On Mon, 15 May 2017 03:28:56 GMT, Steve Wilson <no@spam.com> wrote:>John Larkin <jjlarkin@highlandtechnology.com> wrote: > >> 12 db gain. It tended to oscillate at large swings. > >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.We used the LT6402-12.> >>>I'm surprised you had problems with LTC. They usually have a petty good >>>reputation for quality. > >> We were surprised too. And annoyed that we had to spin the board. > >Did you send your data to LTC?No, we just spun the board. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Fast buffer idea
Started by ●May 14, 2017
Reply by ●May 15, 20172017-05-15
Reply by ●May 15, 20172017-05-15
John Larkin <jjlarkin@highlandtechnology.com> wrote:> We used the LT6402-12.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 http://cds.linear.com/docs/en/datasheet/640212fa.pdf Looks like there is enough information in the datasheet to steer you away from this device.
Reply by ●May 15, 20172017-05-15
On a sunny day (Sun, 14 May 2017 12:54:12 -0700) it happened John Larkin <jjlarkin@highlandtechnology.com> wrote in <auchhcp4h0898t9hatrh3017esf6dve1n5@4ax.com>:>If the object is to buffer a logic level, some 13-cent TinyLogic gate, >like an NL37WZ16US or something. > >If it needs to be linear, there are tons of opamps to consider: >THS4222, AD8007, whatever.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,
Reply by ●May 15, 20172017-05-15
On 05/15/2017 03:56 AM, Jan Panteltje wrote:> On a sunny day (Sun, 14 May 2017 12:54:12 -0700) it happened John Larkin > <jjlarkin@highlandtechnology.com> wrote in > <auchhcp4h0898t9hatrh3017esf6dve1n5@4ax.com>: > >> If the object is to buffer a logic level, some 13-cent TinyLogic gate, >> like an NL37WZ16US or something. >> >> If it needs to be linear, there are tons of opamps to consider: >> THS4222, AD8007, whatever. > > 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 $7 op-amp, just a regular-fast op amp. It'll handle DC to 5MHz say, and then anything higher gets bypassed around it and pushed straight through.
Reply by ●May 15, 20172017-05-15
On a sunny day (Mon, 15 May 2017 06:58:15 -0400) it happened bitrex <bitrex@de.lete.earthlink.net> wrote in <cRfSA.95424$se.10515@fx38.iad>:>On 05/15/2017 03:56 AM, Jan Panteltje wrote: >> On a sunny day (Sun, 14 May 2017 12:54:12 -0700) it happened John Larkin >> <jjlarkin@highlandtechnology.com> wrote in >> <auchhcp4h0898t9hatrh3017esf6dve1n5@4ax.com>: >> >>> If the object is to buffer a logic level, some 13-cent TinyLogic gate, >>> like an NL37WZ16US or something. >>> >>> If it needs to be linear, there are tons of opamps to consider: >>> THS4222, AD8007, whatever. >> >> 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.For video, out of habit let's say: From: http://panteltje.com/panteltje/quadcopter/hud.html the circuit diagram: http://panteltje.com/panteltje/quadcopter/hud-0.1_circuit_diagram_IMG_5984.JPG 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. <disclaimer> </disclaimer>
Reply by ●May 15, 20172017-05-15
On Mon, 15 May 2017 07:56:01 GMT, Jan Panteltje <pNa0nStpealmtje@yahoo.com> wrote:>On a sunny day (Sun, 14 May 2017 12:54:12 -0700) it happened John Larkin ><jjlarkin@highlandtechnology.com> wrote in ><auchhcp4h0898t9hatrh3017esf6dve1n5@4ax.com>: > >>If the object is to buffer a logic level, some 13-cent TinyLogic gate, >>like an NL37WZ16US or something. >> >>If it needs to be linear, there are tons of opamps to consider: >>THS4222, AD8007, whatever. > >THS4222 datasheet says open loop gain 0dB at 100 MHz?Where does it say that?>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... >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.> >But indeed you can do that with a few NPNs of the old days, >Post it. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●May 15, 20172017-05-15
On 05/15/2017 10:01 AM, John Larkin wrote:> 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. > >> >> But indeed you can do that with a few NPNs of the old days, >> > > Post it.With discretes you can have: 1) wide bandwidth 2) low parts count 3) low quiescent current draw Pick two.
Reply by ●May 15, 20172017-05-15
On 05/15/2017 10:09 AM, bitrex wrote:> On 05/15/2017 10:01 AM, John Larkin wrote: > >> 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. >> >>> >>> But indeed you can do that with a few NPNs of the old days, >>> >> >> Post it. > > 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.
Reply by ●May 15, 20172017-05-15
On a sunny day (Mon, 15 May 2017 07:01:06 -0700) it happened John Larkin <jjlarkin@highlandtechnology.com> wrote in <dpcjhclpajjafaf2nq0q7l7nt17jl40spo@4ax.com>:>On Mon, 15 May 2017 07:56:01 GMT, Jan Panteltje ><pNa0nStpealmtje@yahoo.com> wrote: > >>On a sunny day (Sun, 14 May 2017 12:54:12 -0700) it happened John Larkin >><jjlarkin@highlandtechnology.com> wrote in >><auchhcp4h0898t9hatrh3017esf6dve1n5@4ax.com>: >> >>>If the object is to buffer a logic level, some 13-cent TinyLogic gate, >>>like an NL37WZ16US or something. >>> >>>If it needs to be linear, there are tons of opamps to consider: >>>THS4222, AD8007, whatever. >> >>THS4222 datasheet says open loop gain 0dB at 100 MHz? > >Where does it say that? > >>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... >> > >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. > >> >>But indeed you can do that with a few NPNs of the old days, >> > >Post it.Did not I ;-)? Actually when I did read that OP, I scribbled this on my 'desk': http://panteltje.com/pub/maybe_this_amp_IMG_6183.JPG 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.
Reply by ●May 15, 20172017-05-15
On Mon, 15 May 2017 10:09:39 -0400, bitrex <bitrex@de.lete.earthlink.net> wrote:>On 05/15/2017 10:01 AM, John Larkin wrote: > >> 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. >> >>> >>> But indeed you can do that with a few NPNs of the old days, >>> >> >> Post it. > >With discretes you can have: > >1) wide bandwidth > >2) low parts count > >3) low quiescent current draw > >Pick two.But your circuit has 24 parts and doesn't even have a voltage gain of 1. 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
