Fast buffer idea

On Sun, 14 May 2017 16:02:13 -0400, bitrex
<bitrex@de.lete.earthlink.net> wrote:

>On 05/14/2017 03:54 PM, John Larkin wrote:
>
>> The posted circuit drives a lowpass filter to make a small sort of
>> sine wave from a giant square-wave drive, so a single transistor
>> buffer should work. Or maybe no buffer at all.
>
>I doubt the load is a realistic "model" for a length of cable. I saw one 
>on EDN but it looks pretty complicated, about 5 controlled sources in 
>series. It'll take a while to read and understand how they came up with 
>it...

Is your object to drive a length of coax? If it's terminated, you can
just ignore the cable entirely. Or poke in a Spice transmission line
to explore mismatch consequences.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On 05/14/2017 04:10 PM, John Larkin wrote:
> On Sun, 14 May 2017 16:02:13 -0400, bitrex
> <bitrex@de.lete.earthlink.net> wrote:
>
>> On 05/14/2017 03:54 PM, John Larkin wrote:
>>
>>> The posted circuit drives a lowpass filter to make a small sort of
>>> sine wave from a giant square-wave drive, so a single transistor
>>> buffer should work. Or maybe no buffer at all.
>>
>> I doubt the load is a realistic "model" for a length of cable. I saw one
>> on EDN but it looks pretty complicated, about 5 controlled sources in
>> series. It'll take a while to read and understand how they came up with
>> it...
>
> Is your object to drive a length of coax? If it's terminated, you can
> just ignore the cable entirely. Or poke in a Spice transmission line
> to explore mismatch consequences.
>

Ya, exploring the "mismatch" consequences I'd like to do. How do I do 
that? I haven't used Spice transmission line models before...

On 05/14/2017 04:16 PM, bitrex wrote:
> On 05/14/2017 04:10 PM, John Larkin wrote:
>> On Sun, 14 May 2017 16:02:13 -0400, bitrex
>> <bitrex@de.lete.earthlink.net> wrote:
>>
>>> On 05/14/2017 03:54 PM, John Larkin wrote:
>>>
>>>> The posted circuit drives a lowpass filter to make a small sort of
>>>> sine wave from a giant square-wave drive, so a single transistor
>>>> buffer should work. Or maybe no buffer at all.
>>>
>>> I doubt the load is a realistic "model" for a length of cable. I saw one
>>> on EDN but it looks pretty complicated, about 5 controlled sources in
>>> series. It'll take a while to read and understand how they came up with
>>> it...
>>
>> Is your object to drive a length of coax? If it's terminated, you can
>> just ignore the cable entirely. Or poke in a Spice transmission line
>> to explore mismatch consequences.
>>
>
> Ya, exploring the "mismatch" consequences I'd like to do. How do I do
> that? I haven't used Spice transmission line models before...
>

Ideally, yes, the length will be terminated in approximately its 
characteristic impedance.

John Larkin <jjlarkin@highlandtechnology.com> wrote:
 
> The posted circuit drives a lowpass filter to make a small sort of
> sine wave from a giant square-wave drive, so a single transistor
> buffer should work. Or maybe no buffer at all.

The signal before the filter at the junction of R1 and R2 is actually 
pretty good. I don't know of any op amp that can do that.

But you are right about the complexity. It seems a much simpler circuit 
could do the same.

On 05/14/2017 04:26 PM, Steve Wilson wrote:
> John Larkin <jjlarkin@highlandtechnology.com> wrote:
>
>> The posted circuit drives a lowpass filter to make a small sort of
>> sine wave from a giant square-wave drive, so a single transistor
>> buffer should work. Or maybe no buffer at all.
>
> The signal before the filter at the junction of R1 and R2 is actually
> pretty good. I don't know of any op amp that can do that.
>
> But you are right about the complexity. It seems a much simpler circuit
> could do the same.
>

It probably could, but the "gimmick" here that requires the extra parts 
is that it draws very little quiescent current. When there's no signal 
the output followers are biased very low, almost to cut-off.

