Why is the input stage impedance of the preamplifier high?
The high input impedance means that the power absorbed by the circuit (or the output of the previous stage circuit) is low, and the power supply or the front stage can drive more load. For measurement circuits, such as electronic voltmeters, oscilloscopes, etc., high input impedance is required, so that the impact on the circuit under test is as small as possible after accessing the meter.
Improvement method: (1) FET, the input impedance is naturally high. (2) Use the bootstrap method to increase the input impedance. (3) Using a common-amplifier circuit, the input stages of the triode amplifier circuit are generally connected in a common mode.
Under ideal conditions, the voltage-driven rear-stage circuit draws only the voltage from the front stage. Without current, it does not draw power. For the front stage, it is almost no-load, so the larger the impedance, the easier it is to drive. In fact, the input impedance of the latter stage can only be close to infinity. For example, the vacuum tube or CMOS device input can achieve GΩ level, and draw current from the front stage and its tiny.
For example, the FET is a voltage-driven type, and the circuit composed of it is a voltage-driven circuit. Since its input impedance is so large that its input current can be neglected, power consumption is neglected;
The triode is a current-driven type, and the circuit composed of it is a current-driven type circuit because it needs to be injected with current to operate, and although its input impedance is relatively small, a certain power consumption is still generated.
The so-called input impedance mainly considers the power consumed by the circuit itself (can be understood as meaningless loss). For a voltage-driven circuit, the larger the impedance, the smaller the current, the smaller the P=I*I*R, and the smaller the current. In terms of the driving circuit, the smaller the impedance, the smaller the P=I*I*R, and the smaller the power consumption, so that for the latter circuit, more power can be output.
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