Operational Amplifiers

1
Operational Amplifiers

• Op-Amps
• By Roes V. Gamo

2

• An operational amplifier (op-amp) is a
  high-gain direct-coupled multistage amplifier
  with the capability of amplifying signals ranging
  from DC (0 Hz) to several megahertz.
• Op-amps have five basic terminals one for the
  positive supply, one for the negative supply, two
  for the input signals, and one for the output
  signal.
• The positive and negative power supplies
  connected to the op-amp allow the output voltage
  to swing positive or negative (with respect to
  ground).

3
Any voltage applied to the noninverting input
appears at the output with the same polarity.
It should be noted that the () and (-) symbols
at the op-amp input terminals have nothing to do
with polarities of voltages at the inputs. The
voltage at either input can be positive or
negative.
4

• An ideal op-amp has infinite input impedance,
  zero output impedance, and infinite voltage gain.
  
• When an amplifier has a high input impedance, it
  will draw a low value of current from the signal
  source.
• The low output impedance allows the amplifier to
  provide a constant output voltage, independent of
  the current drawn from the source.

5

• An ideal op-amp will produce an output voltage of
  0 V when the voltage applied between the two
  input terminals is zero.
• In a practical op-amp the output is never exactly
  0 V, although it is close enough for most
  applications.
• If the output voltage is required to be exactly
  zero when both inputs are zero, it is possible to
  apply a small DC bias voltage to correct, or
  offset, the discrepancy.
• This compensating voltage is called the input
  offset voltage.

6

• Op-amps often use feedback to establish a
  specific voltage gain.
• Feedback is defined as taking a portion of the
  output signal and feeding it back to the input.
• When an op-amp has its output connected to one of
  its inputs it is referred to as a closed-loop
  configuration.
• An op-amp with no feedback is called an
  open-loop circuit.
• The open-loop voltage gain of an op-amp is
  typically 200,000.
• The two basic types of feedback in electronic
  circuits are negative feedback and positive
  feedback.
• When the output voltage is in phase with the
  input voltage, the feedback signal will add to
  the input. This is called positive feedback.
• When the output signal is out of phase with the
  input, the feedback signal will subtract from the
  input. This is known as negative feedback.
• Because op-amps have very high open-loop gain,
  they are generally designed to operate in
  circuits that develop negative feedback.

7
INVERTING AMPLIFIER

• When an op-amp is used as an inverting amplifier,
  the input signal is fed into the inverting input
  and the noninverting input is grounded.
• Resistor Rf provides a closed loop between the
  output and the input, and the output voltage is
  180 out of phase with the input.
• Because the voltage drop across Rf is the
  opposite polarity to the applied voltage, it is
  said to have negative feedback.
• The voltage between the () and (-) inputs is
  essentially equal to zero volts.
• Because the () terminal is grounded, the (-)
  input terminal is at 0 V and is referred to as a
  virtual ground.
• It is called virtual ground because the terminal
  is at ground potential, but is not physically
  connected to the system or chassis ground.

8
NONINVERTING AMPLIFIERS

• When the op-amp is connected so that there is no
  phase displacement between the input and output
  signals, it is called a noninverting amplifier.
  
• In this circuit, the input signal is applied at
  the noninverting input (). Resistors Rf and R1
  basically form a voltage divider at the inverting
  input (-).
• The circuit uses negative feedback.

9
Voltage Follower
Av 1 Zi 8 Zo 0 The voltage follower
is a impedance matching circuit. The voltage
follower is called a buffer It is used to connect
a high-impedance voltage source to a very low load