Op Amps

Inverting Op Amp

Non-Inverting Op Amp

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.
 

Example:
If you need an amplifier with a gain of, say 10, and you have a gain block (op amp) possessing gain in the neighborhood of >200 K (open loop), you use a feedback network that allows signal cancellation such that all but enough to allow the output to be ten times greater than the input. This results in a very stable and low distortion amplifier with a gain of 10. 

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 Inverting Amp 

Differential Mode = Acceptance

Single Ended Noninverting Amp 

Common Mode = Rejection (CMR)

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. 

Multiple stage "stair-stepping" DC (direct coupled) Amplifier

A resistor is a voltage to current converter.

A resistor is a current to voltage converter. 

Operational Amplifiers
The "Op Amp" is one of the most valuable and versatile integrated circuits to ever come down-the-pike. If you want to spend a Month -of-Sundays abusing yourself, and you've run out of sharp sticks: build an Op Amp, from scratch--one that works as well as the monolithic version; or for that matter, one that works period! There is almost no circuit design that can't benefit from the use of the Op Amp. 

From the 741--the first internally compensated Op Amp, and son of the 709--to the super-fast near GHz Op Amps; these little "boogers" are easy to use: If you bypass, decouple, and use common sense... Ha! There I said it. Ha! 

"How does this Damn thing work?" "Very well thank you." 
Lets keep this simple: The Op Amp is basically three amplifiers or stages. The input differential stage; the gain stage, and the output stage. 
 

Op-Amp-Stages

The differential stage is the heart of the Op Amp, and the most confusing. If you think of the Op Amp as a differential amplifier --because that's what it is--and think of the other stages as parts of the same, then the confusion may drop about 20dB, or go up thirty. 

Showing Differential Mode Input

Showing Common Mode Input

The Op Amp is a current device in, and a voltage device out. The Op Amp's output is not dependent on the amplitude of the input current per se, but on the current difference between the input pins. If you tied both pins together and applied a very large signal to this connection: the output would be unchanged, or at most, a weak crappy replica of the input. This is known as common mode rejection, (CMR). 

Input Stage

Differential Input Stage 
Demonstrating Offset

Of course that's why GOD made PNP transistors (we finally know the reason), but this can be taxing and not always satisfactory.

The reason GOD made PNP transistors 

Gain Stage

Enter the Op Amp: it has the facility of not having--by nature--any offset between input and output. You could DC couple zillions of OP Amps in cascade. When configured as an inverting amplifier with some gain (Gv), its sole aim in life is to not allow any current to flow in the inverting pin. And, because it has in its arsenal, gains on the order of several hundreds of thousands (100k to 600k), it has no problem in doing just that: No Current In! Remember: it is a current input, and a voltage output device. 

Output Buffer Stage

Another Output Buffer Stage