Oscillators

1
Oscillators

• With the addition of a capacitor and resistor,
  the Schmitt trigger can be turned into a simple
  oscillator.
• Technically, its known as an astable
  multivibrator.
• Before looking at the circuit, it is worth
  reviewing the transient response of a first order
  RC network.

2
Transient Response
3
is a separable first order differential equation.
These are the easiest kind to solve!
4
NB. In either case, VC VS as t . VC() is
known as the aiming potential.
5
Oscillator Circuit

• The op-amp, R1 and R2 form an inverting Schmitt
  trigger
• As R1 R2, threshold levels are VSAT/2
• The output is fed-back to the inverting input via
  an RC network
• The aiming potential of the capacitor voltage
  will be either VSAT or VSAT

6
Waveforms
V
VSAT
0
t
-VSAT
7
Analysis
To calculate the time-period, consider just one
half-cycle of the oscillation.
V
VSAT
0
t
-VSAT
8
Analysis (cont)
Half-cycle ends when VC reaches VSAT/2
V
VSAT
0
t
-VSAT
9
Oscillator Summary

• Using only a couple of extra components, the
  Schmitt trigger can be converted into a simple
  oscillator
• R C set the period of oscillation independently
  of VSAT
• Many square wave oscillators work using the same
  basic principles

10
Active Feedback Components

• There are two broad categories of non-linear
  op-amp circuits
• Those that exploit saturation
• Those that use active components in the feedback
  network
• We will concentrate on one example of the latter
  genre the precision rectifier

11
Simple Half-Wave Rectifier

V
V

V
V
IN
OUT
Ideal Output
1

1

0.5

0.5



0

0

t

t

-
0.5

-
0.5

-
1

-
1

12
Precision rectifier

• Note that
• The current through the diode (and therefore also
  through RL) can only flow in one direction
• The load voltage, VOUT, cannot, therefore, go
  below 0 V
• When the diode conducts, VOUT V1 - ½

13
Analysis Negative Input
14
Analysis Positive Input
15
Waveforms

V


V

IN

V

1

V

OUT
0

0

t

t


-
V
SAT
16
Other Non-Linear Applications

• Circuits that effectively change the saturation
  level(s) of the op-amp.
• Precision rectifier
• Limiters
• Circuits that use the exponential V-I curve of PN
  junctions.
• Log and antilog amplifiers
• Multipliers, squarers and dividers

17
Summary

• Non-Linear applications of op-amps
• Essentials
• Comparator
• Schmitt Trigger
• Precision Rectifier
• Additions for interest
• Limiters
• Log/antilog amps
• Multipliers etc.