Topics to be covered
Characteristics of Operational Amplifier
Op-amp Parameter and Idealized Characteristics.
Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration, and differentiation.
An Operational Amplifier, or op-amp for short, is fundamentally a voltage-amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. These feedback components determine the resulting function or “operation” of the amplifier and by the different feedback configurations whether resistive, capacitive or both, the amplifier can perform a variety of different operations, giving rise to its name of “Operational Amplifier”.
There can be four different classifications of operational amplifier gain:-
1. Voltage – Voltage “in” and Voltage “out”
2. Current – Current “in” and Current “out”
3. Transconductance – Voltage “in” and Current “out”
4. Trans- resistance – Current “in” and Voltage “out”
Characteristics of Operational Amplifier
Op-amp Parameter and Idealized Characteristic
Open Loop Gain, (Avo)
Infinite – The main function of an operational amplifier is to amplify the input signal and the more open-loop gain it has the better. Open-loop gain is the gain of the op-amp without positive or negative feedback and for such an amplifier the gain will be infinite but typical real values range from about 20,000 to 200,000.
Input impedance, (ZIN) Infinite – Input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current flowing from the source supply into the amplifiers input circuitry ( IIN = 0 ). Real op-amps have input leakage currents from a few pico-amps to a few mili-amps.
Output impedance, (ZOUT) Zero – The output of the impedance of the ideal operational amplifier is assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it can supply as much current as necessary to the load. This internal resistance is effectively in series with the load thereby reducing the output voltage available to the load. Real op-amps have output impedances in the 100-20kΩ range.
Infinite – An ideal operational amplifier has an infinite frequency response and can amplify any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have infinite bandwidth. With real op-amps, the bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the frequency where the gain of the amplifier becomes unity.
Offset Voltage, (VIO) Zero – The amplifier output will be zero when the voltage difference between the inverting and the non-inverting inputs are zero, the same or when both inputs are grounded. Real op-amps have some amount of output offset voltage.
The amplified output of the signal from the Op Amp is the difference between the two input signals.
The diagram shown above is the Op-Amp simple connection. If both the inputs are supplied with the same voltage, the Op Amp will then takes the difference between the two voltages and it will be 0. The Op-Amp will multiply this with its gain 1,000,000 so the output voltage is 0. When 2 volts is given to one input and 1 volt in the other, then the Op Amp will take its difference and multiply with the gain. That is 1 volt x 1,000,000. But this gain is very high so to reduce the gain, feedback from the output to the input is usually done through a resistor.
A) Inverting Amplifier:
The circuit shown above is an inverting amplifier with the Non-inverting input connected to the ground. Two resistors R1 and R2 are connected in the circuit in such a fashion that R1 feeds the input signal while R2 returns the output to the Inverting input. Here when the input signal is positive the output will be negative and vice versa. The voltage change at the output relative to the input depends on the ratio of the resistors R1 and R2. R1 is selected as 1K and R2 as 10K. If the input receives 1 volt, then there will be 1 mA current through R1 and the output will have to become – 10 volts to supply 1 mA current through R2 and to maintain zero voltage at the Inverting input. Therefore the voltage gain is R2/R1. That is 10K/1K = 10.
B) Non-inverting Amplifier:
The circuit shown above is a Non-inverting amplifier. Here the Non-inverting input receives the signal while the Inverting input is connected between R2 and R1. When the input signal moves either positive or negative, the output will be in phase and keeps the voltage at the inverting input the same as that of Non inverting input. The voltage gain in this case will be always higher than 1 so (1+R2/R1).
C) Voltage Follower
The circuit above is a voltage follower. Here it provides high input impedance, low output impedance .When the input voltage changes, the output, and the inverting input will change equally.
The operational amplifier compares the voltage applied at one input to the voltage applied at the other input. Any difference between the voltages ever if it is small drives the op-amp into saturation. When the voltages supplied to both the inputs are of the same magnitude and the same polarity, then the op-amp output is 0Volts.
A comparator produces limited output voltages which can easily interface with digital logic, even though compatibility needs to be verified.