RDA 112: PRACTICAL ELECTRONICS
AND EMBEDDED SYSTEMS
Lecture 04:
Introducing Electronic Components :
Operational Amplifiers (Op Amps)
Op Amps
What they are
How they work
What they are used for
The operational amplifier is a very high-gain dc-coupled
differential amplifier with a single-ended output. It also
excels in precise input symmetry and nearly zero input
current.
Op Amps
What they are
How they work
What they are used for
Op-amps are designed as “gain engines” for negative
feedback, with such high gain that the circuit performance
is set almost entirely by the feedback circuitry. Op-amps are
small and inexpensive, and they should be the starting point
for nearly every analog circuit you design.
Op-amp symbol
Op Amps
What they are
How they work
What they are used for
Operational amplifiers are available in literally thousands of
types, where the (+) and (−) inputs do as expected: the
output goes positive when the noninverting input (+) goes
more positive than the inverting input (−), and vice versa.
Op Amps
What they are
How they work
What they are used for
The (+) and (−) symbols don’t mean that you have to keep
one positive with respect to the other, or anything like that;
they just tell you the relative phase of the output (which is
important to keep negative feedback negative).
Op Amps
What they are
How they work
What they are used for
Using the words “noninverting” and “inverting,” rather than
“plus” and “minus” helps avoid confusion. Power-supply
connections are frequently not displayed, and there is no
ground terminal. Operational amplifiers have enormous
voltage gain, and they are never (well, hardly ever) used
without feedback.
Op Amps
What they are
How they work
What they are used for
Inside LF411 op-amp in 8-pin DIP
Inside the 411 is a piece of silicon containing 24 transistors (21 BJTs, 3 FETs),
11 resistors, and 1 capacitor.
Op Amps
What they are
How they work
The Golden Rules
1. The output attempts to do whatever is necessary
to make the voltage difference between
the inputs zero. In other words, with
Negative Feedback, the inputs are at the
same voltage (i.e., V+ = V-).
2. The inputs draw no current.
What they are used for
Op Amps
What they are
Example:
How they work
What they are used for
Op Amps
What they are
Example:
How they work
What they are used for
Op Amps
What they are
Inverting Amplifier:
How they work
What they are used for
1. Point B is at ground, so rule 1 implies that
point A is also.
Op Amps
What they are
Inverting Amplifier:
How they work
What they are used for
1. Point B is at ground, so rule 1 implies that
point A is also.
2. This means that (a) the voltage across R2 is Vout
and (b) the voltage across R1 is Vin.
Op Amps
What they are
Inverting Amplifier:
How they work
What they are used for
1. Point B is at ground, so rule 1 implies that
point A is also.
2. This means that (a) the voltage across R2 is Vout
and (b) the voltage across R1 is Vin.
3. So, using rule 2, we have Vout / R2 = − Vin / R1.
In other words, the voltage gain (GV ≡ Vout / Vin) is
GV = − R2 / R1
Op Amps
What they are
Inverting Amplifier:
How they work
What they are used for
1. Point B is at ground, so rule 1 implies that
point A is also.
2. This means that (a) the voltage across R2 is Vout
and (b) the voltage across R1 is Vin.
3. So, using rule 2, we have Vout / R2 = − Vin / R1.
In other words, the voltage gain (GV ≡ Vout / Vin) is
GV = − R2 / R1
It’s sometimes better not to ground B directly,
but through a resistor.
Op Amps
What they are
How they work
What they are used for
Noninverting Amplifier:
1. VA = VoutR1/(R1+ R2)
2. VA = Vin
3. Gv = 1 + (R2/R1)
Op Amps
What they are
How they work
What they are used for
Exercise 1:
Question: What is the output
voltage at terminal A?
Op Amps
What they are
How they work
What they are used for
Exercise 2:
Question: What is the current, i, and output voltage, vo?
End of Lecture 4