power opamp?

[John Larkin]

What's a good medium-power opamp? I need to source/sink at least 60
mA, with say +-7.5 volt supplies, and swing +-6 maybe. No particular
requirements otherwise.

I suppose I could boost a cheap SOT-23 opamp with a couple of cheap
SOT-23 emitter followers....


v+
|
c
\ +----Rb---b npn
\ | e
\ | |
\---------+----Rx----+-------
/ | |
/ | e
/ +----Rb---b pnp
c
|
v-


but a single part would be nice; I have to do this 16 times on a
smallish board.

John

 

[Eeyore]

What's a good medium-power opamp? I need to source/sink at least 60
mA, with say +-7.5 volt supplies, and swing +-6 maybe. No particular
requirements otherwise.

I suppose I could boost a cheap SOT-23 opamp with a couple of cheap
SOT-23 emitter followers....

v+
|
c
\ +----Rb---b npn
\ | e
\ | |
\---------+----Rx----+-------
/ | |
/ | e
/ +----Rb---b pnp
c
|
v-

You like dead bands do you ?

but a single part would be nice; I have to do this 16 times on a
smallish board.

Common up the outputs of 2 op-amps that can drive 30mA though current
sharing resistors.

Graham

 

[John Larkin]

You like dead bands do you ?

What do you think the 'x' in Rx stands for?

John

 

[Jim Thompson]

What's a good medium-power opamp? I need to source/sink at least 60
mA, with say +-7.5 volt supplies, and swing +-6 maybe. No particular
requirements otherwise.

I suppose I could boost a cheap SOT-23 opamp with a couple of cheap
SOT-23 emitter followers....


v+
|
c
\ +----Rb---b npn
\ | e
\ | |
\---------+----Rx----+-------
/ | |
/ | e
/ +----Rb---b pnp
c
|
v-


but a single part would be nice; I have to do this 16 times on a
smallish board.

John

Your suggested approach is probably the least fraught with possible
stability problems.

But when I need lots of near-rail-to-rail swing I put resistors in the
VCC and VEE lines and add the transistors there (e.g., base to VCC
pin, emitter to rail, collector to output point... if you don't need
rail-to-rail, tie OpAmp output to collectors).

...Jim Thompson

 

[John Popelish]

What's a good medium-power opamp? I need to source/sink at least 60
mA, with say +-7.5 volt supplies, and swing +-6 maybe. No particular
requirements otherwise.

I suppose I could boost a cheap SOT-23 opamp with a couple of cheap
SOT-23 emitter followers....


v+
|
c
\ +----Rb---b npn
\ | e
\ | |
\---------+----Rx----+-------
/ | |
/ | e
/ +----Rb---b pnp
c
|
v-


but a single part would be nice; I have to do this 16 times on a
smallish board.

Single:
http://cache.national.com/ds/LM/LMH6640.pdf

Slower dual:
http://cache.national.com/ds/LM/LM8272.pdf

 

[John Larkin]

Single:
http://cache.national.com/ds/LM/LMH6640.pdf

Slower dual:
http://cache.national.com/ds/LM/LM8272.pdf

The 8272 looks good, especially so since I'll be essentially driving a
bypassed power rail, and it doesn't mind capacitive loads. It's $1.30
at 1K, but it would save me a lot of parts, 16 times. Boosting a small
opamp into a bypassed load would add, say, 7 more parts.

16 x 7 = 112!

John

 

[Jim Thompson]

Your suggested approach is probably the least fraught with possible
stability problems.

But when I need lots of near-rail-to-rail swing I put resistors in the
VCC and VEE lines and add the transistors there (e.g., base to VCC
pin, emitter to rail, collector to output point... if you don't need
rail-to-rail, tie OpAmp output to collectors).

Like this....

v+
| v+
R |
| e
\VCC----------------------b pnp
\ c
\ |
\--------------------+-------OUTPUT
/ |
/ c
/VEE---------------------b npn
| e
R |
| v-
v-


...Jim Thompson

 

[Eeyore]

What do you think the 'x' in Rx stands for?

You'll still get a nasty kink in the transfer characteristic.

Graham

 

[John Larkin]

Your suggested approach is probably the least fraught with possible
stability problems.

But when I need lots of near-rail-to-rail swing I put resistors in the
VCC and VEE lines and add the transistors there (e.g., base to VCC
pin, emitter to rail, collector to output point... if you don't need
rail-to-rail, tie OpAmp output to collectors).

