INTRODUCTION The aim of the experiment was to study the application of IC 741 as adder and subtractor. THEORY An op-amp as adder An adder is an electronic circuit that produces an output, which is equal to the sum of the applied inputs. In this section we discussed about the op-amp based adder circuit. An op-amp based adder produces an output equal to the sum of the input voltages applied at its inverting terminal. It is also called as a summing amplifier, since the output is an amplified one. The circuit diagram of an op-amp based adder is as seen below; Adder amplifier can combine numbers of input signal to a single output that is the weighted sum of the applied inputs. Derivation of formula for summing amplifier (adder op-amp) For inverting adder The input signals to be added are applied to the inverting input terminal of op-amp. By assuming currents πΌ1 and πΌ2 are flowing through resistance π 1 and π 2 respectively. Since input current to the op-amp is zero, the two currents are added to get current I which flows through the feedback resistances π π . Thus, by KCL at inverting terminal πΌ = πΌ1+ πΌ2 Substituting for currents 0 − ππ π1 − 0 π2 − 0 = + π π π 1 π 2 −ππ π1 π2 = + π π π 1 π 2 ππ = − [ π π π1 π π π2 + ] π 1 π 2 If π 1 = π 2 = π Then,ππ = − π π π 1 [π1 + π2 ] Thus, the addition of the two input signals obtained with gain [−π π ⁄π ] If π π = π ππ = −[π1 + π2 ] The negative sign indicates that the input and output are having 180Λ phase shift. Subtractor amplifier Is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. A differential amplifier is an analog circuit with two inputs (π1 and π2 ) and one output (π0) in which the output is ideally proportional to the difference between the two voltages. It is also known as differential amplifier. Differential amplifier amplify the difference between the input Voltages and this is it known as difference amplifier The circuit diagram for subtractor op-amp The derivation of formula for op-amp as subtractor For subtractor or difference amplifier The subtraction of two input voltages is possible with the help of subtractor. Let currents πΌ1 and πΌ2 are flowing through resistance π 1 and π 2 respectively. Since input current to the op-amp is zero, the two currents flow through the feedback resistance π π as follows; πΌ2 = [ π2 − π π−0 ]=[ ] π 2 π π From the above equation, voltage V can be calculated as follows π2 π π − = π 2 π 2 π π π2 π π = + π 2 π 2 π π π 2+ π π π2 = π[ ] π 2 π 2 π π π π π = π2 [ ] π 2 + π π The current πΌ1 is given as πΌ1 = π1 − π π − ππ = π 1 π π By simplifying the equation π1 π π ππ − = − π 1 π 1 π π π π ππ π π π1 = + − π π π π π 1 π 1 ππ π π π π 1 π = π [ π π+π ]-π 1 π 1 1 Substituting the voltage V from the equation π π π π+ π 1 ππ π1 = π2 [ ][ ]− π π π π + π 2 π π π 1 π 1 π π π π+ π 1 π π π1 π0= π2 [ ][ ]− π π + π 2 π 1 π 1 If π 1 = π 2 = π π , ππ = π π [π − π1 ] π 1 2 ππ = [π2 − π1 ] Thus, it is the output obtained from subtraction of the two input voltages. The subtractor circuits are used to solve various mathematical equations. METHOD The apparatii used in this experiment were IC 741, Resistors(1kΩ) 4 pieces, functional generator, Regulated power supply, IC bread board trainer, CRO, patch cords and CRO probes. The setup of the experiment was as follows; a) For summing amplifier b) For subtractor amplifier Procedures a) For adder i) The connections were made as per circuit diagram ii) The following input Voltages were applied a) π1 = 5π, π2 = 2π b) π1 = 5π, π2 = 5π c) π1 = 5π, π2 = 7π iii) The DC voltage at the output terminal was measured using the Multimeter. iv) The output voltage was measured for different values of π1 and π2. For a subtractor op-amp i) ii) iii) iv) Connections were made as per circuit diagram The following input Voltages were applied a) π1 = 5π, π2 = 2π b) π1 = 5π, π2 = 5π c) π1 = 5π, π2 = 7π Using the Multimeter the DC output voltage was measured at the output terminal. The output voltage was measured for different values of V1 and V2. DATA RESULTS AND ANALYSIS π 1 =10kΩ π 2 =10kΩ π 3 =10kΩ For Adder op-amp V1 V2 Theoritical, π½πΆ Practical, π½πΆ 2.986 2.986 2.986 2.986 1.623 3.260 4.850 6.430 4.5 6.0 7.5 9.0 4.52 6.03 7.70 9.31 %πππππ π»π½ π»π½ − π·π½ 11.5 0.5 2.7 3.44 For subtractor V1 V2 Theoritical, π½πΆ Practical, π½πΆ 2.985 