Practical OPAMP
Contents
 1 Intro
 2 Op Amp Functions
 3 Linear Configurations
 4 Nonlinear configurations
Intro
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:

 V_{o} = A (V_{2} – V_{1})
Op Amps are usually packaged as an 8pin integrated circuit.
Pin  Usage 

1  Offset Null 
2  Inverted Input 
3  NonInverted Input 
4  V Supply 
5  No use 
6  Output 
7  +V Supply 
8  No use 
Op Amp symbol
 V_{+}: noninverting input
 V_{−}: inverting input
 V_{out}: output
 V_{S+}: positive power supply
 V_{S−}: negative power supply
Op amps amplify AC signal or AC Voltage better than a simple bipolar junction transistor.
Op Amp Functions
Voltage Difference Amplifier
From above

 V_{0} = A (V_{2} – V_{1})
Voltage Comparator
 V_{2} > V_{1} , V_{0} = +V_{ss}
 V_{2} < V_{1} , V_{0} = V_{ss}
 V_{2} = V_{1} , V_{0} = 0
Inverting Amplifier
With one voltage is grounded
 If V_{2} = 0 , V_{0} = A V_{1} . Inverting Amplifier
NonInverting Amplifier
With one voltage is grounded
 If V_{1} = 0 , V_{0} = A V_{2} . NonInverting Amplifier
Linear Configurations
Differential amplifier
 Differential (between the two input pins) =
Voltage Difference Amplifier
Whenever and ,
Voltage Difference
When and (including previous conditions, so that ):
Inverting Amplifier
Inverting Amplification is dictated by the ratio of the two resistors
NonInverting Amplifier
NonInverting Amplification is dictated by the ratio of the two resistors plus one
Voltage Follower
From NonInverting 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 amplifier
When , and independent
When
Integrator
Integrates the (inverted) signal over time
(where and are functions of time, is the output voltage of the integrator at time t = 0.)
Differentiator
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)
Comparator
Từ V_{0} = A (V_{2} – V_{1})
 V_{o} = 0 khi V_{2} = V_{1}
 V_{o} > 0 khi V_{2} > V_{1}
 V_{o} = V_{ss}
 V_{o} < 0 khi V_{2} < V_{1}
 V_{o} = 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
 V_{o} = V_{ss} khi V_{2} > V_{1}
 V_{o} = V_{ss} khi V_{2} < V_{1}
Instrumentation amplifier
Combines very high input impedance, high commonmode rejection, low DC offset, and other properties used in making very accurate, lownoise measurements
 Is made by adding a inverting buffer to each input of the differential amplifier to increase the input impedance.
Schmitt trigger
A comparator with hysteresis
Hysteresis from to .
Gyrator
A gyrator can transform impedances. Here a capacitor is changed into an inductor.
Zero level detector
Voltage divider reference
 Zener sets reference voltage
Negative impedance converter (NIC)
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:
Nonlinear configurations
Rectifier
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 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 output
 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 opamp 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 nonideal effects.
Exponential output
 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: