# op amp cicuits

#### garyp

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

thanks

#### garyp

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

thanks

#### Papabravo

Joined Feb 24, 2006
14,647
Originally posted by garyp@Mar 17 2006, 09:21 AM
can anyone tell me how these three circuits work. i haven't a clue when it comes to opamps.

thanks
[post=15090]Quoted post[/post]​
attachmenesia posting a request to review an attachment without actually attaching the attachment

#### hgmjr

Joined Jan 28, 2005
9,029
Take a look at AAC's opamp tutorial. It can provide a solid basic understanding of opamps.

hgmjr

#### garyp

Joined Feb 17, 2006
14
Originally posted by Papabravo@Mar 17 2006, 08:35 AM
attachmenesia posting a request to review an attachment without actually attaching the attachment
[post=15094]Quoted post[/post]​
if you look it says my pc crashed and if you look i have posted another one with attachments.

#### Papabravo

Joined Feb 24, 2006
14,647
Originally posted by garyp@Mar 17 2006, 12:09 PM
if you look it says my pc crashed and if you look i have posted another one with attachments.
[post=15098]Quoted post[/post]​
It happens to me all the time, and I was trying to be cute. I meant no offense.

#### Dave

Joined Nov 17, 2003
6,970
Originally posted by Papabravo@Mar 17 2006, 01:35 PM
attachmenesia posting a request to review an attachment without actually attaching the attachment
[post=15094]Quoted post[/post]​
Sorry this is my fault. The thread is now merged correctly (I hope!)

Dave

#### garyp

Joined Feb 17, 2006
14
Originally posted by Papabravo@Mar 17 2006, 12:02 PM
It happens to me all the time, and I was trying to be cute. I meant no offense.
[post=15101]Quoted post[/post]​
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.

#### Papabravo

Joined Feb 24, 2006
14,647
Originally posted by garyp@Mar 21 2006, 09:57 AM
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.
[post=15253]Quoted post[/post]​
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
Rich (BB code):
I(R1) = (VS - V2)/R1 = (VS - 0)/R1
This current must exactly balance the current in R2, and this current by Ohms Law is
Rich (BB code):
I(R2) = (VO - V2)/R2 = (VO - 0)/R2
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
Rich (BB code):
VS/R1 + VO/R2 = 0   OR

VO = -(R2/R1)*VS
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?