In this chapter, we deal with Operational Amplifiers (Op Amps) in Proteus. The previous chapter was about Transistor Circuits in Proteus. So let’s begin to learn about Operational Amplifiers in Proteus.
Op-Amp Circuits in Proteus
Operational amplifiers have a lot of application in analog circuits. In the open loop operation it works as a comparator and gives discrete outputs i.e, +Vsat & -Vsat, whereas in a closed loop operation it works as an amplifier and the output swings between +Vsat to –Vsat. The maximum Vcc is not the same for all the Op-Amps. Designer must refer to the datasheet of the specific op amp IC for absolute maximum or minimum values.
Op-Amps in Proteus
Op-Amps in different IC packages are available. For example:- 741 has single Op-Amp and LM 324, LM 339 have 4 Op-Amps each. The frequency of operation, slew rate and voltage ranges are the key parameters to select the right Op-Amp. Some of the basic operation modes and application circuits are presented here.
741 Op-Amp description in Proteus
Op-Amp as a Comparator
The basic application of an Op-Amp is to use it as a voltage comparator. This is open loop operation of Op-Amp. The voltage at the Non-Inverting and Inverting terminals (pins) i.e., V+ and V- are compared, and if V+ > V- then output is +Vsat and if V+ < V- then output is –Vsat.
The circuit above is a voltage comparator. The voltages across V+ and V- are controlled by potentiometers POT1 and POT2. +Vsat is 5V and –Vsat is GND in this circuit. When V+ > V- the output=1.49V which is Logic Low state. When V+ < V- the output=4V which is Logic High state.
Simulation of Op-Amp as a Comparator
Op-Amp as Non-Inverting Amplifier
In this circuit, Op-Amp operates in a closed loop. Input is applied to Non-Inverting terminal and output is fed-back to the Inverting terminal as a voltage shunt feed-back. The polarity of the output voltage is same as that of the input. Below are the circuit diagrams for Single as well as Dual voltage supply conditions.
Op-Amp as a Non-Inverting Amplifier
The output varies according to the gain of the amplifier circuit up to Vsat. So, selection of supply voltage range is important to obtain the desired output voltages. But, Non-Inverting configuration cannot give gain less than or equal to unity. Output of the Op-Amp follows the equation,
Output equation of Non-Inverting Amplifier
Simulation of Op-Amp as Non-Inverting Amplifier
Op-Amp as an Inverting Amplifier
In this circuit, Op-Amp operates in the closed loop. Input is applied to the Inverting terminal and output is fed-back to the inverting terminal as voltage shunt feed-back. The polarity of the output voltage is opposite to that of the input. Below is the circuit diagram of inverting amplifier with Dual voltage supply conditions.
Op-Amp as Inverting Amplifier
Inverting amplifier configuration can give unity gain, i.e., Vout=-Vin. The output varies according to gain of the amplifier circuit between +Vsat and -Vsat. So, selection of supply voltage range is important to obtain the desired output voltages. Output of the Op-Amp follows the equation,
Output equation of Inverting Amplifier
Simulation of Op-Amp as Inverting Amplifier
Op-Amp as Differential Amplifier/Subtractor
The difference of the voltage between V+ and V- is amplified in this circuit. It is similar to the comparator mode, and in addition the Op-Amp is operated in a closed loop, making it possible to control the voltage gain of the circuit.
Op-Amp as Differential Amplifier
The output varies according to the gain of the amplifier circuit between +Vsat and -Vsat. This can be used as an analog Subtractor. If Vout=Va+Vb-Vc-Vd is the desired output, then connect Vc, Vd to V- terminal and Va, Vb to V+ terminal.Output of the Op-Amp follows the equation,
Output equation of Differential Amplifier
Simulation of Op-Amp as Differential Amplifier
Op-Amp as a Summing Amplifier/Adder
Voltages from various sources referred to the common ground are added in this circuit. The gain of the output can be controlled by selecting appropriate resistance values. If R1=R2=R3=R=2*Rf then output, Vout=Va+Vb+Vc. By varying the Rf value, gain of the circuit can be controlled.
