How to use Opamp ?

Discussion in 'General Electronics Chat' started by chicken, Nov 22, 2009.

1. chicken Thread Starter New Member

Nov 22, 2009
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Hi ! in theory Opam.. It's simple to use an opam ?
ex : 1mV_Peak (source) f= 50Hz. if I want the output 1 V_peak. so the gain I need is 1000k. ( If I choose the supply source +-5V, and the value of R1 = 1K, Rf = 1M . in the inverting amplifier the gain = - Rf/R1 = 1000/1 = 1000.
Vout = 1mV_peak*1000 = 1V_peak.

But It's ideal.The Datasheet of any Opam show me a lot information That makes me confuse How I use there information.

EX : LM 324 ..
Low input bias current: 20nA
Low input offset 5mV max
Low input offset current 2nA
Large volt gain 100 db
IF I desig a cuircuit to amplify a signal from a sensor ( ac volt at terminal).
What should I have to pay intention ? How to choose the Resistor ?
Some parameter of LM324 obout What do I do ?

Apr 20, 2004
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3. chicken Thread Starter New Member

Nov 22, 2009
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Yeah ! something It's theory ! I need the practical ! my question is " How to use Opam" ? but not realy I mean !

ex: I have a small signal and I need to amplify it. But with the gain (10 times,
we can cho 1k and 10k or 10k and 1M or more ? so which appropriate value of these resistors that I chose ? And what did It depend on ? etc ..

4. beenthere Retired Moderator

Apr 20, 2004
15,815
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One practical consideration is the amount of current the feedback network will use, vice the current needed for the load. If the op amp has a limited output current, using larger resistances in the feedback network may make sense. At one extreme, 10 ohms and 1 ohm are much too demanding of current to work. 100 megs and 10 megs may easily cause noise problems by the increased input resistance.

The tolerances of the resistors have some effect, too. Some combinations allow closer approximation of the ratio that provides the gain you need.

5. Audioguru New Member

Dec 20, 2007
9,411
896
The LM324 opamp is old and is not good because it was designed for low power which made poor performance in those days.
Its DC voltage gain is a minimum of 15,000 and is typically 320,000 (110dB).
Its max input offset voltage is a max of 7mV at room temperature and it will amplify it up to 320,000 times unless you design your circuit to amplify only AC, not DC. Your simple circuit amplifies both 1000 times so its output could be saturated high or low.

6. chicken Thread Starter New Member

Nov 22, 2009
3
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Thanks ! @ beenthere and Audioguru.
- @ beenthere . Can you explain detail for me ? :
. Sometime we want to amplify a small signal to bigger to properly device etc... (Microcontroller ..) or to trigger the transistor .. So the current requites too small (milivolt) and It's not problem here.. Can use let's me see some example where the resistors were chosen suitable. Is there any reasons without overload current of Opam when we choosing the value of resistor ?
- @ Audioguru. In your point LM324 is a Poor device. So if I want to amplify the signal from the CT (curret transformer) to meansure AC current What type of Opam should I use better ?
- In Datasheet of LM324..
Code ( (Unknown Language)):
1. Low input offset 5mV max
is this a worse case at 100 gain the out will be 0.5 volt + V_input*gain ( depending vollt supply .). some Opams have null offset pin but Other not.