One of the basic
building blocks of analog and digital circuits is the operational amplifier (or
op amp), and these devices could almost be included in the same category along with
transistors, resistors, and capacitors.
They can be designed into a circuit as
a differentiator, integrator, summing amplifier, differential amplifier,
comparator, precision diode, and many other functions. They are also commonly
used as a voltage follower or level shifter, and can convert a current to a
voltage, voltage to a current, etc. In short: they're very handy devices!
Like a transistor,
an op amp is basically a three-terminal device with two high impedance
differential inputs and a low impedance output. One of the two inputs is
inverting and the other is non-inverting, and the difference in voltage levels
on these two pins is amplified with a very high gain value which determines the
voltage (or current) level on the output. In an ideal op amp, this gain would
be infinite, but in real op amps, it is in the order of 20,000 to 10,000,000.
If an op amp had no
feedback components connected to it, even the slightest difference in the
voltage levels on the inputs (a fraction of a millivolt) could swing the output
to either the full positive voltage or full negative voltage depending on which
input is higher. This would be useful in comparator circuits, like those found
in A/D converters.
In most op amp
circuit designs, a feedback component is placed between the output pin and the
inverting input pin, which creates a condition called "negative
feedback". The values of the feedback component (or components) are what determine
the gain characteristics, and the effect of the high open loop gain would
become negligible and even cancelled out in most calculations. This is why it’s
important to have the open loop gain as high as possible compared to the much
lower feedback gain factor.
Besides making the
external circuit behavior more predictable, using negative feedback flattens
the frequency response over a larger frequency range, and allows it to be used
in higher frequency applications. Negative feedback also tends to cancel out
distortion and nonlinearity problems and make the circuit more temperature independent.
on an op amp has the feedback component connected between the output pin and
the non-inverting input pin. The main purpose here is to create an oscillating
circuit where the op amp output is set by the value of the feedback components
and becomes independent of any input voltage applied. The finite open loop gain
of op amps makes it possible for oscillations to occur.
specifications found in modern op amps comes so close to the ideal
characteristics that designers can sometimes ignore the actual specs design the
circuit based on these ideal parameters.
For an ideal op amp with perfect characteristics,
it would have infinite input impedance, zero output impedance, infinite voltage
gain, infinite bandwidth, zero input offset voltage, and infinite CMRR. When
analyzing circuits with op amps, usually only the passive components around the
circuit are considered to determine the circuit behavior. Designers would not
have to worry about details like those in transistor circuit design, such as
biasing and operating range.
In more critical
applications, the designer would have to consider the actual specifications of
the op amp.
Real op amps can have input resistances which can be in the order
of a few megohms with input leakage currents from a few pico-amps to a few
milli-amps. The input offset voltage is typically around a milli-volt, and the
output resistance can vary from 20-1000 Ω.
The bandwidth is usually limited by something
called the gain-bandwidth product (GB), which is equal to the frequency where
the amplifiers gain becomes unity, usually from 1-20 MHz. The designer would
need to consider phase shift limitations in the voltage gain, also.
In an ideal op amp, if
both input voltages changed at precisely the same amount, it should have no
effect on the output. But in real applications, there is a slight change on the
output, so a designer needs to consider a parameter called the common mode
rejection ratio or CMRR. The common mode voltage gain in an ideal op amp would
be zero, but typically it would be about .0001. This is low enough that it
could be ignored in most applications.