A Gain "Block" (ideally possessing infinite gain)
A Network that sets the amount of feedback (e.g., resistors)
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
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
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.
Ended Inverting Amp
Mode = Acceptance
Ended Noninverting Amp
Mode = Rejection (CMR)
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
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
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.
"stair-stepping" DC (direct coupled) Amplifier
A resistor is
a voltage to current converter. A resistor is also a current
to voltage converter. How does it know which it is at the time?
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.
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,
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
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
Output Buffer Stage
Gain Bandwidth Product
Misc Op Amp Circuits
Single Supply Operation
Can you Draw the Waveforms?