2.985 2.985 2.985 1.636 3.188 4.81 6.43 1.5 0.0 1.5 3.0 1.445 0.175 1.610 3.250 %πππππ π»π½ π»π½ − π·π½ 3.7 0.0 7.3 8.33 DISCUSSION AND EVALUATION QUESTIONS 1. What are an adder and subtractor An adder Is a digital circuit that performs addition of numbers. In many computers and other kinds of processors, adders are used in the arithmetic logic circuits. Op-amp as an adder An Op-amp based adder produces an output equal to the sum of the input voltages applied at its inverting terminal. It is also called as summing amplifier since the output is an amplified one. Subtractor Is an electronic logic circuit for calculating the difference between two binary number, the minuend and the number to be subtracted, the subtrahend. A full subtractor performs this calculation with three inputs: minuend bit, subtrahend bit and borrow bit. Op-amp as an subtractor An op-amp based subtractor produces an output equal to the difference of the input voltages applied at its inverting and non-inverting terminals. It is also called as a difference amplifier, since the output is an amplified one. 2. Write the formula for sum of three inputs for an Op-amp From the summing amplifier equation −ππ = π π [π + π2 + π3 … … … … + ππ ] π ππ 1 For the three inputs π π π π0 = − π π (π 1 + π 2 + π 3 ) 1 2 3 Where: π0 = ππ’π‘ππ’π‘ π£πππ‘πππ π π = ππππππππ πππ ππ π‘ππ π1 , π2 πππ π3 represents individual input voltages respectively π 1 , π 2 πππ π 3 represents individual resistors 3. What are the various DC characteristics of op-amp i) Input Bias current The average value of the two currents flowing into the op-amp input terminals is called Input Bias current (πΌπ ). For 741, a bipolar op-amp, the bias current is 500nA or less. Ideally it must be zero. ii) Input Offset Current The input stage of the op-amp is dual input differential amplifier and the inputs are base terminals of the two transistors. Hence the input currents of op-amp are the base currents of the transistors used in the input stage. Due to transistor mismatch these currents differ. The algebraic difference between the currents flowing into the two input terminals of the op-amp is called input offset current. Ideally called input offset current. is zero while for op-amp 741, its value is 200nA iii) Input Offset Voltage. Ideally, if the input voltages are zero then the output Voltage becomes zoro. A small voltage applied to the input terminals to make the output voltage as zero when the two input terminals are grounded is called input offset voltage. iv)Thermal Drift Bias current, offset current and offset voltage change with temperature. A circuit carefully nulled at 25β may not remain so when the temperature rises to 35β. 4. What are the various AC characteristics of an op-amp i) Frequency Response: Frequency response is the condition in which the gain of the op-amp responds to different frequencies. The variation in operating frequency will cause variations in gain magnitude and its phase angle ii) Circuit Stability A circuit or a group of circuit are said to be stable when they are connected together as a system. iii) Open loop voltage gain iv Output offset voltage v Common Mode Rejection ratio vi) Slew rate CONCLUSION AND RECOMMENDATIONS The aim of the experiment was to study the applications of IC 741 as adder and subtractor. The experiment was useful in learning applications of op-amp as adder and subtractor. For example Op-amp can be used to sum the input voltage of two or more into sources into a single output voltage, but also a summing amplifier can be used to provide an output voltage that is equal to the difference of two voltages. The slight errors that occurred during the experiment might be due to resistance difference and decrease in power supply intensity. Recommendations In order to improve any Op-amp related experiment, the compensating networks are used to modify the performance of an op-amp circuit over the desired frequency range by controlling its gain and phase shift. And the overall gain can be much controlled (reduced) by applying negative feedback hence it can be accurately tailored to the required level or to produce the required output format as in the case of filters. Also, few electronic components can be added to the opamp circuit to provide the required feedback. REFERENCES
