Understanding OP AMP Parameters Definition Tutorial
OP AMP Parameters Definitions Tutorial
Operational Amplifiers - An Introduction:
- Operational Amplifiers (in short op amps) are made from discrete components.
- The opamp is a direct coupled high gain(negative feedback) amplifier.
- It can amplify signals having frequency ranging from 0Hz to 1MHz.
- It is used to perform a wide variety of linear functions and some non linear functions too.
- Thus it is also called as basic linear integrated circuit.
This device is named as Operational amplifier because it was originally designed to perform mathematical operations like addition, subtraction, differentiation, integration and multiplication etc in an analog computer.
The main properties of an op amp are:
- A very high open loop voltage gain AO = 105 for d.c. and low-frequency a.c., which decreases with frequency increase
- A very high input impedance (Ri = 106 to 1012) so that the input voltage is passed on to the op amp with little loss
- A very low output impedance(RO around 100Ω) , such that efficiently the output voltage(VO) is transferred to any load greater than a few kΩ.
Opamp Circuit Diagram Symbol:
Basically an opamp is a differential voltage amplifier. It means it amplifies the difference between input voltages V1 and V2. Here 3 situations are possible:
- If V2 > V1 --> Vo is positive
- If V2 < V1 --> Vo is negative
- If V2 = V1, Vo = 0
Ideal Operational Amplifier:
The ideal opamp is a differential input, single ended output device. It has the following characteristics
- Infinite input resistance [Ri = Infinity]
- Zero output resistance [RO = 0]
- Infinite voltage gain [AV = Infinity]
- Infinite bandwidth [BW = Infinity]
- Infinite Common Mode Rejection Ratio
- Infinite slew rate
- Zero offset [ ie,V1 = V2 , VO =0]
- Characteristics do not drift with temperature
Op amp Parameters:
Input Bias Current:
- The input bias current (IB) is the average of the currents enter into the two input terminals with the output at zero volts.
- Typically the input bias current is around 80nA.
- This input bias current makes a voltage drop across the equivalent source impedance seen from the input side of opamp.
Input Offset Current:
- The input offset current is the difference between the two input currents of the opamp with the output at zero volts.
- Typically the input offset current for a 741 op-amp is 20 nA .
Input Offset Voltage
- In the ideal op amp when both inputs are at zero volts the output should be zero volts.
- Due to imbalances within the device a small amount of voltage will appear at the output.
- This extra voltage can be eliminated by giving a small voltage called Input offset voltage (VOS) to the amplifier.
- Typically the input offset voltage for a 741 op-amp is around 1mV.
Common-Mode Rejection Ratio:
- In OPAMP, the output voltage is proportional to the difference between the voltages applied to its two input terminals.
- When the two input voltages are equal ideally the output voltages should be zero.
- A signal applied to both input terminals of the opamp is called as common-mode signal. Usually it is an unwanted noise voltage.
- The ability of an op amp to suppress common-mode signals is expressed in terms of its common-mode rejection ratio (CMRR).
- Typically the CMRR for a 741 op-amp is around 90 dB.
CMRR=20 log10[Differential Voltage Gain/Common Mode Gain] dB
- The slew rate is the maximum rate of change of output voltage for a step input voltage.
- The slew rate makes the output voltage to change at a slower rate than the applied input.
- Eventually the output waveform is a distortion of the input waveform.
- The typical value for the slew rate is 0.5V/μs.
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