Parameters of Operational Amplifier (Common Mode Gain, CMRR, SVRR, Slew Rate & Thermal Drift)

Operational amplifiers have several key parameters that define their performance. These include output offset voltage, which affects the accuracy of the output signal; input offset voltage and current, which influence how closely inputs match; and input bias current, critical for understanding circuit behavior. Additionally, common mode gain measures how well an op-amp can reject signals present on both inputs simultaneously while maintaining a high common mode rejection ratio is essential for effective noise reduction in applications.

Output offset voltage occurs in operational amplifiers when there is a non-zero output despite zero input. Ideally, with both the non-inverting and inverting inputs grounded, the output should be zero since there's no difference between the two signals. However, due to imperfections and mismatched components within differential amplifier stages of an op-amp circuit, some voltage appears at the output even when it shouldn't. This discrepancy leads to what is known as output offset voltage.

Input offset voltage is the necessary voltage applied to achieve zero output when the input is grounded. Ideally, with both inputs of a differential amplifier grounded, the output should be zero; however, practical scenarios yield a small output due to imperfections in the amplifier's two stages. To counteract this unwanted output and attain true nullification at the output, an input offset voltage must be introduced either at V_in+ or V_in-. This adjustment ensures that any deviation from ideal conditions can be corrected for accurate performance.

Input offset current is the difference between currents flowing into the inverting and non-inverting terminals of an operational amplifier (OPM). Ideally, this difference should be zero due to infinite input resistance, which would prevent any current from flowing. However, since practical OPMs do not have infinite input resistance, a small amount of current does flow at the inputs. This results in a measurable input offset current that deviates from ideal conditions.

Input bias current is defined as the average of the currents flowing into the inverting (I-) and non-inverting (I+) terminals of an operational amplifier, expressed mathematically as (I+ + I-)/2. Ideally, this value should be zero due to infinite input impedance; however, practical limitations prevent achieving infinite impedance. Consequently, there will always be some measurable values for I+ and I-, leading to a non-zero average input bias current.

Common mode gain refers to the output voltage divided by the common input voltage applied to both inverting and non-inverting terminals of an operational amplifier (op-amp). Ideally, this gain should be zero because when a common signal is applied, there is no difference between the inputs. However, due to practical limitations and imperfections in op-amps, some output will still occur despite having a common input signal.

Common Mode Rejection Ratio (CMRR) is defined as the ratio of differential gain to common mode gain. Differential gain measures how much output voltage changes in response to a difference between input signals, while common mode gain assesses the effect of identical signals applied at both inputs. Ideally, differential gain should be infinite and CMRR should be maximized for optimal performance. In practical scenarios, CMRR can also be calculated using input offset voltage divided by common mode voltage.

Supply Voltage Rejection Ratio (SVRR) measures how changes in supply voltage affect an operational amplifier's input offset voltage. It is defined as the ratio of differential gain to common mode gain, indicating sensitivity to power supply variations. Different manufacturers may refer to this concept using terms like Power Supply Rejection Ratio (PSRR) or Power Supply Sensitivity (PSS), but they all essentially describe SVRR, which quantifies the change in input offset voltage relative to fluctuations in supply voltages.

Thermal drift refers to the changes in operational amplifier parameters such as input offset voltage, input offset current, and input bias current due to variations in temperature. Ideally, these parameters should remain constant; however, they fluctuate with temperature changes, supply variations, and even over time as components age. To quantify thermal drift: thermal voltage drift measures how much the input offset voltage varies with temperature change; similarly for currents. The goal is to minimize this variation—ideally achieving zero—with acceptable values expressed in microvolts per degree Celsius for voltages and picamperes per degree Celsius for currents.

Slew rate measures the maximum change in output voltage over time, typically expressed in volts per microsecond. It indicates how quickly an operational amplifier (op-amp) can respond to input signals. For example, when a square wave is applied as input, the op-amp's output should ideally mirror this waveform but may take time to reach its peak value due to slew rate limitations. A higher slew rate results in faster response times for changes from high to low and vice versa; thus, an ideal scenario would have infinite slew rates for immediate signal reaction.