Op - Amps

Other Hardware General Electronics
1

Hi
I just have a quick question regarding op amps .Increasing the voltage of an op amp is easier. It is said that the inputs of an op amp have high impedence. It is also said that op amps can be used instead of transistors to increase the current value. Could someone let me know how.
Thankyou
Regards

2

You can drive an op-amp with a low-current capable device, it can then control a transistor to control higher currents.
Some op-amps are capable of driving high current themselves, such as LM386, it is set up as a power amplifier.

3

I am sill not sure on how to increase the amps using an Opamp.

Is there some kind of an example on how it is done??

Thankyou
Regards

4

Here is an example of a "High current" op amp
http://www.ti.com/lit/ds/symlink/opa561.pdf

If you want more power, than an amplifier chip is better suited

For most digital-related applications tho, PWM into a transistor, or transistor array such as ULN2803, is more than sufficient.
The arduino doesn't really put out an analog level that can be amplified in a linear manner by an op amp. Need to convert PWM into analog with bandpass filter, or use an external DAC.
For driving an LED string on/off, just use a transistor.

5

Possibly the OP is referring to an opamp ckt where it's hooked up in a
unity-gain follower amp configuration [Vin to + pin, output tied back
to - pin], but then you insert an NPN transistor in the output connection
[ie, tie output to NPN base, tie NPN emitter back to - pin, connect +Vdd
to NPN collector, take output at the emitter].

6

FelixFelicis,

Typical op-amps are limited to a few milliamps. You can boost microamps to milliamps, but you can't get lots of current (amps) or lots of power (watts) out of them.

Op-amps can be used for lots of applications and one very simple circuit is a [u]buffer amplifier[/u]. A buffer has a voltage gain of 1.0 (you get out exactly what you put in), but it can boost the current capability of the signal. The actual current you get out depends on the signal voltage and the load resistance/impedance (within limits of the chip).

You can also consider a buffer to be a circuit that turns a high impedance source (megohms) into a low-impedance source (kohms).

Technically a buffer is not a "current amplifier", since the output current depends on the voltage and the load... It's does not simply a multiply the input current.

7

I posted links to two op amps that TI considers High Current op amps. They have a few others in that same category.

For most uses associated with Arduino tho, an transistor setup as a simple on/off switch is all that's needed.
Perhaps a high current op-amp would be useful in an audio amplification situation, similar to an LM386.