This is Non-Inverting summing amplifier. If the common node of resistors is connected to V- terminal then it is called Inverting summing amplifier. The output of inverting amplifier for above mentioned resistor values is, Vout=-(Va+Vb+Vc). The output of Non-Inverting Summing amplifier follows the equation,
Output equation of Summing Amplifier
Simulation of Op-Amp as Summing Amplifier
Op-Amp as a Window voltage detector
To detect the arrival of a particular range of voltage(s) from the swing of an input signal, this circuit is used.Two Op-Amps in comparator mode are used in this circuit. Lower comparator monitors the minimum value of the desired range and upper comparator monitors the maximum value of the desired range. POT Upper and POT Lower are presets for minimum and maximum values.
Op-Amp as Window Voltage Detector
- As in the above image, when the input voltage is less than the Minimum value, then output of Lower Comparator and Upper Comparator are low. The voltages at cathodes of the diode AND Gate are High and Low, so the output is low.
Op-Amp as Window Voltage Detector
- When the input voltage is between the preset range the output of LC is High and UC is low.
The voltages at cathodes of the diode AND Gate are High and High, so the output is High.
Op-Amp as Window Voltage Detector
- When the input voltage is higher than the maximum preset value, the output of LC is High and UC is High.The voltages at cathodes of the diode AND Gate are Low and High, so the output is Low.
Window Voltage Detector with Dual Voltage supply
Thus this circuit behaves as a window voltage detector. This circuit can be used with dual voltage supply and including negative input voltages also.
Op-Amp as Zero-Crossing detector
Sinusoidal voltages signals are often converted to square waves using Zero-Crossing detectors. In this circuit, Op-Amp operates in the open loop comparator mode. The supply voltage can be single or dual. This circuit is generally used to start a process after zero-crossing, like turning ON the load or generating firing pulse through a microcontroller by starting the timers or counting the cycles or frequency of alternating voltage signal etc.
When the input signal is connected to V+ and V- is grounded, as the input voltage rises above the ground voltage i.e, when the input is positive, the output of the Op-Amp is +Vsat and output is –Vsat while the input is negative.
Op-Amp as Non-Inverting Zero-Crossing detector
Alternatively, when the input signal is connected to V- and V+ is grounded, as the input voltage rises above the ground voltage i.e., when the input is positive, the output of the Op-Amp is -Vsat and the output is +Vsat while the input is negative. This difference is used when input signal is obtained from the secondary of a transformer, and square wave output is to be used for some other applications.
Op-Amp as Inverting Zero-Crossing detector
However, suitable filter circuits must be inserted between input signal and the Op-Amp circuit in order to eliminate harmonics.
PWM-Pulse Width Modulation
Combination of astable multi vibrator, Integrator, Comparator gives the PWM circuit. But if the frequency of operation is to be variable, then it becomes difficult, as it is necessary to change the knobs of frequency adjustment and resistors of the integrator to be adjusted to match with input frequency in addition to the duty cycle knob.
Op-Amp as Duty Cycle Controller
To simplify this requirement, Voltage Controlled Oscillator is used as above. This is provided by the manufacturer in the data sheet of LM 324 IC. By making some modifications and adding a few components, we can design a PWM circuit for motor speed control or Light brightness control etc.
Simulation of Op-Amp as Duty Cycle Controller
The square wave whose frequency can be adjusted with the frequency knob, is converted to a triangular wave. The Duty cycle adjusting knob is used to vary the duty cycle (which is Red line on the triangular wave form in the above image).
When the instantaneous triangular wave voltage is lesser than the dc voltage of duty cycle knob, the output is in High state and vice versa. This can be reversed by reversing the V+ and V- inputs of the output stage Op-Amp.
Op-Amps cannot work without feedback resistors because of offset currents. Thus the comparator circuit is wrong.