Operational Amplifier Applications
Operational amplifiers (“op-amp”) are high gain electronic voltage amplifiers, which are the significant building blocks for most electronic circuits. In addition to this, they are still the most widely used microelectronic devices nowadays, being used in vast applications for industrial and individual users. The aim of this experiment is to demonstrate how the operational amplifier operates and show its imperfections via constructing various kind of circuit such as non-inverting/inverting amplifier circuits, filter circuits, differentiator and integrator circuits.
In this report, we will go through two experiments, which are the fundamental circuits of operational amplifiers: non-inverting and inverting amplifier circuits, to analyze the difference between ideal and real op-amps. For the following section, the relevant theory will be introduced, and then the detail and results of the experiments will be discussed before proceeding to conclusion.
Figure 1 : The op amp and its ideal attributes
As the Figure1 shown, operational amplifier has two inputs labeled (+) and (-) with positive and negative power supply, and a single output. It is primarily a high gain differential amplifier which amplifies the difference of voltages between two inputs. The output voltage of the amplifier Vout is given by the following formula:
Vout = A (V+ - V-) --------------- (1)
Where A is the open loop voltages gain of the amplifier, which typically is very large about 105 at low frequency. V+ and V- are the non-inverting and inverting input voltage respectively. From the equation, output voltage is entirely governed by the difference between the two input voltages. However for real op-amps inputs do draw a small amount of current and the output voltage is affected by the output current drawn. For the analysis, both inverting and non-inverting amplifiers are applying negative feedback. It cause the V- to increase, hence voltages of the two input terminals will be much closed together. And the input draw current is assumed to be zero. Therefore Kirchhoff’s first (current) Law and Kirchhoff’s second (voltage) Law could be applied.
The main apparatus for this experiment are the test board with ±15V power supply, Kenwood CS4125 oscilloscope, Hameg DVMs, and the input signal function generator is Hameg HM80030-2. Inverting amplifier:
Figure 2 : Inverting Amplifier
Constructing the circuit of an inverting amplifier as shown in figure 2 on the test board. In order to make an amplifier with a gain of -10, setting R1 = 2.7 kΩ and RF = 27 kΩ.Applying a Hameg signal generator, a 1KHz sine wave was supply into the amplifier input, the amplitude should be adjusted to low values to prevent waveform distortion occur. Moreover, connecting the input and output of amplifier to X-Y channels of the Oscilloscope, to check the waveform and verify the amplification.
If both inputs are held at a common zero, the offset voltage will not be zero as ideally owing to a small amount of bias currents and internal imbalances of a real amplifier. Setting the oscilloscope to X-Y mode, a graph like Figure 3 will be display in the screen.
The output offset voltage which is the sum of two independent variables, one is Input offset voltage (Vin off), the other one is input bias current (Iin bias ).The equation of the Vout off is given below: Vout off=Vin off1+RFR1+Iin bias RF --------------- (2)
For the experimental purpose, the values of R1 and RF should be varied to form simultaneous equations, as a result, Vin off and Iin bias could be derived separately. When applying R1 = 2.7kΩ and RF = 27kΩ , the value of offset voltages obtained was 8mV; furthermore, the value of Vout off increased to 10mV while R1 = 0.1kΩ and RF = 1kΩ.Hence the simultaneous equation could be solved: