LM324差分放大

首先声明，本文转自

http://intro2electro.blogspot.com/2011/10/operational-amplifier-op-amp-is-dc.html

非常好的介绍LM324的文章，非常通俗

OP-AMP (non-inverting) - LM324

An operational amplifier (op-amp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. 

Amplifier is a device which takes small voltage in input terminal and spits out large voltages in output terminal...

the gain of amplifier is the amount multiplied with input to produce the desired output...

for example if u take a 1V input and get 5v output then the gain of amlifier is =5

an OP-AMP has generally 5 important pins

i> inverting input 
ii> non-inverting input
iii> +Vcc
iv> -Vcc
v> OUTPUT

in input terminal only use signals ...

OP-AMPS can't creat voltage out of nothing!!! so we have to use a bit larger supply volatage than the desired output voltage

so if i have to get a 10V oupput then i have to use atleast 12V supply voltages... dont try to apply high voltage it may damage your device so always reffer the DATA SHEET


if your supply voltage s very less then you will get clipped OUTPUTS



smooth voltage sources are important for a amplifier to give a uniform output signal so use two suopply batteries, hooked up in series  with capacitors given below :

CLASSIC NON-INVERTING AMPLIFIER:


here we use a LM324 OPAMP with max current rating = 20mA

you can set the gain using resistors ...
the circuit diagram is given below

THE GAIN FORMULA :

for simplicity use R1=1k (fixed) and then choose R2 according to you gain......

CLICK HERE for the derivation of the gain formula ( find it in thr 9th page )

APPLICATION:

SPY-MICROPHONE

 a simple small microphone has a output signal of 20mV
and to run a EAR-BUD we need atleast 2V
so gain must be 100

using gain fomula r2= 100k
so gain= 101 nearly equal to 100

OK TIME TO UNDERSTAND THE CIRCUIT BLOCK BY BLOCK :

BLOCK A
use 9V supply to power up the mic and use R1 to limit the current

BLOCK B
is a high pass filter and the capacitor blocks the supplied 9v DC and passes the signal

BLOCK C
r2 and r1 is set to a gain of 100
so in output pin we get a 2V signal 

BLOCK D
10K variable resistor is used to change the supply voltage of LS(ear-bud) to control the volume 



2V output is sufficient to run the earbuds but can't run a LOUD SPEAKER as a loudspeaker needs high power supply which will kill the LM324 ..... so i have googled for a home theater OPAMP and found LM1875 OPAMP, which is great for loud sounds in your home.....you can supply more voltages and use high valued resistors to get LOOUUDDDDD SOUNDDDDDD!!!!!!!!!! the circuit diagram is same for both LM324 N' LM1875

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Single Supply Op Amps

    When working with a unipolar (0-Vcc, as opposed to bipolar +/-Vcc) supply, certain design considerations must be made.  For instance, the 411 is no longer a viable choice for an op-amp.  Recall that the ouptut of the 411 only gets within about 1.5 volts of the power supply (and this can be as much as 3 volts).  If we were to power this chip by 0 and 5 volts this would only allow us to have outputs from about 1.5 to 3.5 volts (and it might not even work if we got a chip that only got within 3 volts of the supply voltage).  

    We also have to worry about the fact that we cannot generate negative outputs.

    There are several techniques for dealing with this.  

Rail-to-rail op amps

When doing single supply design it is common to use what are called rail to rail op amps.  These amplifiers can output voltage very near the power supply voltages (or rails).  You have already seen an example of one such amplifier, the LM324, whose output can go almost to the negative rail (ground), but still can't get close to the positive rail (Vcc).  If you do you design carefully, you could use a 324 in this lab.  It has the advantage of being quite cheap.  

Another option is to use an op amp that goes near both supply rails.  These tend to be much more expensive, but easier to work with.  In the lab you will find two rail to rail op amps from Texas Instruments. the TLCV2772, and the TLC2774.  The op amps on the chips are identical - the 2772 is an 8 pin DIP with 2 amplifiers, the 2774 is a 14 pin DIP with 4 amplifiers. 

Virtual ground

To cope with the problem of not being able to generate negative voltages, a virtual ground is used.  The virtual ground is simply a voltage reference that is typically half way between Vcc and ground.  One way of generating a virtual ground is shown below.  The two resistors form a voltage divider so that Vcc/2 appears at the non-inverting input of the op amp.  Since it is set up as a follower, Vcc/2 also appears at the output, as shown.

An easier way to do this  is to use a virtual ground circuit, such as the TLE2426 virtual ground (the "Rail Splitter") from Texas Instruments.  It generates an output precisely midway between the two supply rails, has noise reducing circuitry and can source and sink about 20 mA.  You can also find these in the lab.

Considerations when using a virtual ground.

The virtual ground can now be used just like a normal ground in your circuit, but you have to remember that the virtual ground has limited output current.  For this reason it is usually advisable to use the op-amp in its inverting configuration (which requires no ground current), rather than the non-inverting configuration (which requires ground current).  However, as long as you are careful not to exceed the manufacturer's specifications on maximum current from the virtual ground, you can use either.

In the diagrams below, virtual ground is depicted by a green symbol with thick lines, ground by a black symbol.

Inverting Amplifier (Virtual ground current = 0)	Non-inverting Amplifier (Virtual ground current != 0)
Virtual Ground Physical Ground