The important Parameters of op-amp are defined and explained as follows. Note that, all parameters are about general term “OP-amp” and not any specific Op-amp such as LM 741.
Parameters of Op-amp
1. Differential Input Resistance
It is denoted by R_{i } and often referred as input resistance. The equivalent resistance that is measured at either the inverting or non-inverting input terminal with the other terminal connected to ground is called input resistance.
2. Input Capacitance
The Input capacitance, C_{i} is the equivalent capacitance that is measured at either the inverting or non-inverting terminal with the other terminal connected to ground.
3. Output Resistance
Output resistance, R_{o} is the equivalent resistance which is measured between the output terminals of the op-amp and the ground (or common point).
4. Input Offset Voltage
The input offset voltage is the voltage that must be applied between the two input terminals of an op-amp to null the output.
5. Input Offset Current
The algebraic difference between the currents into the non-inverting and inverting terminals is referred to as input offset current.
In the form of equation,
I_{io} =| I_{B+}-I_{B}-|
Where I_{io} is the input offset current, I_{B}+ is the current into the non-inverting input and I_{B}– is the current into the inverting input.
6. Input Bias Current
Input bias current is the average of the currents that flow into the inverting and non-inverting input terminals of an op-amp. In the equation form,
I_{B} = (I_{B+}+I_{B}-)/2
7. Offset Voltage Adjustment Range
The offset voltage adjustment range is the range through which the input offset voltage can be adjusted by varying the 10KΩ potentiometer.
8. Input Voltage Range
When the same voltage is applied to both input terminals, the voltage is called a common-mode voltage V_{CM}, and the op-amp is said to be operating in the common-mode configuration.
For IC LM741 the range of input common mode voltage is ± 13 V maximum. This means that the common mode voltage applied to both input terminals can be as high as + 13V to as low as – 13V without disturbing proper functioning of the op-amp.
9. Common-Mode Rejection Ratio (CMRR)
The common-mode rejection ratio (CMRR) is defined as the ratio of differential voltage gain A_{d} to the common-mode voltage gain A_{CM}.
i.e. CMRR = A_{d}/ A_{CM}
10. Supply Voltage Rejection Ratio (SVRR)
The change in op-amp’s input offset voltage V_{io} caused by variations in supply voltages is called the supply voltage rejection ratio (SVRR). It is also called Power Supply Rejection Ratio(PSRR) and Power Supply Sensitivity(PSS).
If we denote the change in supply voltages by ∆v and the corresponding change in input offset voltage by ∆V_{io} .
Then, SVRR is defined as,
SVRR = ∆V_{io} / ∆V .
The lower the value of SVRR in microvolts/volt, the better the op-amp performance.
11. Large-Signal Voltage Gain
Since the op-amp amplifies difference voltage between two input terminals, the voltage gain of the amplifier is defined as ratio of output voltage to differential input voltage.
Voltage gain = (output voltage) / (Differential input voltage)
That is, A = V_{o}/V_{id}
Because the output signal amplitude is much larger than the input signal, the voltage gain is commonly called large-signal voltage gain.
12. Output Voltage Swing
The difference between positive saturation voltage and negative saturation voltage is called output voltage swing.
13. Output Short-Circuit Current
The output short circuit current is the value of output current that is allowed to flow by the internal short circuit protection circuitry, if the output is shorted to ground. It is denoted by I_{SC.}
14. Slew Rate
Slew rate is defined as the maximum rate of change of output voltage per unit of time and is expressed in volts per microseconds. In equation,
SR = dV_{o}/dt | maximum V/µs
Slew rate indicates how rapidly the output of an op-amp can change in response to changes in the input frequency.
15. Gain Bandwidth Product
Gain bandwidth product (GB) is the bandwidth of the op-amp when the voltage gain is 1.
For 741 op-amp GB is 1MHz. GB is also known as closed-loop bandwidth, unity gain bandwidth and small signal bandwidth.
16. Supply Voltages
It is defined as voltage applied to op-amp from power supply. Some op-amps use dual power supply and some use a single polarity supply voltage.
The dual supply voltage is typically ±15V while a single polarity supply is typically +12V or +15V.
17. Supply Current
It is defined as current drawn by the op-amp from the power supply.
Ideal Op-amp Electrical Characteristics
An ideal op-amp would exhibit the following electrical characteristics.
- Infinite input resistance (R_{i}=∞) so that almost any signal source can drive it and there is no loading of the preceding stage.
- Zero output resistance (R_{O}=∞) so that the output can drive an infinite number of other devices.
- Infinite voltage gain (A= ∞).
- Zero offset voltage (V_{io}=0) so that zero output voltage when input voltage is zero.
- Infinite bandwidth (BW=∞) so that any frequency signal from 0 to ∞ Hz can be amplified without attenuation.
- Infinite common-mode rejection ratio (CMRR=∞) so that the output common-mode noise voltage is zero.
- Zero Supply Voltage Rejection Ratio (SVRR=0) so that output voltage does not change due to fluctuation in supply voltage.
- Infinite slew rate (SR=∞) so that output voltage changes occur simultaneously with input voltage changes.
Ideal and Typical Values of IC LM741
Ideal characteristics of Op-amps are the expected values from any Op-amp. Typical values are practical values because ideal parameters never exist in reality. Practically a good op-amp should have minimum value, if ideal value is zero and should have very very high value of parameter if ideal value is infinite.
Sr. No. | Characteristics | Ideal Value | Typical Value |
1 | Input Resistance | ∞ | 2MΩ |
2 | Output resistance | 0 | 75Ω |
3 | Voltage gain | ∞ | 200000 |
4 | Bandwidth | ∞ | 1 MHz |
5 | CMRR | ∞ | 90 dB |
6 | Slew rate | ∞ | 0.5 V/µs |
7 | Offset voltage | 0 | 2 mV |
8 | SVRR/ PSRR | 0 | 30 µV/V |
To know more about the parameters of Op-amp, try to read datasheet of IC 741.
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