Common source

In electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

, a common-source amplifier

Electronic amplifier

An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...

 is one of three basic single-stage field-effect transistor

Field-effect transistor

The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...

 (FET) amplifier topologies, typically used as a voltage or transconductance amplifier

Amplifier

Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

. The easiest way to tell if a FET is common source, common drain

Common drain

In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field effect transistor amplifier topologies, typically used as a voltage buffer. In this circuit the gate terminal of the transistor serves as the input, the source is the output, and the...

, or common gate

Common gate

In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a current buffer or voltage amplifier...

 is to examine where the signal enters and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit. The analogous bipolar junction transistor

Bipolar junction transistor

|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 circuit is the common-emitter amplifier

Common emitter

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor amplifier topologies, typically used as a voltage amplifier...

.

The common-source (CS) amplifier may be viewed as a transconductance amplifier or as a voltage amplifier. (See classification of amplifiers). As a transconductance amplifier, the input voltage is seen as modulating the current going to the load. As a voltage amplifier, input voltage modulates the amount of current flowing through the FET, changing the voltage across the output resistance according to Ohm's law

Ohm's law

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...

. However, the FET device's output resistance typically is not high enough for a reasonable transconductance amplifier (ideally infinite), nor low enough for a decent voltage amplifier (ideally zero). Another major drawback is the amplifier's limited high-frequency response. Therefore, in practice the output often is routed through either a voltage follower (common-drain or CD stage), or a current follower (common-gate or CG stage), to obtain more favorable output and frequency characteristics. The CS–CG combination is called a cascode

Cascode

The cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance,...

 amplifier.

Characteristics

At low frequencies and using a simplified hybrid-pi model

Hybrid-pi model

The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. The model can be quite accurate for low-frequency circuits and can easily be adapted for higher frequency circuits with the addition of appropriate...

, the following small-signal characteristics can be derived.

Bandwidth

The bandwidth of the common-source amplifier tends to be low, due to high capacitance resulting from the Miller effect

Miller effect

In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...

. The gate-drain capacitance is effectively multiplied by the factor , thus increasing the total input capacitance and lowering the overall bandwidth.

Figure 3 shows a MOSFET common-source amplifier with an active load

Active load

An active or dynamic load is a component or a circuit behaving as a current-stable nonlinear resistor. This term may refer to a component of circuit design, or to a type of test equipment.-Circuit design:...

. Figure 4 shows the corresponding small-signal circuit when a load resistor R_L is added at the output node and a Thévenin driver

Thévenin's theorem

In circuit theory, Thévenin's theorem for linear electrical networks states that any combination of voltage sources, current sources, and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem...

 of applied voltage V_A and series resistance R_A is added at the input node. The limitation on bandwidth in this circuit stems from the coupling of parasitic transistor capacitance

Parasitic capacitance

In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...

 C_gd between gate and drain and the series resistance of the source R_A. (There are other parasitic capacitances, but they are neglected here as they have only a secondary effect on bandwidth.)

Using Miller's theorem

Miller effect

In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...

, the circuit of Figure 4 is transformed to that of Figure 5, which shows the Miller capacitance C_M on the input side of the circuit. The size of C_M is decided by equating the current in the input circuit of Figure 5 through the Miller capacitance, say i_M , which is:

,

to the current drawn from the input by capacitor C_gd in Figure 4, namely jωC_gd v_GD. These two currents are the same, making the two circuits have the same input behavior, provided the Miller capacitance is given by:

.

Usually the frequency dependence of the gain v_D / v_G is unimportant for frequencies even somewhat above the corner frequency of the amplifier, which means a low-frequency hybrid-pi model

Hybrid-pi model

The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. The model can be quite accurate for low-frequency circuits and can easily be adapted for higher frequency circuits with the addition of appropriate...

 is accurate for determining v_D / v_G. This evaluation is Miller's approximation and provides the estimate (just set the capacitances to zero in Figure 5):

,

so the Miller capacitance is

.

The gain g_m (r_O//R_L) is large for large R_L, so even a small parasitic capacitance C_gd can become a large influence in the frequency response of the amplifier, and many circuit tricks are used to counteract this effect. One trick is to add a common-gate (current-follower) stage to make a cascode

Cascode

The cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance,...

 circuit. The current-follower stage presents a load to the common-source stage that is very small, namely the input resistance of the current follower (R_L ≈ 1 / g_m ≈ V_ov / (2I_D) ; see common gate

Common gate

In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a current buffer or voltage amplifier...

). Small R_L reduces C_M. The article on the common-emitter amplifier

Common emitter

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor amplifier topologies, typically used as a voltage amplifier...

 discusses other solutions to this problem.

Returning to Figure 5, the gate voltage is related to the input signal by voltage division as:

.

The bandwidth (also called the 3dB frequency) is the frequency where the signal drops to 1/ √ 2 of its low-frequency value. (In decibel

Decibel

The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

s, dB(√ 2) = 3.01 dB). A reduction to 1/ √ 2 occurs when ωC_M R_A = 1, making the input signal at this value of ω (call this value ω_3dB, say) v_G = V_A / (1+j). The magnitude of (1+j) = √ 2. As a result the 3dB frequency f_3dB = ω_3dB / (2π) is:

.

If the parasitic gate-to-source capacitance C_gs is included in the analysis, it simply is parallel with C_M, so

.

Notice that f_3dB becomes large if the source resistance R_A is small, so the Miller amplification of the capacitance has little effect upon the bandwidth for small R_A. This observation suggests another circuit trick to increase bandwidth: add a common-drain (voltage-follower) stage between the driver and the common-source stage so the Thévenin resistance of the combined driver plus voltage follower is less than the R_A of the original driver.

Examination of the output side of the circuit in Figure 2 enables the frequency dependence of the gain v_D / v_G to be found, providing a check that the low-frequency evaluation of the Miller capacitance is adequate for frequencies f even larger than f_3dB. (See article on pole splitting

Pole splitting

Pole splitting is a phenomenon exploited in some forms of frequency compensation used in an electronic amplifier. When a capacitor is introduced between the input and output sides of the amplifier with the intention of moving the pole lowest in frequency to lower frequencies, pole splitting causes...

to see how the output side of the circuit is handled.)

External links

• JFET Common Source Amplifier, Physics Lecture Notes, D.M. Gingrich, University of Alberta Department of Physics
• Common-Source Amplifier Stage