The first stage of every op amp is a differential amplifier.
A differential amplifier has two inputs, an inverting (-) input and a noninverting (+) input.
A differential amplifier amplifies the difference between its two input signals.
A differential amplifier rejects or severely attenuates signals that are common to both inputs.
The common-mode rejection ratio (CMRR) is defined as the ratio of differential voltage gain, Ad, to common-mode voltage gain, ACM.
Op amps are high gain, direct-coupled, differential amplifiers.
The open-loop voltage gain of an op amp is its voltage gain without negative feedback.
When the output voltage of an op amp lies between ± Vsat, the differential input voltage, Vid is so small it can be considered zero.
The open-loop cutoff frequency is the frequency where the open-loop voltage gain of an op amp is down to 70.7% of its maximum value at dc.
The frequency where the open-loop voltage gain equals one is called funity.
The slew-rate specification of an op amp indicates how fast the output voltage can change. The slew rate is specified in V/μs.
The slew-rate distortion of a sine wave makes the output waveform appear triangular.
Most op amp circuits use negative feedback. With negative feedback, a portion of the output signal is fed back, 180° out of phase, to oppose the input signal.
Negative feedback stabilizes the voltage gain of an amplifier and improves the bandwidth and input and output impedances.
An inverting amplifier has a voltage gain, ACL, of -RF/Ri. The minus sign indicates that Vin and Vout are 180° out of phase.
A noninverting amplifier has a voltage gain, ACL, of RF/Ri + 1.
A voltage follower is a noninverting amplifier with a voltage gain of one or unity.
An op amp summing amplifier is a circuit whose output voltage is the negative sum of the input voltages.
An op amp differential amplifier is a circuit that amplifies the difference between two input voltages whose values may be several volts or more.
An active filter uses an op amp to provide voltage gain in addition to filtering.
A first-order, active filter uses only one reactive component. The output voltage of a first-order, active filter rolls off at the rate of 6 dB/octave beyond the cutoff frequency.
An op amp comparator is a circuit that compares the signal voltage on one input with a reference voltage on the other. A comparator uses no negative feedback and its output voltage is at either ±Vsat.
A Schmitt trigger is an op amp comparator that uses positive feedback to eliminate the erratic operation caused by undesired noise.
Op amps are often used in conjunction with diodes to rectify and filter small signals in the millivolt region.
