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Unit 6 - Notes

ECE221 6 min read

Unit 6: Operational amplifier applications II

Introduction to Comparators and Converters

Operational amplifiers (op-amps) are versatile components used widely beyond linear amplification. In this unit, we explore non-linear applications such as comparators, converters, timers, and phase-locked loops. Comparators are used to compare two voltages and provide a digital output indicating which is larger. Converters translate signals between analog and digital domains or between voltage and frequency.

Basic Comparator

A basic comparator is an op-amp operating in an open-loop configuration (without negative feedback).

  • Function: It compares a signal voltage applied to one input with a reference voltage applied to the other.
  • Operation:
    • If the non-inverting input voltage () is greater than the inverting input voltage (), the output goes to positive saturation ().
    • If is greater than , the output goes to negative saturation ().
  • Applications: Level shifting, analog-to-digital conversion, and threshold detection.

Zero Crossing Detector

A zero-crossing detector is a specific type of comparator where the reference voltage is set to (ground).

  • Function: It detects when an input AC signal crosses the zero-voltage level.
  • Operation:
    • When the input signal is above , the output is saturated in one direction.
    • When the input drops below , the output swings to the opposite saturation level.
    • Converts a sine wave input into a square wave output.
  • Applications: Phase meters, frequency counters, and AC power control (triac switching).

Schmitt Trigger

A Schmitt trigger is an inverting or non-inverting comparator with positive feedback.

  • Function: It introduces hysteresis to prevent noise from causing multiple false triggers as the input signal slowly crosses the threshold.
  • Upper Threshold Point (UTP) & Lower Threshold Point (LTP): The positive feedback creates two distinct threshold voltages.
    • The output changes state only when the input exceeds the UTP or falls below the LTP.
    • The difference between UTP and LTP is the hysteresis voltage.
  • Applications: Signal conditioning, debouncing switches, and relaxing oscillators.

Voltage Limiters

Voltage limiters (or clippers) are circuits used to restrict the output voltage swing of an op-amp to predetermined limits, regardless of the input signal level.

  • Operation: Typically achieved by placing Zener diodes in the feedback loop of an op-amp.
    • When the output reaches the Zener breakdown voltage, the diode conducts, drastically reducing the closed-loop gain and clipping the output waveform.
  • Applications: Protecting subsequent circuitry from overvoltage, shaping waveforms.

Voltage to Frequency (V/F) and Frequency to Voltage (F/V) Converters

  • Voltage to Frequency (V/F) Converter:
    • Function: Converts a continuous analog input voltage into a train of pulses whose frequency is directly proportional to the input voltage.
    • Mechanism: Usually consists of an integrator and a threshold detector (comparator or 555 timer). As input voltage increases, the integrator charges faster, triggering the comparator more frequently.
    • Applications: Telemetry, analog-to-digital conversion, and FM modulation.
  • Frequency to Voltage (F/V) Converter:
    • Function: Converts a continuous train of input pulses (frequency) into a proportional DC analog output voltage.
    • Mechanism: Often uses a monostable multivibrator triggered by the input frequency, followed by an integrator or low-pass filter to average the pulses into a DC voltage.
    • Applications: Motor speed control (tachometers), demodulation of FM signals.

Analog to Digital (ADC) and Digital to Analog Converters (DAC)

  • Digital to Analog Converter (DAC):
    • Function: Converts digital binary codes into a continuous analog voltage or current.
    • Common Types: Weighted Resistor DAC, R-2R Ladder DAC.
    • Key Specs: Resolution (number of bits), settling time, accuracy.
  • Analog to Digital Converter (ADC):
    • Function: Converts a continuous analog signal into a discrete digital binary number.
    • Common Types:
      • Flash ADC: Very fast, uses multiple comparators.
      • Successive Approximation (SAR): Good balance of speed and resolution.
      • Dual Slope ADC: Slow but highly accurate and immune to noise.
    • Key Specs: Resolution, sampling rate, conversion time.

Sample and Hold (S/H) Circuit

  • Function: Samples an analog input signal at a specific instant in time and holds that voltage level constant until the next sampling period.
  • Operation:
    • Consists of an analog switch (like a FET), a holding capacitor, and buffer op-amps.
    • Sample Mode: The switch closes, and the capacitor charges/discharges to the input voltage.
    • Hold Mode: The switch opens, and the capacitor holds the charge. The high input impedance of the output buffer prevents the capacitor from discharging.
  • Applications: Essential at the input of ADCs to keep the voltage stable during the conversion process.

The 555 Timer

The 555 timer is a highly stable integrated circuit capable of producing accurate timing pulses or oscillations.

  • Internal Structure: Comprises two comparators, a flip-flop, a discharge transistor, and a resistive voltage divider (three resistors).
  • Operating Modes:
    1. Monostable (One-Shot): Produces a single output pulse of a specific duration when triggered. Duration .
    2. Astable (Free-Running): Operates as an oscillator, outputting a continuous train of square waves.
      • Frequency
      • Duty Cycle can be adjusted via resistor ratios.
    3. Bistable: Acts as a basic flip-flop with two stable states.

Phase Locked Loops (PLL)

A Phase-Locked Loop is a control system that generates an output signal whose phase is related to the phase of an input signal.

  • Block Diagram Components:
    1. Phase Detector: Compares the phase of the input signal with the phase of the oscillator signal and outputs an error voltage.
    2. Low Pass Filter (LPF): Removes high-frequency noise from the error voltage and determines the loop's dynamic characteristics (capture and lock ranges).
    3. Voltage Controlled Oscillator (VCO): Generates a frequency proportional to the filtered error voltage.
  • Key Terms:
    • Lock Range: The range of frequencies over which the PLL can maintain lock with the input signal.
    • Capture Range: The range of frequencies over which the PLL can acquire a lock on an input signal initially.
  • Applications: Frequency synthesis, FM demodulation, clock recovery, and motor speed control.

Reading Datasheet of 555

Understanding the datasheet is crucial for practical circuit design using the 555 timer. Key parameters to look for include:

  • Supply Voltage (): Usually ranges from to (or up to depending on the variant).
  • Supply Current: Current consumed by the IC (typically to ).
  • Output Current: Maximum sink or source current capability (typically around ).
  • Threshold Voltage: Usually .
  • Trigger Voltage: Usually .
  • Maximum Operating Frequency: Practical limit for astable operation (typically a few hundred kHz for standard bipolar 555, higher for CMOS versions like TLC555).
  • Pin Configuration: Knowing the function of Ground (1), Trigger (2), Output (3), Reset (4), Control Voltage (5), Threshold (6), Discharge (7), and (8).
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