Operational Amplifiers (Op Amps)

Operational Amplifiers (Op Amps)

• Amplifies a signal to a higher level
• Often referred to as Op Amps
• Usually takes the form of an integrated circuit
• One common op amp is the 741 Op Amp
• Five pin connections which function as follows:
  • Vs+: Positive voltage supply
  • Vs-: Negative voltage supply
  • V+: Input signal
  • V-: Reference signal
  • Vout: Output signal
• For simplicity, Vs+ and Vs- connections are omitted from schematics as op amps always need a voltage supply
• Removing these connections reduces clutter

Op Amp Function

• Compares voltages across two input signals
• Amplifies this into an output voltage
• Signal applied to the non-inverting input (+ input)
• The inverting input (- input) typically serves as the reference
• Reference is often connected to ground (0V)
• Input naming convention stems from the phase relationship between input and output signals
• Input signals can be connected in opposite fashion
• This results in an output signal with phase reversal
• The original signal is amplified through the operational amplifier

Gain

• Gain refers to the amount a signal is amplified
• Calculated as the ratio of output signal to input signal
• Input signal is typically in millivolts (mV)
• Output signal is typically in volts
• Example calculation with input signal of 50mV (0.005V) and output signal of 5V:
  • Gain = Vout / Vin = 5 / 0.05 = 100

Clipping

• Output voltage from an op amp is limited by the positive and negative supplies
• Op amps often have gain values in the order of several hundred thousand
• Clipping is when the output voltage is limited by the power supply
• When gain from input signal exceeds the supply voltages, the output signal is driven to saturation
• In saturation the output waveform no longer resembles the input signal
• The signal's maximum and minimum portions are flattened
• Referred to as distortion
• Common example is a speaker system with the volume turned up too high
• With an opamp with a gain of 200,000 and an input signal of 1mV (0.001V), the output voltage would be:
  • Output = Input x Gain = 0.001 x 200,000 = 200V
• Clipping occurs because most op amps operate on positive and negative supply voltages (e.g. +15V and -15V)
• Feedback is used to control the gain of the op amp and prevent clipping

Feedback

• Feedback is used to control the output signal of an op amp
• Prevents the op amp from reaching saturation
• Achieved by feeding a portion of the output signal back to the input signal
• Types of feedback:
  • Positive
  • Negative
• Negative feedback is most commonly used to reduce the output signal to levels without saturation
• Positive feedback strengthens the input signal, which is generally undesirable
• Positive feedback has the feedback in phase with the input signal
• Negative feedback has the feedback signal 180 degrees out of phase with the input signal.

Feedback Circuit

• Output signal is fed back through resistor Rf to the reference input
• This cancels a portion of the input signal, reducing its amplitude
• Controlled by the value of resistors Rf and Rin
• Increasing the value of Rf reduces negative feedback and boosts the output signal

Comparator Circuit

• Op amp circuits generally fall under the category of linear circuits
• Linear circuits have a linear relationship between input and output values
• Logic circuits have one of two output states: low or high
• Logic circuits are desirable for detecting if an input voltage is above or below a set level
• Common in digital systems
• Can be achieved using the 741 opamp integrated circuit, without feedback
• A reference voltage is set using the Vref input
• The Vin input monitors the signal input
• As long as the voltage applied to Vin is below that applied to Vref, the output will be low
• Once the voltage at Vin exceeds that at Vref, the output will be high
• This 741 device in this form is known as a comparator
• By inverting the inputs (signal on Vref and reference on Vin), the output logic levels will be inverted
• A high output occurs when the signal voltage is below a set threshold