Transcript What is an Operational Amplifier?

Operational Amplifier
What is an Operational Amplifier?
1) Differential amplifier - amplifies difference
between two signals.
2) Can amplify very small voltage signals to a
useful level.
3) Typically an 8 lead IC.
4) Op Amps can require one power supply
(single supply) or a positive and a negative
power supply (dual supply)
Basic Op Amp Function
1) If (inverting input) > (non-inverting input)
output saturates towards -Vss
2) If (inverting input) < (non-inverting input)
output saturates towards +Vss
3) Without feedback maximum saturation occurs
with the slightest difference between inputs.
4) Ideal op-amp has infinite impedance on the
inputs, infinite gain, and zero resistance on
output.
Contnd….
5) In real life gain is limited to voltage present
at +Vss/-Vss
6) For practical purposes there is no current
flow from the inputs to the outputs. The
input voltage serves only as a reference to
control the output.
Feedback
1) Degree of amplification can be controlled
using feedback.
2) Feedback can be applied to the inverting or
non-inverting input.
3) Feedback applied to the inverting input is
more common and is called negative
feedback.
4) Gain (A) = 1 + (Rf/Rin)
Feed back
Voltage follower
1) Rf = 0, therefore no amplification. Gain = 1 +
(Rf/Rin)
2) Output voltage is equal to input voltage.
3) Use to buffer/isolate circuit. Voltage is the
same, but current is supplied by Vss rather
than Vin
Comparator
1) Amplifies small difference between two
signals.
2) Output voltage will "switch" whenever the
input voltage (at the inverting input)
reaches the reference voltage Vref (at the
non-inverting input)
3) Very useful for comparing signals and
working with sensors
4) Note that R2 and R1 form a voltage divider. Use a
potentiometer in place of R2 for an adjustable
reference voltage.
5) Comparator cicuits can be built with opamps, but
there are also comparator ICs with large slew rates
and short propagation delays - good for high speed
switching.
Non-Inverting Amplifier
1) Inverting input is connected to ground +
feedback.
2) Noninverting input serves as signal.
3) Output is in phase with input.
Negative power supply
1) Some opamps need a dual power supply.
2) If we're only interested in getting positive
output from opamp, -Vss can be connected to
ground.
3) Dual power supply options include building a
dual power supply, using two batteries, or
purchasing a dual polarity power supply.
Slew Rate
1) Slew rate defines how fast the opamp can
react to changes at the inputs. Different
opamps have different slew rates.
2) If voltage chages faster than opamp's slew
rate, signal will be attentuated.
3) The higher the gain, the more deliterious
effect from slow slew rate.
4) This is only an issue with high frequencey
signals.
Construction tips
1) Don't reverse the power leads. This will kill the
opamp. A diode can be placed in series with -Vss
to avoid this.
2) +Vss and -Vss must be greater than the
noninverting and inverting inputs.
3) Couple signals and output to ground with caps
(1.0uf - 0.1uf) if needed to filter noise and
prevent oscillation.
4) Keep leads from power supply to +/- Vss short
and direct.
Thanking You!