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From: Hammy on 14 Apr 2010 08:05
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
From: mook johnson on 14 Apr 2010 08:40

"Hammy" <spam(a)spam.com> wrote in message
news: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
>
> 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


Put the bases on the output of the opamp and pick the feedback two emitters.
That will minimize the crossover distortion. But may not completely
eliminate it. Be careful driving reactive loads or you may get oscillation.

Why not just use a part with two independent NPNs?



From: Jan Panteltje on 14 Apr 2010 09:07
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.
From: MooseFET on 14 Apr 2010 09:37
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.
From: John Larkin on 14 Apr 2010 10:06
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
> |

Yup, that's a good one, inside a feedback loop.

John

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