope does not go that high. Never underestimate El Pante

Current does not increase from 5 to 10 V, did not try higher,, The audio generator is Linux sgen.

Now for the spice by those who have it!