# Current amplifier

Discussion in 'General Electronics' started by Tyron, Apr 15, 2004.

1. ### TyronGuest

heya ppl!
ne one out there know how to amplyfy the current comming outta the
opamp?
the opamp is hooked up to a 8 bit DAC (DAC0832 ) if i could post a
picture of my circuit i would.. ne ways the current current that i am
gettin out of the opamp is about 0.32mA and i need the current to be
between 4mA and 16mA ive seen a coupla circuits online and and if
tried a transitor as a buffer and op amp curent amplifier cicuit.. but
it did not seem to help much..
any help wpuld be appreciated!
if necessary email me @
Tyron

2. ### Karl UppianoGuest

According to the spec sheet, a garden variety 741 will source 25mA into a
short circuit without any external buffering. With an external
emitter-follower buffer circuit, you could get several amps if you have the
right opamp gain, supply voltages and load impedance. You didn't say what
your supply voltage is, nor the gain of the opamp, the frequencies involved,
or the impedance of whatever you're trying to drive with the opamp, so it's
kind of difficult to say what you need to do.

Ohm's law says E = I * R, so if you solve for I, then I = E/R. If the load R
is not negotiable, then you must adjust the opamp gain to provide enough
voltage E to bring I up to the desired level. That also means the power
supply voltage needs to be larger than the maximum voltage you expect the
opamp to deliver. The power supply also needs to be able to provide at least
16mA.

3. ### Robert BaerGuest

....or use the LH0002 which is made specifically for that purpose.
It is a push-pull complimentary darlington emitter follower; the
op-amp output pin drives it, and its output goes to the load and
feedback.
Since it is inside the feedback loop, the small crossover distortion
it makes is reduced by the loop gain, and is not seen by the load.

4. ### vinay sharmaGuest

u can use darlington pair as a buffer, as it has current gain
corresponding to b^2 which is huge.

5. ### Robert BaerGuest

Then one has to have a standby current larger than the load current.
With the LH0002, *NO* standby current is needed.

6. ### Karl UppianoGuest

I'm not sure I understand your remarks about standby current. I have
built-out buffers on opamps using discrete parts for years, using various
circuits, including darlingtons as well as topologies like the LH0002, and
standby current has never been an issue.

Incidentally, the OP was only getting 0.32mA into the load. Almost any opamp
can top that, assuming the opamp's output voltage is high enough and/or the
load impedance is low enough, which leads me to believe external buffering
will not solve the problem (although it might be necessary to solve other
performance problems). I'm assuming the load impedance is a non-negotiable
characteristic of the load, in which case the only solution would be to
increase the opamp output voltage. That probably means increasing the opamp
gain, assuming the necessary output voltage is within the absolute maximum
Vcc for the opamp, and/or the voltage available from the power supply.

7. ### Robert BaerGuest

It is a foregone conclusion that an emitter follower (ie:
single-ended) needs standby current to operate. Beta, current gain,
darlington, whatever makes no difference to that basic requirement.
And the greater the load current, the higher the standby current must
be.
Take a theoretical NPN emitter follower with a beta of 100,000 or
more.
Make the load 10 ohms and the requirement that the output signal be
+/- 10V peak to peak.
Use supplies of +/- 15V. That emitter rsistor gotza be small to allow
1 amp to flow from the -15V supply to the load (5 ohms).
So with zero volts output (standby), the NPN standby current ain't
exactly peanuts (3 amps).

Now a class B push-pull complimentary darlington (like the LH0002) is
a totally different kettle of fish.

8. ### Karl UppianoGuest

OK, I was assuming class B or class A/B push-pull the whole time. If the OP
required only pull-up or only pull-down (DC only), they could go with the
top or bottom half of a push pull arrangement, with minimal standby current,
i.e., the load *is* the emitter resistor.

9. ### Robert BaerGuest

Almost all loads on the output of an op amp aer driven symetrically.
so that is not a viable option.

10. ### Karl UppianoGuest

OK, I was assuming class B or class A/B push-pull the whole time. If the
OP
I would rephrase that: *If* the load needs to be driven symmetrically,
*then* it is not a viable option, which is more or less what I said: "*If*
the OP required only pull-up or pull-down (DC only)...". Since we don't know
the OP's application, it's impossible to say what's viable or not.