On 05/14/2017 04:26 PM, Steve Wilson wrote:
> John Larkin <jjlarkin@highlandtechnology.com> wrote:
>
>> The posted circuit drives a lowpass filter to make a small sort of
>> sine wave from a giant square-wave drive, so a single transistor
>> buffer should work. Or maybe no buffer at all.
>
> The signal before the filter at the junction of R1 and R2 is actually
> pretty good. I don't know of any op amp that can do that.
>
> But you are right about the complexity. It seems a much simpler circuit
> could do the same.
>

I've lowered the frequency to 5MHz, and changed the risetime to 1ns, 
more in line with what those jellybean transistors are used to. Here are 
the waveforms:

http://imgur.com/a/QJgL8

Input is in red and output into 50 ohm load is in green. Blue shows the 
current pulses the "boosting" transistors are pushing into the output 
devices to speed the edges - there's a positive-going pulse as the 
increase in output current drives them into conduction, and then a 
negative-going pulse as the output swings the other way and they conduct 
briefly in the reverse direction, until enough charge is pulled out of 
their bases for them to snap off again.

On Sun, 14 May 2017 16:16:29 -0400, bitrex
<bitrex@de.lete.earthlink.net> wrote:

>On 05/14/2017 04:10 PM, John Larkin wrote:
>> On Sun, 14 May 2017 16:02:13 -0400, bitrex
>> <bitrex@de.lete.earthlink.net> wrote:
>>
>>> On 05/14/2017 03:54 PM, John Larkin wrote:
>>>
>>>> The posted circuit drives a lowpass filter to make a small sort of
>>>> sine wave from a giant square-wave drive, so a single transistor
>>>> buffer should work. Or maybe no buffer at all.
>>>
>>> I doubt the load is a realistic "model" for a length of cable. I saw one
>>> on EDN but it looks pretty complicated, about 5 controlled sources in
>>> series. It'll take a while to read and understand how they came up with
>>> it...
>>
>> Is your object to drive a length of coax? If it's terminated, you can
>> just ignore the cable entirely. Or poke in a Spice transmission line
>> to explore mismatch consequences.
>>
>
>Ya, exploring the "mismatch" consequences I'd like to do. How do I do 
>that? I haven't used Spice transmission line models before...

It's fun to play with. Just plop the TLINE thing onto your sheet and
edit the impedance and time delay. It's easiest at first to ground
both ends of the lower side, or things get a little more complex.

https://dl.dropboxusercontent.com/u/53724080/Circuits/Fast_Stuff/EQR_50_350_MAX.jpg

You can see the pulse reflection from the 100 ohm mismatch. Jeroen's
lowpass filter absorbs that nicely.

TLINE is lossless, usually good enough. There is also LTLINE, a lossy
transmission line.

Set the sim DT smaller than the rise times you expect. LT Spice can do
dumb things with time steps.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Sun, 14 May 2017 20:26:57 GMT, Steve Wilson <no@spam.com> wrote:

>John Larkin <jjlarkin@highlandtechnology.com> wrote:
> 
>> The posted circuit drives a lowpass filter to make a small sort of
>> sine wave from a giant square-wave drive, so a single transistor
>> buffer should work. Or maybe no buffer at all.
>
>The signal before the filter at the junction of R1 and R2 is actually 
>pretty good. I don't know of any op amp that can do that.
>
>But you are right about the complexity. It seems a much simpler circuit 
>could do the same.

The TI THS series really screams. There are parts with 1.8 GHz GBW and
huge output currents. Or the Analog Devices AD800x parts.




-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

John Larkin <jjlarkin@highlandtechnology.com> wrote:
 
> The TI THS series really screams. There are parts with 1.8 GHz GBW and
> huge output currents. Or the Analog Devices AD800x parts.

Both are slow. 2GHz or so.

These look interesting:

  LTC6268 4GHz
  http://www.linear.com/docs/46371

  LTC6409 10GHz  1.1nV/rt(Hz)
  http://www.linear.com/docs/29893

On Sun, 14 May 2017 21:53:32 GMT, Steve Wilson <no@spam.com> wrote:

>John Larkin <jjlarkin@highlandtechnology.com> wrote:
> 
>> The TI THS series really screams. There are parts with 1.8 GHz GBW and
>> huge output currents. Or the Analog Devices AD800x parts.
>
>Both are slow. 2GHz or so.

But the TI parts have huge voltage swing and current capability.
Everyone else is ratcheting down voltage as speed goes up.

>
>These look interesting:
>
>  LTC6268 4GHz
>  http://www.linear.com/docs/46371

Impressive, except for the 5 volt supply.

>
>  LTC6409 10GHz  1.1nV/rt(Hz)
>  http://www.linear.com/docs/29893
>
>

I used a similar part, LTC6402 differential ADC driver. It was flakey,
so we had to redesign with an ADA4950.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics