Comparator circuits

1
Comparator circuits

• An ideal comparator compares two input voltages
  and produces a logic output signal whose value
  (high or low) depends on which of the two input
  is larger. The circuit symbol for a comparator is
  identical to the one for OpAmp.
• If Vi is positive, the output is high. If Vi is
  negative, the output is low
• The output logic levels may be
• symmetrical or asymmetrical,
• depending on the needs of the
• digital circuits that follows the
• comparator.

2
Comparator circuits

• Real comparator circuits do not display an abrupt
  transition in the output.
• Voltage gain is extremely high in the transition
  region.
• Internal circuitry of the an integrated
  comparator is similar to that of an OpAmp. One
  important difference is that frequency
  compensation is not needed for comparator because
  it is not operated in negative feedback.
• So, roughly, OpAmp can be used as a comparator
  circuit, but not vice versa due to frequency
  compensation.
• Comparator is usually designed to have small
  transition region and small propagation delay for
  output signal in response to inputs.
• Comparator also suffer from non-idealities, such
  as bias current, offset current and offset
  voltage.

3
Open-collector output

• Comparators are often used as the interface
  between analog and digital circuits, since it
  converts analog signal into logic levels.
• Power supply for analog signal in many systems
  are 10 to 15V. But power supply for digital
  signal is 1.8 to 5V.
• Suppose for digital signal, 0V stands for logic 0
  and 5V for logic 1, then a comparator needs to
  produce an output of 0V and 5V with input and
  internal signal at 0 to 15V. An open-collector
  output stage are very useful in the case.
• For example, in open-collector output stage, the
  output terminals are the emitter and collector of
  a NPN transistor. If the output is intended to be
  low (0), the transistor is driven to saturation
  so the output voltage is about 0.2V (saturation
  voltage of BJT). On the other hand, if the output
  should be high, the output transistor is cutoff
  and current does flow through transistor,
  resulting about 5V output

4
Open-collector output circuit
5
Schmitt Trigger I

• Comparators are often used to compare an input
  signal with a reference. But it suffers from a
  few problems, such as noise in the input signal,
  unintentional feedback, relatively slow change of
  logic level.
• Due to these problems, comparators are usually
  used with positive feedback. The type of circuit
  is also called a Schmitt Trigger.
• Notice that the feedback returns to the
  non-inverting terminal of the comparator.

6
Schmitt Trigger II

• One interesting thing about Schmitt Trigger is
  the transfer characteristic.
• Suppose the output level are 10V or -10V. If the
  input voltage is negative and of sufficient
  magnitude, the comparator output is 10V (high).
  Because of the feedback path, the voltage at
  non-inverting input is 1V, thus the input
  voltage has to increase to 1V before the output
  switches to -10V.
• On the other hand, if the output is -10V (low),
  the voltage at non-inverting input is -1V, so the
  input must decrease to -1V before the output
  switches to 10V (high).
• For input voltage between -1V and 1V, the output
  could be either high or low, depending on the
  past history of the input voltage.
• The difference between the switching thresholds
  for an increasing input and decreasing input is
  called hysteresis.

7
Noise Suppression using Schmitt Trigger
8
Schmitt Trigger III

• The good thing with hysteresis is that noise
  added to the input signal does not cause
  undesired multiple transitions of the output (as
  long as the peak-to-peak noise is less than the
  width of the hysteresis zone).
• Due to the positive feedback in Schmitt Trigger,
  the transition time of the output is very fast.
• The Schmitt Trigger
• shown before is a
• inverting because the
• output is low for a
• positive input and vice
• versa. A non-inverting
• Schmitt Trigger can be
• obtained as follows.

A non-inverting Schmitt Trigger
9
Schmitt Trigger IV

• Schmitt Trigger can be designed to have specified
  thresholds.
• One concern with using these triggers is that
  resistors should be properly chosen so that
  excessive power consumption is avoided.