Op-Amps

Op-Amps

Inverting amplifier                                              non-inverting amplifier

special fetures:

An Amplifier is made of: 
1) A Gain "Block" (ideally possessing infinite gain) 
2) Feedback 
3) A Network that sets the amount of feedback (e.g., resistors)

If the above Carved-in-Stone requirements are met, the characteristics of the Amplifier are determined by the feedback network only, not the gain block, nor the transistors used in the gain block's construction. That is, the more "raw" gain that is available to the amplifier, the less effect components have on fidelity. 

Saying it another way: 
Feedback combined with >> Gain, reduces Distortion--improves Fidelity!

Of course, the world is not made that way; there are no Ideal Amplifiers, no infinite gain Gain Blocks; so the name of the game is to do the best you can--up to the point of satisfying the Amplifier design requirements.

Think of feedback as a continuous comparison between the input signal and what the amplifier is putting out. As this comparison is made, ERRORS between the real signal and any lack of faithfulness of the amplifier output tend to be corrected. These corrections are made as a result of the feedback and the LARGE open loop GAIN of the opamp.

NOTE :  

The absolute gain of an amplifier is a function of the feedback network precision, not the open loop gain of the op amp (within limits). This statement is more true, the greater the open-loop gain of the op amp device. 

single ended inv & non-inv amp

 

Diff mode-acceptance & common mode=rejection

the above are the modes of circuit connection for various applications

let us now move to the willing and chilling concept of  virtual ground 

Anyway, because of the aforementioned (I've always wanted to use that word in a sentence), let's say a current of 1 milliamp is caused to flow to the inverting input pin through the 1000 ohm input resistor, R1, the Op Amp tries to maintain equilibrium, i.e., no current flow in that input pin. To do this marvelous feat, it generates an output voltage of the opposite polarity, which maintains that 1 milliamp to flow through the 10 K feedback resistor, R2 to the output. Because the feedback resistor is ten times the value of the input resistor, it will require ten times the voltage to cause that same 1 ma to flow. The view from the input pin: there is a current of 1 milliamp coming down the input resistor, and at the same time, there is a current of 1 milliamp coming from the feedback resistor. there is no current left over for the input pin; therefore satisfying the zero current requirement of the Op Amp. "Eureka!" you have a signal ten times larger, than you started with, and boys and girls, there's not a mirror in sight! 

OK. OK. If you're so smart: what the Hell is Virtual Ground ? Explain that if you can! I just did! Because no matter (within reason) how much current was made to flow in the input resistor, no voltage change was seen at the other end--the input pin. If the resistor had been attached to ground, the effect would have been the same: current flow into the resistor; no voltage at the other end. I know it sounds silly, but hang on for one more point. Let us say you use a CMOS Op Amp having an input impedance of tens of thousands of megohms--with the same resistor values as the example above. You apply a signal generator that has a output impedance of 1000 ohms. We know that if we apply that generator to a 1000 ohm load, the output voltage of the generator will drop by 6dB (50%). Now apply this generator to our CMOS Op Amp and measure the generator output level before and after: the output will be down by--you guessed it--6dB. In fact, nothing has changed, whether its a CMOS or a BJT Op Amp, the principle is the same: if the Op Amp has enough gain, the device itself has no discernible effect on the circuit.  Now! The non-inverting input is another story altogether! Its input impedance is affected by the device type. In the CMOS case the non-inverting input pin--as mentioned earlier--has the impedances approaching tens of thousands of megohms. This high impedance input can be used to great advantage: in sample & hold circuits, peak detectors--you name it.    If you've ever designed a DC (direct coupled) amplifier with more than two stages, you've discovered the stair-step phenomenon: as each collector is connected to the base of the succeeding stage, the higher the emitter/base bias or offset must be. opamp also has applications of integrator,differentiator,filter,wave generator n instrument amplifier which you can get in any reference book so i am skipping those 2 discuss.                     Multiple stage "stair-stepping" DC (direct coupled) Amplifier