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The standing current in Q14/Q20 will be a factor exp(100 mV mm/ V T) ≈ 36 smaller than the 1 mA quiescent current in the class A portion of the op amp. This (small) standing current in the output transistors establishes the output stage in class AB operation and reduces the crossover distortion of this stage.
Representative schematic of a current-feedback op-amp or amplifier. The current-feedback operational amplifier (CFOA or CFA) is a type of electronic amplifier whose inverting input is sensitive to current, rather than to voltage as in a conventional voltage-feedback operational amplifier (VFA).
Class-A amplifiers may be used in output stages of op-amps [11] (although the accuracy of the bias in low cost op-amps such as the "741" may result in class A or class AB or class B performance, varying from device to device or with temperature). They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers.
Consequently, an op-amp makes a sloppy comparator with propagation delays that can be as long as tens of microseconds. Since op-amps do not have any internal hysteresis, an external hysteresis network is always necessary for slow moving input signals. The quiescent current specification of an op-amp is valid only when the feedback is active.
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Referring to the above diagram, if the op-amp is assumed to be ideal, then the voltage at the inverting (-) input is held equal to the voltage at the non-inverting (+) input as a virtual ground. The input voltage passes a current V in / R 1 {\displaystyle V_{\text{in}}/{R_{1}}} through the resistor producing a compensating current flow through ...
Figure 5: Op-amp differential amplifier. An operational amplifier, or op-amp, is a differential amplifier with very high differential-mode gain, very high input impedance, and low output impedance. An op-amp differential amplifier can be built with predictable and stable gain by applying negative feedback (Figure 5).
For transistors, the current-gain–bandwidth product is known as the f T or transition frequency. [4] [5] It is calculated from the low-frequency (a few kilohertz) current gain under specified test conditions, and the cutoff frequency at which the current gain drops by 3 decibels (70% amplitude); the product of these two values can be thought of as the frequency at which the current gain ...