Practical Electronics/Operational amplifiers

IntroEdit

Op Amp is a short hand term for Operational Amplifier. An operational amplifier is a circuit component that amplifies the difference of two input voltages:

Vo = A (V2 - V1)

Op Amps are usually packaged as an 8-pin integrated circuit.

 
Operational Amplifier IC Chip
Pin Usage
1 Offset Null
2 Inverted Input
3 Non-Inverted Input
4 -V Supply
5 No use
6 Output
7 +V Supply
8 No use


Op Amp symbol

 
op-amp
  • V+: non-inverting input
  • V: inverting input
  • Vout: output
  • VS+: positive power supply
  • VS−: negative power supply

Op amps amplify AC signal or AC Voltage better than a simple bipolar junction transistor.

Op Amp FunctionsEdit

Voltage Difference AmplifierEdit

From above

V0 = A (V2 - V1)

Voltage ComparatorEdit

V2 > V1 , V0 = +Vss
V2 < V1 , V0 = -Vss
V2 = V1 , V0 = 0

Inverting AmplifierEdit

With one voltage is grounded

If V2 = 0 , V0 = -A V1 . Inverting Amplifier

Non-Inverting AmplifierEdit

With one voltage is grounded

If V1 = 0 , V0 = A V2 . Non-Inverting Amplifier

Linear ConfigurationsEdit

Differential amplifierEdit

 
Differential amplifier
 
  • Differential   (between the two input pins) =  

Voltage Difference AmplifierEdit

Whenever   and  ,

 

Voltage DifferenceEdit

When   and   (including previous conditions, so that  ):

 

Inverting AmplifierEdit

 
Inverting amplifier
 

Inverting Amplification is dictated by the ratio of the two resistors

Non-Inverting AmplifierEdit

 
Non-inverting amplifier
 

Non-Inverting Amplification is dictated by the ratio of the two resistors plus one

Voltage FollowerEdit

 
Voltage follower

From Non-Inverting Amplifier's formula. If the resistors has the same value of resistance then output voltage is exactly equal to the input voltage

 

From Inverting Amplifier's formula. If the resistors has the same value of resistance then output voltage is exactly equal to the input voltage and inverted

 

Summing amplifierEdit

 
Summing amplifier
 

When  , and   independent

 

When  

 

IntegratorEdit

 
Integrating amplifier

Integrates the (inverted) signal over time

 

(where   and   are functions of time,   is the output voltage of the integrator at time t = 0.)

DifferentiatorEdit

 
Differentiating amplifier

Differentiates the (inverted) signal over time.

The name "differentiator" should not be confused with the "differential amplifier", also shown on this page.

 

(where   and   are functions of time)

ComparatorEdit

 
Comparator
  •  

Từ V0 = A (V2 - V1)

  • Vo = 0 khi V2 = V1
  • Vo > 0 khi V2 > V1
Vo = Vss
  • Vo < 0 khi V2 < V1
Vo = V-ss

When two input voltages equal. The output voltage is zero . When the two input voltages different and if one is greater than or less than the other

  1. Vo = Vss khi V2 > V1
  2. Vo = V-ss khi V2 < V1

Instrumentation amplifierEdit

 
Instrumentation amplifier


Combines very high input impedance, high common-mode rejection, low DC offset, and other properties used in making very accurate, low-noise measurements

Schmitt triggerEdit

 
Schmitt trigger

A comparator with hysteresis

Hysteresis from   to  .

GyratorEdit

 
Inductance gyrator

A gyrator can transform impedances. Here a capacitor is changed into an inductor.

 

Zero level detectorEdit

Voltage divider reference

  • Zener sets reference voltage

Negative impedance converter (NIC)Edit

 
Negative impedance converter


Creates a resistor having a negative value for any signal generator

  • In this case, the ratio between the input voltage and the input current (thus the input resistance) is given by:
 

Non-linear configurationsEdit

RectifierEdit

 
Super diode

Behaves like an ideal diode for the load, which is here represented by a generic resistor  .

  • This basic configuration has some limitations. For more information and to know the configuration that is actually used, see the main article.

Peak detectorEdit

 
Peak detector

When the switch is closed, the output goes to zero volts. When the switch is opened for a certain time interval, the capacitor will charge to the maximum input voltage attained during that time interval.

The charging time of the capacitor must be much shorter than the period of the highest appreciable frequency component of the input voltage.

Logarithmic outputEdit

 
Logarithmic configuration
  • The relationship between the input voltage   and the output voltage   is given by:
 

where   is the saturation current.

  • If the operational amplifier is considered ideal, the negative pin is virtually grounded, so the current flowing into the resistor from the source (and thus through the diode to the output, since the op-amp inputs draw no current) is:
 

where   is the current through the diode. As known, the relationship between the current and the voltage for a diode is:

 

This, when the voltage is greater than zero, can be approximated by:

 

Putting these two formulae together and considering that the output voltage   is the inverse of the voltage across the diode  , the relationship is proven.

Note that this implementation does not consider temperature stability and other non-ideal effects.

Exponential outputEdit

 
Exponential configuration
  • The relationship between the input voltage   and the output voltage   is given by:
 

where   is the saturation current.

  • Considering the operational amplifier ideal, then the negative pin is virtually grounded, so the current through the diode is given by:
 

when the voltage is greater than zero, it can be approximated by:

 

The output voltage is given by: