# op amp cicuits

Discussion in 'General Electronics Chat' started by garyp, Mar 17, 2006.

1. ### garyp Thread Starter Member

Feb 17, 2006
14
0
can anyone tell me how these three circuits work. i haven't a clue when it comes to opamps.

thanks

2. ### garyp Thread Starter Member

Feb 17, 2006
14
0
can anyone tell me how these three circuits work. i haven't a clue when it comes to opamps.

thanks

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
attachmenesia posting a request to review an attachment without actually attaching the attachment

4. ### hgmjr Moderator

Jan 28, 2005
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Take a look at AAC's opamp tutorial. It can provide a solid basic understanding of opamps.

hgmjr

5. ### garyp Thread Starter Member

Feb 17, 2006
14
0
if you look it says my pc crashed and if you look i have posted another one with attachments.

6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
It happens to me all the time, and I was trying to be cute. I meant no offense.

7. ### Dave Retired Moderator

Nov 17, 2003
6,960
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Sorry this is my fault. The thread is now merged correctly (I hope!)

Dave

8. ### garyp Thread Starter Member

Feb 17, 2006
14
0
sorry mate.

I've had a look at the tutorial but i couldn't find out how an inverting amp, non-inverting amp or a comparator worked. maybe i'm blind but it was late last night that i had a look.

9. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
I fear that if the tutorial was insufficient for your needs, then after rearranging the deck chairs, the ship will still sink. In any case let's try the second of the three diagrams. This is the inverting amplifier

The circuit node labeled "2", at the junction of R1 and R2 is called a virtual ground. What does this mean? It means that the point is not directly connected to ground, but it is at a potential so close to ground as to be indistinguishable from ground. Why does this happen? It happens because the (+) input of the amplifier is connected to actual ground, and the amplifier will adjust its output voltage in such a way as to make the voltage difference between the two inputs zero. In order for the voltage difference to be zero, the (-) input must be very close to ground.

This is like one on one basketball with neither side able to score. For every move made by the input VS the amplifier makes a countering move VO. Now for the calculations. If the voltage at the minus input is zero then the current in R1 will be by Ohms Law
Code ( (Unknown Language)):
1.
2. I(R1) = (VS - V2)/R1 = (VS - 0)/R1
3.
This current must exactly balance the current in R2, and this current by Ohms Law is
Code ( (Unknown Language)):
1.
2. I(R2) = (VO - V2)/R2 = (VO - 0)/R2
3.
Amplifier inputs are high impeadance, so no current flows into the amplifier inputs for an ideal amplifier. In a real amplifier, very small currents on the order of picoamperes flow into the inputs, but these are negligible in our analysis.

So if we sum the three currents at the minus input and neglect the current flowing into the amplifier, we get
Code ( (Unknown Language)):
1.
2. VS/R1 + VO/R2 = 0   OR
3.
4. VO = -(R2/R1)*VS
5.
It is called an inverting amplifier because the output has the opposite sign as the input. The ratio of the resistors is the gain and it can be less than one, equal to one, or greater than one.

The deck chairs have been rearranged; are we sailing or sinking?