Control.com - Nerds in Control

Generally transistor acts as an amplifier in active state so we use transistor for amplification. But op-amp is also used for amplification purpose. Then why don't we use op-amp instead of transistor? please tell what characteristics made them different? give me complete information. Whats the difference between an op-amp and transistor?

Taking us to the basics of college!!!

Truly, a transistor doesn't have a voltage gain. It has transconductance, i.e. it varies its current (depending on configuration) according to the applied voltage. Voltage gain may be realized at some place depending upon how the circuit is realized around the said transistor.

But it is likely that:
1. Gain is limited.
2. Temperature sensitivity to gain is high
3. May not achieve high input and low output impedence.

Op-Amp in turn is a ready-made solution in a single package consisting of multiple transistors and other components in different configurations to meet your specific requirements.

An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with differential inputs and a single output (usually).

You may follow nice animated presentation on Op-Amp as reference,
http://www.eas.asu.edu/~holbert/ece201/opamp.html

By the way, request you to refer text books on this subject for better clarity on understanding. Forum will always be ready to answer if you are stuck with certain specific issues rather than spending time on generic questions.

Regards.

I think of a bipolar transistor as a high gain current controlled current device, and an OP-Amp as an infinite gain voltage controlled voltage device.

OPAMPS are having many advantages comparatively with transistors in input stages, so why should we select a transistor?

1) Doesn't load the source (high input impedence)
2) High gain, easy to set the gain level by just varying feed back.
3) Low noise and common mode rejection.
4) Easy to design and implement.
5) Wide range of applications.

It all depends on where it is to be implemented.

mbarazeen at yahoo. com

One place would be at high frequencies. And actually, you are always selecting transistors, it's just that opamps include many transistors and take advantage of close thermal and parametric matching to produce differential amplifiers that are closer to ideal than can be accomplished easily with discrete transistors. But, indeed there are far fewer discrete designs in the DC to high kHz spectrum than there are IC designs. It is simply much easier to get stable, predictable gain with a feedback amplifier.

Regards

cww