Transcript Voltage Follower Schematic

VOLTAGE FOLLOWER

The OP-AMP can be used as a VOLTAGE FOLLOWER.
The output voltage follows the input.

In this arrangement the OPerational AMPlifier is called a
BUFFER and has unity gain.
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Definition

The voltage follower, also called a buffer, provides a high
input impedance, a low output impedance, and unity gain.
As the input voltage changes, the output and inverting
input will change by an equal amount.

Vout = Vin
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Voltage Follower Schematic
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Cont..,

This schematic shows an op-amp (the AD8515 from Analog
Devices) wired up as a Voltage Follower.

The Sensor is essentially a potentiometer with the outer leads
connected to power and ground and the inner tap running into the
positive input of the op-amp.

You could easily substitute a voltage divider or many other sensors
here. The op-amp’s output from pin 1 is connected directly back into
the negative input to set it to voltage follower mode, as well as
heading out to the Teabox through the center pin on J1 (either a cable
or a three pin header).
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Cont..,

Notice that pins 5 and 2 of the op-amp (somewhat
disembodied at the far left of the circuit) are connected to
power and ground and are bypassed with the 0.1µF Cap.
This is very important as it smooths out fluctuations in the
power supply.

One final note is that the 5V and Ground indicators are not
necessarily from a second power source.
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Cont..,

Most likely this entire circuit will get its power from Pin 1
on J1 (the cable or header), and ground from Pin 3, which
means you can ignore the 5V/Gnd connections.

If you do need to power the device externally (for
instance if you are using a few sensors that need a lot of
power (like the Sharp IR sensors), you must be careful to
have a very clean power supply.
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Cont..,

The way sensor analog to digital convertors work is that they
compare the sensor input to a reference voltage.

If the power supply is fluctuating, it can’t compare accurately
and you will end up with noisy data. We have a Power Injector
and Powered Expander that can be used to add 500mA of
power to an interface, although you can make one on your own
using a wall wart and a decent voltage regulator: something
like the LT1763 from Linear Technology.
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Voltage Follower Example

Here a zener diode is providing a reference voltage such as 9
volts. The current available from this reference voltage circuit
is minimal.
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The voltage follower outputs the same reference voltage but
with enough current to supply the motor.

If the motor load increases, the voltage follower maintains the
same output voltage but allows more current to flow in the
motor. Stabilised power supplies use this principle.
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Cont..,
The zener diode needs to have at least 5mA breakdown flowing through it. This
ensures that the zener voltage will be correctly established.
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