Basic FET amplifier circuit

Chapter Basic FET amplifier circuit

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • fraction of a microfarad, down to picofarads at the highest frequencies and impedances.The exact values are determined by the design engineers, working to optimize circuit per-formance in the lab.The resistor values likewise depend on the application. Typical values are R1 470Ω, R2 4.7 KΩ, R3 10KΩ, and R4 4.7 KΩ for a weak-signal, broadband am-plifier.If the circuit is used as a power amplifier, such as in a radio transmitter or a stereohi-fi amplifier, the values of the resistors will be different. It might be necessary to biasthe base negatively with respect to the emitter, using a second power supply with a volt-age negative with respect to ground.Basic FET amplifier circuitIn Fig. 24-3, an N-channel JFET is hooked up as a common-source amplifier. The inputsignal passes through C2 to the gate. Resistor R2 provides the bias. Resistor R1 and ca-pacitor C1 give the source a dc voltage relative to ground, while grounding it for ac sig-nals. The ac output signal goes through capacitor C3. Resistor R3 keeps the ac outputsignal from being short-circuited through the power supply.438 Amplifiers24-3An amplifier using an FET. Component designators and valuesare discussed in the text.Concerning the values of the capacitors, the same considerations apply for this am-plifier, as apply in the bipolar circuit. A JFET amplifier almost always has a high inputimpedance, and therefore the value of C2 will usually be small. If the device is a MOS-FET, the input impedance is even higher, and C2 will be smaller yet, sometimes as littleas 1 pF or less.