line driver with automatic bias

On Sat, 28 Aug 2010 20:11:11 -0700, "BobW"
nimby_GIMME_SPAM__at_roadrunner.com> wrote:


"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:mr9j76d9l19nqmf8bed72e794b04ohbhsv_at_4ax.com...
On Sat, 28 Aug 2010 19:02:30 -0500, Vladimir Vassilevsky
nospam_at_nowhere.com> wrote:



Ban wrote:

features AB/operation of the output transistors with a bias current of
1.5mA
(set by 15k). The complementary pairs should have a similar beta. To
have a
symmetrical clipping at around +/-11.5V a zener in the positive supply
of
the opamp overcomes the bootstrapping action of the 100u capacitor.
Keeping the output transistors conducting reduces distortion by a whole
magnitude. There is no current limit, so an 100R output resistor
protects
the transistors. Temperature compensation is intrinsic to the circuit,
no
thermal runaway should occur.
.--------o------------o--o
| | | +15V
| .-. |
| | | |
| | |15k |
z '-' |
2.7V A | |
| .-----o |
| | \| |/
| --- PNP |--------| NPN
| --- >| |
| |100u | |
___ | | | ___ | ___
o--|___|-o---)--)-----)---|___|----o-|___|-o
in 10k | | | | 10k | out
| | | | |
| | | \| |/
| | | NPN |--------| PNP
| |\| | <| |
'-|-\ | | |
| >-o-----o |
o--------o-|+/ .-. |
GND | |/| OP27 | | |
| | | |15k |
=== | '-' |
GND | | | -15V
'--------o------------o--o

This one is simpler and better in all ways:

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


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



This is nice and simple.

ftp://jjlarkin.lmi.net/OpampBoost.JPG

The resistor lets the opamp do some of the work at low level,
eliminating the deadband. Sort of. No thermal problems at all.

We could have a 500+ post thread about which "class" this is.

I don't understand Ban's circuit.


John


Neither do I. How do the output devices' bases get their drive?

Bob

It's an old input bias current compensation circuit gone awry by
amateur application

...Jim Thompson

John Larkin wrote:
Ban wrote:

features AB/operation of the output transistors with a bias current
of 1.5mA (set by 15k). The complementary pairs should have a
similar beta. To have a symmetrical clipping at around +/-11.5V a
zener in the positive supply of the opamp overcomes the
bootstrapping action of the 100u capacitor.
Keeping the output transistors conducting reduces distortion by a
whole magnitude. There is no current limit, so an 100R output
resistor protects the transistors. Temperature compensation is
intrinsic to the circuit, no thermal runaway should occur.
.--------o------------o--o
| | | +15V
| .-. |
| | | |
| | |15k |
z '-' |
2.7V A | |
| .-----o |
| | \| |/
| --- PNP |--------| NPN
| --- >| |
| |100u | |
___ | | | ___ | ___
o--|___|-o---)--)-----)---|___|----o-|___|-o
in 10k | | | | 10k | out
| | | | |
| | | \| |/
| | | NPN |--------| PNP
| |\| | <| |
'-|-\ | | |
| >-o-----o |
o--------o-|+/ .-. |
GND | |/| OP27 | | |
| | | |15k |
=== | '-' |
GND | | | -15V
'--------o------------o--o


This is nice and simple.

ftp://jjlarkin.lmi.net/OpampBoost.JPG

The resistor lets the opamp do some of the work at low level,
eliminating the deadband. Sort of. No thermal problems at all.

We could have a 500+ post thread about which "class" this is.

I don't understand Ban's circuit.


The resistor onthe upper left drives 1mA into the emitter of the PNP, from
which most will be taken by the collector, only 1/beta will flow out of the
base into the base of the NPN O/P transistor. This causes the same quiescent
current flowing in the output pair.
Temperature variation is minimal, since we do not have a bias voltage but
current drive, and the first transistor adjusts its base current
accordingly.

ciao Ban

Build it _exactly_ as drawn, measure it and then tell us about it.

On Mon, 30 Aug 2010 08:19:25 +1000, Grant <omg_at_grrr.id.au> wrote:

On Sun, 29 Aug 2010 23:06:38 +0200, "Ban" <bansuri_at_web.de> wrote:

...
uploaded my simulation file, has some beefier driver in sziklay
configuration.
The PNPs are in fact upside down.
http://i33.tinypic.com/20kx3lv.gif

Well at least your PNPs are right way up, but circuit looks too busy,
have you built this or just simulating? Looks to me as if you expect
some matching that's not going to happen in real life?

Thanks for putting up a clearer circuit!

Grant.

It's one of those PhD circuits... Piled higher and Deeper... nice
theory/simulation (†), but crap in practice :-)

(†) Which would catch the issues if the guy in the cockpit had any
clue as to what to test for

...Jim Thompson

On Sun, 29 Aug 2010 16:01:13 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon_at_On-My-Web-Site.com> wrote:

On Mon, 30 Aug 2010 08:19:25 +1000, Grant <omg_at_grrr.id.au> wrote:

On Sun, 29 Aug 2010 23:06:38 +0200, "Ban" <bansuri_at_web.de> wrote:

...
uploaded my simulation file, has some beefier driver in sziklay
configuration.
The PNPs are in fact upside down.
http://i33.tinypic.com/20kx3lv.gif

Well at least your PNPs are right way up, but circuit looks too busy,
have you built this or just simulating? Looks to me as if you expect
some matching that's not going to happen in real life?

Thanks for putting up a clearer circuit!

Grant.

It's one of those PhD circuits... Piled higher and Deeper... nice
theory/simulation (†), but crap in practice :-)

(†) Which would catch the issues if the guy in the cockpit had any
clue as to what to test for

...Jim Thompson

That was uncharastically kind. The circuit is fubar. It may even
work but it is evil and wrong.

On Fri, 03 Sep 2010 23:20:02 -0700,
"JosephKK"<quiettechblue_at_yahoo.com> wrote:

On Sun, 29 Aug 2010 16:01:13 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon_at_On-My-Web-Site.com> wrote:

On Mon, 30 Aug 2010 08:19:25 +1000, Grant <omg_at_grrr.id.au> wrote:

On Sun, 29 Aug 2010 23:06:38 +0200, "Ban" <bansuri_at_web.de> wrote:

...
uploaded my simulation file, has some beefier driver in sziklay
configuration.
The PNPs are in fact upside down.
http://i33.tinypic.com/20kx3lv.gif

Well at least your PNPs are right way up, but circuit looks too busy,
have you built this or just simulating? Looks to me as if you expect
some matching that's not going to happen in real life?

Thanks for putting up a clearer circuit!

Grant.

It's one of those PhD circuits... Piled higher and Deeper... nice
theory/simulation (†), but crap in practice :-)

(†) Which would catch the issues if the guy in the cockpit had any
clue as to what to test for

...Jim Thompson

That was uncharastically kind. The circuit is fubar. It may even
work but it is evil and wrong.

Sonuvagun! I usually catch hell for being blunt.

Burr-Brown used to do something like that to compensate input stage
bias current in their bipolar OpAmps.

...Jim Thompson