...Jim Thompson

Yeah, that's neat but harder to analyze for stability. One would
*really* have to trust the Spice models, or test the heck out of it.

I can imagine way too many oscillation modes!

We recently did something similar, using opamp supply currents to
drive transistors sort of like that, but current-mirror mode, to turn
the opamp into a fairly precise current source. The sim we did didn't
work at all, apparently because the opamp model doesn't actually pull
current from the rails!

John

 

[Jim Thompson]

Yeah, that's neat but harder to analyze for stability. One would
*really* have to trust the Spice models, or test the heck out of it.

I can imagine way too many oscillation modes!

If you tie "Output" back to the OpAmp out it resolves to just a
compound follower.

We recently did something similar, using opamp supply currents to
drive transistors sort of like that, but current-mirror mode, to turn
the opamp into a fairly precise current source. The sim we did didn't
work at all, apparently because the opamp model doesn't actually pull
current from the rails!

John

Spice models can be treacherous, which is why I roll so many of my
own.

...Jim Thompson

 

[Joerg]

The 8272 looks good, especially so since I'll be essentially driving a
bypassed power rail, and it doesn't mind capacitive loads. It's $1.30
at 1K, but it would save me a lot of parts, 16 times. Boosting a small
opamp into a bypassed load would add, say, 7 more parts.

You can't use a regular amp with biased npn/pnp into the rail and then
sum the 16 others into its input?

16 x 7 = 112!

Why not? I once had to cram over 50 parts onto the size of a standard
40c postage stamp. Now wait, that'll be 41c soon. 0201, SC75, oh what
fun to debug.

 

[Joerg]

What do you think the 'x' in Rx stands for?

Pharmacy?

 

[Joerg]

Yeah, that's neat but harder to analyze for stability. One would
*really* have to trust the Spice models, or test the heck out of it.

I can imagine way too many oscillation modes!

We recently did something similar, using opamp supply currents to
drive transistors sort of like that, but current-mirror mode, to turn
the opamp into a fairly precise current source. The sim we did didn't
work at all, apparently because the opamp model doesn't actually pull
current from the rails!

If I had trusted SPICE every time many of my design would not exist
today. Can't remember the last time I fired up SPICE. Some time last
year I guess.

 

[Jim Thompson]

If I had trusted SPICE every time many of my design would not exist
today. Can't remember the last time I fired up SPICE. Some time last
year I guess.

But you aren't building a very complex structure from individual
devices. My chip designs are almost always way over 1000 devices.

...Jim Thompson

 

[Joerg]

But you aren't building a very complex structure from individual
devices. My chip designs are almost always way over 1000 devices.

True. However, some designs are, let's say, highly unorthodox. For
example SPICE often isn't very useful when you have to use the
saturation behavior of a custom-mix ferrite as a design parameter. Or in
detecting and handling mode jumps of a laser diode. Or mapping sound
wave reflections from a biopsy needle in a diseased area of human tissue.

 

[Joerg]

Seems to stand for the Latin "ecipe"

Yep. So that would be a prescription resistor.

 

[Spehro Pefhany]

Pharmacy?

Seems to stand for the Latin "ecipe"


Best regards,
Spehro Pefhany

 

[Tim Williams]

Why not? I once had to cram over 50 parts onto the size of a standard
40c postage stamp. Now wait, that'll be 41c soon. 0201, SC75, oh what
fun to debug.

It sounds like, pretty soon, you and Jim are going to have to
collaborate -- your scales of construction are constantly converging!

Tim

 

[John Larkin]

True. However, some designs are, let's say, highly unorthodox. For
example SPICE often isn't very useful when you have to use the
saturation behavior of a custom-mix ferrite as a design parameter. Or in
detecting and handling mode jumps of a laser diode. Or mapping sound
wave reflections from a biopsy needle in a diseased area of human tissue.

Anybody got decent models for ferrite beads?

John

 

[Jim Thompson]

On Sun, 02 Mar 2008 14:55:02 -0800, Joerg
[snip]

True. However, some designs are, let's say, highly unorthodox. For
example SPICE often isn't very useful when you have to use the
saturation behavior of a custom-mix ferrite as a design parameter. Or in
detecting and handling mode jumps of a laser diode. Or mapping sound
wave reflections from a biopsy needle in a diseased area of human tissue.

Anybody got decent models for ferrite beads?

John

I seem to recall seeing one... maybe Google ?

...Jim Thompson