# AC Analysis of FET Circuits

## Frequency Response of FET Amplifier

Frequency Response of FET Amplifier: Low-Frequency Response – The low Frequency Response of FET Amplifier circuits is determined by exactly the same considerations as for BJT circuits. The lower cutoff frequency is normally set by a source bypass capacitor, and it can be affected by coupling capacitors. High Frequency Response: Unlike BJTs, a device cutoff …

## FET and BJT Difference

FET and BJT Difference: FET and BJT Difference (CS, CD, and CG Circuit Comparison) – Table 11-1 compares Zi, Zo and Av for CS, CD, and CG circuits. As already discussed, the CS circuit has voltage gain, high input impedance, high output impedance, and a 180° phase shift from input to output. The CD circuit has high …

## Common Gate Circuit

Common Gate Circuit: The FET Common Gate Circuit (CG) shown in Fig. 11-19 uses voltage divider bias. The ac output is taken from the drain terminal, and an external load (RL) is capacitor-coupled to the drain, exactly as in the case of a common-source circuit. Unlike a CS circuit, the ac input for the CG …

## Common Drain Amplifier Circuit Diagram

Common Drain Amplifier Circuit Diagram: The FET Common Drain Amplifier Circuit Diagram shown in Fig. 11-14 has the output voltage developed across the source resistor (RS). The external load (RL) is capacitor-coupled to the source terminal of the FET, and the gate bias voltage (VG) is derived from VDD by means of voltage divider resistors …

## FET Common Source Amplifier with Unbypassed Source Resistors

FET Common Source Amplifier with Unbypassed Source Resistors: Equivalent Circuit – When an unbypassed source resistor (RS) is present in a FET Common Source Amplifier circuit, as shown in Fig.11-10(a), it also appears in the ac equivalent circuit, [Fig. 11-10(b)]. In the complete equivalent circuit RS must be shown connected between the FET source terminal …

## Common Source Circuit Analysis

Common Source Circuit Analysis: A FET Common Source Circuit Analysis is shown in Fig. 11-­6. With the capacitors treated as ac short-circuits, the circuit input terminals are the gate and source, and the output terminals are the drain and the source. So, the source terminal is common to both input and output, and the circuit …

## FET Equivalent Circuit Model

FET Equivalent Circuit Model: The complete FET Equivalent Circuit Model is shown in Fig. 11-5(a). It is seen that tilt source terminal is common to both input and out, so this is a common-source equivalent circuit. Resistor RGS between the gate and source terminals is the resistance of the reverse-biased gate-source junction, and Cgs is …

## Coupling Capacitors

Coupling Capacitors: Coupling Capacitors are required at a circuit input to couple a signal source to the circuit without affecting the bias conditions. Similarly, loads are capacitor-coupled to the circuit output to avoid the change in bias conditions produced by direct coupling. Input and output Coupling Capacitors (C1 and C3) and are shown in the FET circuit …

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