Electrical
Instrumentation - Operational Amplifiers - Class Notes
1. Operational
Amplifiers
-
Why use operational amplifiers?
They can be used for the following (not an exhaustive list!)
-
To amplify
signals (make signals larger, but of the same form, carrying the same information).
-
To isolate
and buffer signals (Some signal sources -
many sensors, for example - cannot provide enough current to "drive" circuits
(loading!). In that case, operational amplifiers can be used to provide
close to an exact copy of the signal, but the copy can come from an operational
amplifier that can provide more current.)
-
To integrate,
add, subtract or filter signals (There are
numerous mathematical operations that can be performed by operational amplifier
circuits.)
-
What makes an amplifier
an operational amplifier?
Operational amplifiers generally have the following characteristics
-
Very high gain
-
very high input resistance
-
very low output resistance
-
What does the circuit
diagram representation look like? And what does an operational amplifier
itself look like?
-
What else is necessary?
-
If you want to use an
operational amplifier, you need an external power supply. Actually,
you normally need a power supply that can provide both a positive voltage
and a negative voltage.
-
Which leads to the Golden
Rules of Op-Amps.
-
Rule #1 - Operational
amplifiers have a gain so high that you can consider it to be infinite
- the Infinite Gain Assumption
-
A consequence of this
rule is that the inverting and noninverting inputs both have the same voltage.
If the noninverting input is grounded, the inverting input is virtually
ground (for all practical purposes).
-
Practically, the gain
of an operational amplifier is something like 200,000
-
That means Vout
= 200,000(V+ - V-)
-
Rule #2 - Operational
amplifiers have such a high input resistance (impedance) that they draw
no current at the input terminals (inverting and non-inverting).
(usually the resistance is many megohms) - the Infinte
Input Resistance Assumption
-
Rule #3 - Operational
amplifiers have a very low output resistance, so in most cases there are
no loading effects. (usually around 50 ohms) - the Zero
Output Resistance Assumption
2. Some
simple operational amplifier circuits - Examples of analysis of op-amp
circuits
-
What we will assume.
-
We assume that you have
an operational amplifier (connected to a circuit board) and that you have
connected the power supplies to the proper terminals of the op-amp.
-
We will examine this circuit
in class
-
We will apply the Golden
Rules of Op-Amps.
-
Rule #1 - Operational
amplifiers have a gain so high that you can consider it to be infinite.
-
A consequence of this
rule is that the inverting and noninverting inputs both have the same voltage.
If the noninverting input is grounded, the inverting input is virtually
ground (for all practical purposes). That's the way it is in the
circuit above. The inverting input (at the minus sign) is - for all
practical purposes - at zero volts. If the output voltage is 10 volts,
for example, with a gain of 200,000, that means that V-
= - Vout/200,000 or around 50 microvolts.
-
Rule #2 - Operational
amplifiers have such a high input resistance (impedance) that they draw
no current at the input terminals (inverting and non-inverting).
So, that 50 microvolts across the input terminals is going to produce pico
amps flowing into the op-amp. Forget about it.
-
Rule #3 - Operational
amplifiers have a very low output resistance, so in most cases there are
no loading effects.
-
For details of the analysis,
click
here.
3. Useful
Op-Amp Circuits
-
The Summing Amplifier
-
This circuit can be extended
so that you can add more than two signals.
-
This circuit has a sign
change (the output is the negative of a weighted sum) that you may have
to correct for.
