looking for a op-amp current booster for low voltage

[mook Johnson]

I have +/-2.5V supply rails an am looking for a minimum parts count 1 amp
peak current booster for a 200Hz sine wave generator. The opamp is rail to
rail and can supply 10mA to drive B=100 transistors to deliver the 1 amp
peek.

Max output voltage is 2V peak so the output doesn't need to go rail to rail.

Class B with opamp feedback?

I'm looking for low crossover distortion. low being arbitrarily defined as
if I look on a scope at 1v/div the distortion won't be clearly noticeable.
Its absolute level is not that critical.

 

[[email protected]]

I have +/-2.5V supply rails an am looking for a minimum parts count 1 amp
peak current booster for a 200Hz sine wave generator. The opamp is rail to
rail and can supply 10mA to drive B=100 transistors to deliver the 1 amp
peek.

Max output voltage is 2V peak so the output doesn't need to go rail to rail.

Class B with opamp feedback?

I'm looking for low crossover distortion. low being arbitrarily defined as
if I look on a scope at 1v/div the distortion won't be clearly noticeable.
Its absolute level is not that critical.

+/-2.5V seems to be a bit low for 1A peak current. Linear Technology
have a couple of current boosters aimed at the ADSL market, and an RF
op amp. presumably ained at video distribution, but they are specified
for supplies from +/-5V to +/-12V.

You haven't specified the bandwidth that you hope to achieve - this is
a pretty important parameter. Some of the more psychotic of our regular
posters would take this as evidence that you were a clueless newby, who
ought to be posting on aci.electronics.basics.

 

[mook Johnson]

bandwidth should be sufficient to produce a 200Hz sine wave with low
distortion. BW shouldnt be too big of an issue.

Load will the tranformer coupled to some very low impedance resistive loads.

magnitizing current for the transformer primary will be less than 50mA.

 

[Winfield Hill]

mook Johnson wrote...

bandwidth should be sufficient to produce a 200Hz sine wave
with low distortion. BW shouldnt be too big of an issue.
Load will the tranformer coupled to some very low impedance
resistive loads. magnitizing current for the transformer
primary will be less than 50mA.

An emitter follower has about 700mV voltage drop (more at 1A),
so even a perfect rail-rail opamp wouldn't be able to drive it.
You can use an odd-looking circuit Tony Williams and I like to
design with every now and then, in which the output transistors
are operated from the opamp's power rails. Here's an example,
ftp://ftp.rowland.org/pub/hill/ris-496-1.pdf

That's a complicated circuit, but it works to +/-250 volts (in
bridge) with up to 125 watts peak, operates in precision-biased
class AB, and has foldback power limiting.

At low voltages you can make a simple low-parts-count version.

.. +2.5 ---+-----------,
.. | |
.. 150 |
.. | |/V pnp
.. +---------|
.. ,-- | |\
.. | __| |
.. --+--|- \ |
.. | >--+--75--+---
.. ----|+__/ | |
.. | 22 |
.. | | |
.. | gnd |/
.. +---------|
.. | |\V npn
.. 150 |
.. | |
.. -2.5 ----+-----------'

Zetex makes suitable high-gain transistors. The amplifier
operates class B at high currents, but at low frequencies
it can have low distortion, if the opamp is fast enough.
However, the opamp must be a low-voltage type, and will be
operating from +/-1.7 volts, which restricts your choices.

I wonder about your 2.5 volts, will that sometimes sag down
even lower?

 

[Jim Thompson]

mook Johnson wrote...

An emitter follower has about 700mV voltage drop (more at 1A),
so even a perfect rail-rail opamp wouldn't be able to drive it.
You can use an odd-looking circuit Tony Williams and I like to
design with every now and then, in which the output transistors
are operated from the opamp's power rails. Here's an example,
ftp://ftp.rowland.org/pub/hill/ris-496-1.pdf

That's a complicated circuit, but it works to +/-250 volts (in
bridge) with up to 125 watts peak, operates in precision-biased
class AB, and has foldback power limiting.

At low voltages you can make a simple low-parts-count version.

. +2.5 ---+-----------,
. | |
. 150 |
. | |/V pnp
. +---------|
. ,-- | |\
. | __| |
. --+--|- \ |
. | >--+--75--+---
. ----|+__/ | |
. | 22 |
. | | |
. | gnd |/
. +---------|
. | |\V npn
. 150 |
. | |
. -2.5 ----+-----------'

Zetex makes suitable high-gain transistors. The amplifier
operates class B at high currents, but at low frequencies
it can have low distortion, if the opamp is fast enough.
However, the opamp must be a low-voltage type, and will be
operating from +/-1.7 volts, which restricts your choices.

I wonder about your 2.5 volts, will that sometimes sag down
even lower?

I was using that scheme in 1970-73 in thick film hybrids at Dickson
Electronics, Scottsdale, AZ ;-)

And there was a custom driver chip plus PNP & NPN chips at Motorola
before that... something like an MC1538 plus (IIRC) 2N2222 & 2N2906
chips in the same package.

...Jim Thompson

 

[[email protected]]

mook Johnson wrote...

An emitter follower has about 700mV voltage drop (more at 1A),
so even a perfect rail-rail opamp wouldn't be able to drive it.
You can use an odd-looking circuit Tony Williams and I like to
design with every now and then, in which the output transistors
are operated from the opamp's power rails. Here's an example,
ftp://ftp.rowland.org/pub/hill/ris-496-1.pdf

That's a complicated circuit, but it works to +/-250 volts (in
bridge) with up to 125 watts peak, operates in precision-biased
class AB, and has foldback power limiting.

At low voltages you can make a simple low-parts-count version.

. +2.5 ---+-----------,
. | |
. 150 |
. | |/V pnp
. +---------|
. ,-- | |\
. | __| |
. --+--|- \ |
. | >--+--75--+---
. ----|+__/ | |
. | 22 |
. | | |
. | gnd |/
. +---------|
. | |\V npn
. 150 |
. | |
. -2.5 ----+-----------'

Zetex makes suitable high-gain transistors. The amplifier
operates class B at high currents, but at low frequencies
it can have low distortion, if the opamp is fast enough.
However, the opamp must be a low-voltage type, and will be
operating from +/-1.7 volts, which restricts your choices.

The venerable LM10 might be interesting in this application - the
positive end of the input common mode range is 1V below the positive
rail. but the input common mode range does extend down to the negative
rail. +/-1.7V rails would be fine - Widlar claimed that it works down
to +/-0.65V.

http://cache.national.com/ds/LM/LM10.pdf

 

[Tony Williams]

. +2.5 ---+-----------,
. | |
. 150 |
. | |/V pnp
. +---------|
. ,-- | |\
. | __| |
. --+--|- \ |
. | >--+--75--+---
. ----|+__/ | |
. | 22 |
. | | |
. | gnd |/
. +---------|
. | |\V npn
. 150 |
. | |
. -2.5 ----+-----------'

Hello Win. What is the purpose of that 75 ohm resistor
back to the output of the opamp. I can only see it as
having a negative effect. ie, It increases the output
voltage swing required from the opamp output pin.
Is that what you are doing, deliberately reducing the
overall voltage gain?