Friday, September 26, 2014
IC 741 Opamp Parameters Explained
It is important to know the typical parameters of the op-amp before trying it in a new application so as to get the best results.
Some important parameters and their significance are mentioned below: Open loop voltage gain Ao: This is the basic voltage gain of the internal circuitry as a whole, and may be expressed as a multiple or in decibels. A typical value of A0 for 741 is 1.00,000 or 100 dB.
Input impedance Zm: This is a measure of the impedance, looking into the input terminals of the op-amp, and is expressed in terms of resistance. A typical value of Zm for 741 is 1 megohm
Output imped¢mce—Zo: Tliis is a measure ot the impe- dance offered by the output terminals of the op-amp. It is also expressed in terms of resistance. A typical value of Zo for 741 is 150 ohms.
Input bias current-Ir: Op-amps utilising bipolar transistor input stages draw a small bias current from the input terminals. A typical value of Ib for 741 is 200 nA (200x 10^-9 amperes).
Input voltage range- Vi(max): This is the maximum value of input voltage that can be connected to the input without causing any damage. It is usually lower than the operating supply voltages. A typical value of Vi(max) for 741 is +/-13V (for the supply voltages +/-15V).
Output voltage range- Vo (max): If the op-amp is over- driven, its output will saturate and be limited by the operating supply voltages. So, Vo(max) is usually specified as being one or two volts less than Vs. A typical value of Vo(max) 741 is +/- 14V (for the supply voltages +/-15V).
Supply voltage range- Vs: Op—amps are usually operated from dual supplies that are anywhere between their minimum and maximum limits. lf the supply voltages are very » high, the op-amp may get damaged. lf they are very low, the op-amp may not work satisfactorily. Typical supply limits for 741 are +/- 6V to +/- 18V.
Differential input offset voltage- Vio: Due to slight imbalances within their input circuitry and due to high gain of the op-amp, it is observed that the output does not register zero voltage even when both inputs are grounded. This small imbalance voltage appearing in the input, which may even cause the output to swing to saturation, is known as the differential input offset voltage. A typical value of Vio for 741 is 2 mV. lt is often required in some particular. op-amp applications to nullify this offset voltage by using external offset nulling circuit, as shown in Fig. (m).
Common mode rejection ratio-CMRR: In an ideal op- amp, if an identical signal is applied to both the inputs, the resultant output is zero. However, in practical op-amps, common mode signals do not entirely cancel out and produce a small signal at the output terminals. The ability of the op-amp to reject common mode signals is usually expressed in terms of common mode rejection ratio. CMMR is the ramp of the op-amp gain with differential signals to the· op-—-amp gain with common mode signals. CMRR value for 741 is typically 90 dB.
Transition frequency-fr: This is the frequency at which the gain of the op-amp falls to unity, Though an op-amp has a very high gain at low frequencies, inthe interest of stability its open loop frequency response is tailored so that the gain falls off as the frequency rises. The gain falls to unity at a transition frequency known as fr. The value of fr for 741 is 1 — MHZ.
Some important parameters and their significance are mentioned below: Open loop voltage gain Ao: This is the basic voltage gain of the internal circuitry as a whole, and may be expressed as a multiple or in decibels. A typical value of A0 for 741 is 1.00,000 or 100 dB.
Input impedance Zm: This is a measure of the impedance, looking into the input terminals of the op-amp, and is expressed in terms of resistance. A typical value of Zm for 741 is 1 megohm
Output imped¢mce—Zo: Tliis is a measure ot the impe- dance offered by the output terminals of the op-amp. It is also expressed in terms of resistance. A typical value of Zo for 741 is 150 ohms.
Input bias current-Ir: Op-amps utilising bipolar transistor input stages draw a small bias current from the input terminals. A typical value of Ib for 741 is 200 nA (200x 10^-9 amperes).
Input voltage range- Vi(max): This is the maximum value of input voltage that can be connected to the input without causing any damage. It is usually lower than the operating supply voltages. A typical value of Vi(max) for 741 is +/-13V (for the supply voltages +/-15V).
Output voltage range- Vo (max): If the op-amp is over- driven, its output will saturate and be limited by the operating supply voltages. So, Vo(max) is usually specified as being one or two volts less than Vs. A typical value of Vo(max) 741 is +/- 14V (for the supply voltages +/-15V).
Supply voltage range- Vs: Op—amps are usually operated from dual supplies that are anywhere between their minimum and maximum limits. lf the supply voltages are very » high, the op-amp may get damaged. lf they are very low, the op-amp may not work satisfactorily. Typical supply limits for 741 are +/- 6V to +/- 18V.
Differential input offset voltage- Vio: Due to slight imbalances within their input circuitry and due to high gain of the op-amp, it is observed that the output does not register zero voltage even when both inputs are grounded. This small imbalance voltage appearing in the input, which may even cause the output to swing to saturation, is known as the differential input offset voltage. A typical value of Vio for 741 is 2 mV. lt is often required in some particular. op-amp applications to nullify this offset voltage by using external offset nulling circuit, as shown in Fig. (m).
Common mode rejection ratio-CMRR: In an ideal op- amp, if an identical signal is applied to both the inputs, the resultant output is zero. However, in practical op-amps, common mode signals do not entirely cancel out and produce a small signal at the output terminals. The ability of the op-amp to reject common mode signals is usually expressed in terms of common mode rejection ratio. CMMR is the ramp of the op-amp gain with differential signals to the· op-—-amp gain with common mode signals. CMRR value for 741 is typically 90 dB.
Transition frequency-fr: This is the frequency at which the gain of the op-amp falls to unity, Though an op-amp has a very high gain at low frequencies, inthe interest of stability its open loop frequency response is tailored so that the gain falls off as the frequency rises. The gain falls to unity at a transition frequency known as fr. The value of fr for 741 is 1 — MHZ.
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