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From: Hammy on 14 Apr 2010 10:41
On Wed, 14 Apr 2010 13:07:55 GMT, Jan Panteltje
<pNaonStpealmtje(a)yahoo.com> wrote:

>On a sunny day (Wed, 14 Apr 2010 08:05:48 -0400) it happened Hammy
><spam(a)spam.com> wrote in <ugbbs5djgh1n3a9do8nssiutlrivfaqeg0(a)4ax.com>:
>
>>Is it possible to reduce crossover distortion on a push-pull buffer in
>>a package where the bases are tied internally?
>>
>>The package I'm talking about is here
>>
>>http://i44.tinypic.com/2rhxo2q.png
>>
>>The input signal is low Hz to 10MHz sine wave from a THS4222. I'm
>>after an output of 10Vpp into 50 ohm.
>>
>>I can't use diodes like you typically would because of the way the
>>package ties the bases.
>>
>>Typical diode compensated Push-Pull.
>>
>>http://i41.tinypic.com/dh8uo7.png
>
>Depends on how you drive it.
>Sometimes you can just put a resistor from the bases to the emittors,
>at low signal levels (where the transistors are still off)
>the driver then drives the load via the resistor.
>
> +
> |
> c
> ---b
> | e
>----- drive ---------- R1-|
> | |---------- out
> | e
> --b
> c
> |
> -
>>I'm just wondering if anyone knows of any tricks.

Thank you Jan. I'll try that.
From: Jim Thompson on 14 Apr 2010 10:42
On Wed, 14 Apr 2010 08:05:48 -0400, Hammy <spam(a)spam.com> wrote:

>Is it possible to reduce crossover distortion on a push-pull buffer in
>a package where the bases are tied internally?
>
>The package I'm talking about is here
>
>http://i44.tinypic.com/2rhxo2q.png
>
>The input signal is low Hz to 10MHz sine wave from a THS4222. I'm
>after an output of 10Vpp into 50 ohm.
>
>I can't use diodes like you typically would because of the way the
>package ties the bases.
>
>Typical diode compensated Push-Pull.
>
>http://i41.tinypic.com/dh8uo7.png
>
>I'm just wondering if anyone knows of any tricks.
>
>I know I could use other BJT's but these are pretty nice single digit
>nano-second rise/fall and storage times,high gain low Vce sat.
>
>
>http://www.nxp.com/acrobat/datasheets/PMD3001D_1.pdf

Can you stand the dissipation if you class-A biased the output by
pulling a DC load current?

I used to do LM324's that way, so they could be used in active
filters.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

The only thing bipartisan in this country is hypocrisy
From: Hammy on 14 Apr 2010 11:40
On Wed, 14 Apr 2010 07:42:52 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote:

>On Wed, 14 Apr 2010 08:05:48 -0400, Hammy <spam(a)spam.com> wrote:
>
>>Is it possible to reduce crossover distortion on a push-pull buffer in
>>a package where the bases are tied internally?
>>
>>The package I'm talking about is here
>>
>>http://i44.tinypic.com/2rhxo2q.png
>>
>>The input signal is low Hz to 10MHz sine wave from a THS4222. I'm
>>after an output of 10Vpp into 50 ohm.
>>
>>I can't use diodes like you typically would because of the way the
>>package ties the bases.
>>
>>Typical diode compensated Push-Pull.
>>
>>http://i41.tinypic.com/dh8uo7.png
>>
>>I'm just wondering if anyone knows of any tricks.
>>
>>I know I could use other BJT's but these are pretty nice single digit
>>nano-second rise/fall and storage times,high gain low Vce sat.
>>
>>
>>http://www.nxp.com/acrobat/datasheets/PMD3001D_1.pdf
>
>Can you stand the dissipation if you class-A biased the output by
>pulling a DC load current?
>
>I used to do LM324's that way, so they could be used in active
>filters.
>
> ...Jim Thompson
I'm trying to keep the power consumption to a minimum both to minimize
heat and to keep the power supply small. I'd like to get as wide a
swing (as close to +/- 5V) from a +/- 6V rails or less.

The THS422 can accept 15V max but the lower the rail voltage the less
dissipation in the output transistors. The less current the THS422
sinks/sources the wider the swing for the push/pull input which is why
I was looking at the dual NPN/PNP high gain low Vce sat duals.

This is for the output of a AD9834 10MHz signal generator I built a
while ago. Its only using a AD8057 to bump the filtered output up to
about 2Vpp. I'm considering ways to get this upto 10Vpp into 50 ohms
which seems to be the standard max output on most of the commercial
ones I looked at. I plan on using the THS422 as an adjustable gain
stage so I can have some amplitude adjustment. I haven't got that far
yet though ;-).
From: Vladimir Vassilevsky on 14 Apr 2010 15:35


Hammy wrote:

> Is it possible to reduce crossover distortion on a push-pull buffer in
> a package where the bases are tied internally?

Bridge feed forward compensation:

http://www.abvolt.com/misc/feedfwdbridge.jpg

If the bridge is balanced (R1 x R2 x C = L), then the compensation is
perfect.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com




From: JIMMIE on 14 Apr 2010 16:06
On Apr 14, 9:37 am, MooseFET <kensm...(a)rahul.net> wrote:
> On Apr 14, 5:05 am, Hammy <s...(a)spam.com> wrote:
>
>
>
> > Is it possible to reduce crossover distortion on a push-pull buffer in
> > a package where the bases are tied internally?
>
> > The package I'm talking about is here
>
> >http://i44.tinypic.com/2rhxo2q.png
>
> > The input signal is low Hz to 10MHz sine wave from a THS4222. I'm
> > after an output of 10Vpp into 50 ohm.
>
> > I can't use diodes like you typically would because of the way the
> > package ties the bases.
>
> > Typical diode compensated Push-Pull.
>
> >http://i41.tinypic.com/dh8uo7.png
>
> > I'm just wondering if anyone knows of any tricks.
>
> > I know I could use other BJT's but these are pretty nice single digit
> > nano-second rise/fall and storage times,high gain low Vce sat.
>
> >http://www.nxp.com/acrobat/datasheets/PMD3001D_1.pdf
>
> You can reduce the distortion and push it up in frequency like this:
>
>                           ---! NPN
>  -------!+\      R1      !   !\e
>         !  >--+--/\/\----+     !
>       --!-/   !          !     +-------------+------------
>      !        +--/\/\----------+             !
>      !        !  R2      !     !             !
>      +---!!---           !   !/e             !
>      !                    ---! PNP           !
>      !      R3                               !
>       -----/\/\------------------------------
>
> You need a very fast slewing op-amp like the LT1351 or 1354
>
> The local feedback capacitor needs to be just enough to prevent
> oscillation at the gain cross over frequency.
>> R1 is perhaps as low as 10 Ohms.  At very high frequencies the
> output of the op-amp looks like a weird LC circuit.  You need
> to deQ this a little.
>
> R2 lets the op-amp drive the load during the cross over.

I used to build some audio line drivers similar to that. Transistors
were a push-pull emitter follower following an op-amp, Negative feed
back for the op-amp was picked off after the transistors. This worked
better than several other biasing schemes I tried.

Jimmie
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