This article discusses the three basic configurations of Field Effect Transistor (FET) amplifier—common-source, common-gate, and common-drain. It highlights their characteristics, circuit behaviors, and typical applications.
FET amplifiers play a crucial role in modern electronics, providing efficient signal amplification with high input impedance and low output capacitance. Understanding the various configurations of these amplifiers is key to designing circuits that meet specific performance requirements. This article explores the fundamental characteristics and applications of FET amplifier circuits, highlighting their importance in both low and high-frequency applications.
Common-Source JFET Amplifier
FET amplifiers, like their bipolar counterparts, can be connected in three different circuit configurations: common-source, common-gate, and common-drain. One lead of an FET is connected to the input, and a second lead is connected to the output. The third lead is commonly connected to both the input and output. This lead is used as a circuit reference point and is often connected to the circuit ground; hence, the terms grounded or common source, grounded or common gate, and grounded or common drain. The terms common and ground refer to the same type of connection.
The common-source amplifier is the most widely used FET circuit configuration. This circuit is similar in many respects to the common-emitter bipolar amplifier. The input signal is applied to the gate-source, and the output signal is taken from the drain-source. The source lead is common to both input and output.
Figure 1. Common-source JFET amplifier circuit diagram
Figure 1 shows a practical common-source amplifier. This circuit is self-biased. The device can be a JFET, a D-MOSFET, or an E-MOSFET. Circuit characteristics are very similar for all three devices. The signal processed by the common-source amplifier is applied to the gate-source. Self-biasing of the circuit is achieved by the source resistor $R_{2}$. This voltage establishes the static operating point. The incoming signal voltage is superimposed on the gate voltage. This causes the gate voltage to vary at an AC rate, which causes a corresponding change in drain current. The output voltage developed across the source and drain is inverted 180°.
Voltage gain ($A_{V}$) for a common-source JFET amplifier:
$$A_{V}=\frac{V_{DS}}{V_{GS}}$$
Typical $A_{V}$ values for a common-source FET amplifier circuit are 5–10. The input impedance is extremely high for nearly any signal source. One to several megohms is common. The output impedance ($Z_{out}$) is moderately high. Typical values are in the range of 2–10 k$\Omega$. $Z_{out}$ is dependent primarily on the value of $R_{L}$.
A common-source amplifier has very high input impedance and relatively high output impedance. Circuits of this type are extremely valuable as impedance-matching devices. Common-source amplifiers are used exclusively as voltage amplifiers. They respond well in radio-frequency signal applications.
Common-Gate JFET Amplifier
The common-gate amplifier is similar in many respects to the common-base bipolar transistor circuit. The input signal is applied to the source-gate, and the output appears across the drain-gate. Note that the gate is connected to the ground. The common-gate amplifier is capable of a rather significant amount of voltage gain. Current gain is not an important circuit consideration. The gate does not ordinarily have any current flow. JFETs, E-MOSFETs, and D-MOSFETs may all be used as common-gate amplifiers.
Figure 2. Common-gate JFET amplifier circuit diagram
Figure 2 shows a practical common-gate amplifier. This circuit employs an N-channel JFET. The operating voltages are the same as those of the common-source circuit. Self-biasing is achieved by the source resistor $R_{1}$. This voltage is used to establish the static operating point. An input signal is applied to $R_{1}$ through capacitor $C_{1}$. A variation in signal voltage causes a corresponding change in source voltage. Making the source more positive has the same effect on $I_{D}$ as making the gate more negative. The positive alternation of the input signal makes the source more positive. This, in turn, reduces drain current. With less $I_{D}$, there is a smaller voltage drop across the load resistor $R_{2}$. The drain or output voltage, therefore, swings positive. The negative alternation of the input reduces the source voltage by an equal amount. This is the same as making the gate less negative. As a result, $I_{D}$ is increased. The voltage drop across $R_{2}$ similarly increases. This, in turn, causes the drain or output to be less positive or negative going. The input signal is, therefore, in phase with the output signal.
The common-gate amplifier exhibits several unusual characteristics. Its voltage gain is somewhat less than that of the common-source amplifier. Representative values are 2–5. The common-gate circuit has very low input impedance ($Z_{in}$). The output impedance ($Z_{out}$) is rather moderate. Typical $Z_{in}$ values are 200–1500 $\Omega$, with $Z_{out}$ being 5–15 k$\Omega$. This type of circuit configuration is often used to amplify radio-frequency signals. Amplification levels are very stable for radio frequencies, without feedback between the input and output.
Common-Drain JFET Amplifier
A common-drain amplifier has the input signal applied to the gate and the output signal removed from the source. The drain is commonly connected to one side of the input and output. Common-drain amplifiers are also called source followers. This circuit has similar characteristics to those of the common-collector bipolar amplifier.
Figure 3 shows a practical common-drain amplifier using an N-channel JFET. The input of this amplifier is primarily the same as that of the common-source amplifier. The input impedance is, therefore, very high. $Z_{in}$ is determined largely by the value of $R_{1}$. The operating point of the amplifier is determined by the value of $R_{2}$. Essentially, this circuit has the same operating point as other circuit configurations. Resistor $R_{3}$ has been switched from the drain to the source in this circuit. Resistors $R_{2}$ and $R_{3}$ are combined to form the load resistance. The output impedance is based on this value.
Figure 3. Common-drain JFET amplifier circuit diagram
When an AC signal is applied to the input of the amplifier, it changes the gate voltage. The DC operating point is established by the value of source resistor $R_{2}$. The positive alternation of an input signal makes the gate less negative. This causes an N-channel device to be more conductive. With more current through $R_{3}$ and $R_{2}$, the source swings positive. The negative alternation of the input then makes the gate more negative. This action causes the channel to be less conductive. A smaller current, therefore, causes the source to swing negative. In effect, this means that the input and output voltage values of the amplifier are in step with each other. These signals are in phase in a common-drain amplifier.
Common-drain amplifiers are primarily used as impedance-matching devices. They have a high-input impedance and a low-output impedance. They are frequently used to match a high-impedance device to a low-impedance load. This type of circuit is capable of handling a high-input signal level without causing distortion. The input impedance places a minimum load on the signal source. Common-drain amplifiers are used frequently to match high-impedance devices such as microphones and phonograph pickups to the input of an audio amplifier.
Review Questions
- In a common-________ amplifier, the input is applied to the gate-source, and the output is developed across the source-drain.
- The input and output signals of a common-source amplifier are inverted by ________ degrees.
- The input signal of a common-________ amplifier is applied to the source-gate, and the output appears across the drain-gate.
- The input signal of a common-________ amplifier is applied to the gate, and the output signal is removed from the source.
- JFET amplifiers are considered to be ________ sensitive devices.
- When a common-source JFET circuit is in cutoff, $I_{D}$ is at _______, and $V{DS}$ is at ________.
Answers
- source
- 180
- gate
- drain
- voltage
- 0 A, supply $V_{DD}$
Key Takeaways
Understanding the different FET amplifier configurations—common-source, common-gate, and common-drain—is essential for designing effective analog circuits. Each FET amplifier configuration offers unique characteristics in terms of impedance, gain, and signal phase, making them suitable for specific applications like voltage amplification, impedance matching, and RF signal processing.