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Field Effect Tube amplifier circuit design

release time:2022-03-17Author source:SlkorBrowse:1151

1. What factors should be considered when measuring the input resistance of the FET.


DA) Due to the high input resistance of the FET, generally up to 10M or more, it is necessary to use a high internal resistance meter to measure, pay attention to preventing electrostatic breakdown of the grid, and pay attention to isolation of external signals to prevent induced signals parasitic in the measurement loop, affecting the measurement.


2. Why is the coupling capacitor at the input end of the FET generally only about 0.02uf?


DA) The size of the coupling capacitor is related to the output and input resistance. Because the input resistance of the FET is high, it is generally enough to use 0.01-0.02uF (this is very similar to the electronic tube in the 1950s)


3. Why is the magnification of the FET amplifier generally not as large as the voltage magnification of the transistor?


DA) The general triode is a current-type amplifier, so Hfe (β) is used as the unit of current control current, while the FET is a voltage-type amplifier, and the indicator it uses is Gm (transconductance). ) is the unit of voltage-controlled current.

What are the advantages and disadvantages of field effect transistor amplifier circuits

The advantages of the FET amplifier circuit are: (1) The input resistance is large.


Using ordinary triodes to make an amplifier circuit, the input resistance of the common emitter circuit is about a few KΩ (we generally call it 10^3 level), and the input resistance of the common collector circuit can only be tens of KΩ to more than one hundred. K ohms (level 10^5), and the use of junction field effect transistors (JFETs) can achieve input resistance of level 10^6, and the use of MOS tubes can achieve levels above 10^8.


(2) The temperature stability is good. Since there is no drift current in the FET, it is basically not affected by temperature changes.


Disadvantages: (1) The magnification is small, and the first-level amplification can only be achieved several times (maybe less than 10 times), (2) The input terminal is prone to breakdown due to electrostatic induction.


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