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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.
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.
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). The CFA was invented by David Nelson at Comlinear ...
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.
The operating point of a device, also known as bias point, quiescent point, or Q-point, is the DC voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied. A bias circuit is a portion of the device's circuit that supplies this steady current or voltage.
[3] [4] In the case of the ideal op-amp, with A OL infinite and Z dif infinite, the input impedance is also infinite. In this case, though, the circuit will be susceptible to input bias current drift because of the mismatch between the impedances driving the V + and V − op-amp inputs. The feedback loop similarly decreases the output impedance:
Figure 6: Differential amplifier with non-ideal op-amp: input bias current and differential input impedance. In case the operational amplifier's (non-ideal) input bias current or differential input impedance are a significant effect, one can select a feedback network that improves the effect of common-mode input signal and bias.
Optimal quiescent current in an amplifier using complementary feedback pairs also tends to be much lower than in Darlington-based output stages, on the order of 10 mA vs 100 mA or more for some emitter follower output stages.