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Objectives:
To analyze the inverting input operational amplifier circuit and its gain
Equipments:
- Resistors: a. R 1 = 1kΩ b. R (^) f = 10kΩ
- IC LM-324 (quad opamp)
- Oscilloscope
- Function generator
- DMM
- Power Supply dual
- Wires
- A Bread Board
- digital trainer board
Operational Amplifier (Op-Amp) Basics
The op-amp is basically a differential amplifier having a large voltage gain, very high input impedance and low output impedance. The op-amp has a "inverting" or (-) input and "noninverting" or (+) input and a single output. The op-amp is usually powered by a dual polarity power supply in the range of +/- 5 volts to +/- 15 volts. Following are the properties of an ideal Op-Amp It is a DC Amplifier (direct coupled / direct input) Have very high voltage gain Av = ∞ Very high gain differential amplifier Have very high input impedance Zi = ∞ Should have very low output impedance The output should be zero @ input = 0V Output must be capable of positive and negative voltage swing
It should have very large Common Mode Rejection Ration
CMRR
Inverting Amplifiers:
The Inverting Amplifier
Figure 1. shows the circuit diagram of an inverting amplifier. The input signal is applied through resistor R1 to the inverting op amp input. Resistor RF is the feedback resistor which connects from the output to the inverting input. The circuit is called an inverting amplifier because its voltage gain is negative. This means that if the input voltage is increasing or going positive, the output voltage will be decreasing or going negative, and vice versa.
The non-inverting input to the op amp is not used in the inverting amplifier circuit. The figure shows this input grounded so that v+=0.
Procedure:
The op-amp is connected using two resistors R 1 and R (^) f such that the input signal is applied in series with R 1 and the output is connected back to the inverting input through R (^) f. The non-inverting input is connected to the ground reference or the center tap of the dual polarity power supply. In operation, as the input signal moves positive, the output will move negative and visa versa. The amount of voltage change at the output relative to the input depends on the ratio of the two resistors R 1 and R (^) f. As the input moves in one direction, the output will move in the opposite direction, so that the voltage at the inverting input remains constant or zero volts in this case. If R 1 is 1k and Rf is 10 k and the input is +1v volt then there will be 1 mA of current flowing through RA and the output will have to move to -10 volts to
I (^) i
Rf
I 1
Vi-
Vi+
I (^) f
Vo
Vi
Figure-1:
R 1
Observations:
Input Voltage = Vi = 0.11 V
For Av Calculation: Av = - R (^) f /R (^1)
For Av Measurement: Av = Vo/Vi
R (^) f R 1 Vo^ Av (Calculated)
Av (Measured) 6.7 kΩ 476 Ω -1.6 V -14.07 -14. 3.3 kΩ 476 Ω -0.8 V -6.93 -7. 3.3 kΩ 330 Ω -1.7 V -10 - 10 kΩ 330 Ω -3 V -30.3 - 1 kΩ 330 Ω -0.4 V -3.03 -3. 1 kΩ 674 Ω -0.2 V -1.48 -1.
Result:
The results of the observations show that the inverting op-amp does invert the output and amplifies the input, as the relation:
Av = - R (^) f /R 1
