Fast buffer idea

The LTC6402 is nowhere near similar to the LTC6409.

Fixed 20db gain, 300MHz BW vs unity gain stable, 10GHz GBW. Totally different devices.

I'm surprised you had problems with LTC. They usually have a petty good reputation for quality.

Devices that are off, or mostly off, are slow to turn on. You suffer from a t = C V / i delay time. SPICE fails to give you the proper value for C, and the delay for i. This is especially true if you invoke any MOSFET models.

You can make SPICE devices do all kinds of amazing things. This is NOT to say the real device can do the same thing.

--
 Thanks, 
    - Win

LMH5401 and LTC6409 are both good parts.

Or an AD8000 for that matter, depending on how many harmonics you want to keep.

-- john, KE5FX

A part like that would generally feed an $80 ADC.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

12 db gain. It tended to oscillate at large swings.

We were surprised too. And annoyed that we had to spin the board.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

The Spice models for discrete transistors are so rough that they don't even model the device capacitances and charge storage effects correctly?

Analog ICs can use basically unlimited numbers of radical parts, and have essentially no interconnect parasitics. And they can spend a lot of money on design.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Are you talking about a different device? There doesn't seem to be any way to change the gain of the LTC6402 except by adding external feedback. This may cause enough phase shift to produce the oscillations.

Did you send your data to LTC?

We used the LT6402-12.

No, we just spun the board.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Aha! I didn't even know that version existed. Thanks.

The response curves show peaking around 200MHz, especially with capacitive load. The small signal and large signal response shows severe ringing and overshoot with square wave input on Page 7 and Page 8 in

Looks like there is enough information in the datasheet to steer you away from this device.

On a sunny day (Sun, 14 May 2017 12:54:12 -0700) it happened John Larkin wrote in :

THS4222 datasheet says open loop gain 0dB at 100 MHz? With a free 80 degrees phase shift? Fig 17, see also Fig 1 for 500 Ohm load.

It won't drive a 50 Ohm coax either.. Maybe good for a 5 to 10 MHz wide video amp...

But indeed you can do that with a few NPNs of the old days,

Yeah, and a fast diamond buffer has a very small amount of phase shift, maybe a couple 10s of degrees in the worst case.

You can certainly wrap a a fast unity gain op-amp feedback loop around such a structure without any extra compensation; it doesn't even need to be a blindingly-fast $7 op-amp, just a regular-fast op amp. It'll handle DC to 5MHz say, and then anything higher gets bypassed around it and pushed straight through.

On a sunny day (Mon, 15 May 2017 06:58:15 -0400) it happened bitrex wrote in :

For video, out of habit let's say: From:

the circuit diagram: bottom left: that has a gain of 2, so can drive a 75 Ohm cable with reasonable Zi. The DC path requires an other 75 Ohm at the end of the cable. No specs.. What's a few MHz among video freaks...:-) I had it in spice once and it oscillated.... Not in reality :-). That was actually the first time I ever used spice.... many decades ago. Wonder how high it would go with some of them modern transistors. Spice not online here ATM...

Now I was in the garden sitting in the sun, the wind, good time to get a sailboat and forget about it all.

Where does it say that?

BW is 230 MHz as a unity-gain buffer. Each half of the dual will source and sink 88 mA. And there are lots of faster THS-series parts.

Post it.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

With discretes you can have:

1) wide bandwidth

2) low parts count

3) low quiescent current draw

Pick two.

The two transistor circuit Jan posted does have a pretty wide bandwidth and a low parts count. It has a pretty nice small signal output impedance, but its large-signal output impedance is bad. It also draws a relatively large amount of quiescent current.

On a sunny day (Mon, 15 May 2017 07:01:06 -0700) it happened John Larkin wrote in :

Did not I ;-)?

Actually when I did read that OP, I scribbled this on my 'desk':

that is a dual complementary emitter follower.

The reason I did not jump into the discussion with that, is that _in this form_ I am not sure about zero temp drift, it may need some resistors. But it should have zero output offset versus input, low output impedance, high input impedance, temp tracking, and I do not have spice running to give an idea, so not a tested thing. So if it melts you mega $ trannies so be it.

But your circuit has 24 parts and doesn't even have a voltage gain of

2. Snoop the input current of that buffer. At 20 MHz, the overall current gain is less than 1. At 100 MHz, the current gain is about 0.1. You'd be better off with a piece of wire.

Designing discrete stuff if fun, but engineers need to be pragmatic, too. Buying an opamps gets things done.

It's disappointing that, when you design a really good discrete circuit, the parts go EOL and some smart-alec makes an IC that does it better.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics