MOSFET Amplifiers: Small-Signal Analysis and Equivalent Circuits | Study notes Basic Electronics

Ch. 6 Basic FET Amplifiers

In the last chapter, we described the operation of the FET, in particular the MOSFET, and analyzed and

designed the dc response of circuits containing these devices. In this chapter, we emphasize the use of FETs

in linear amplifier applications. Although a major use of MOSFETs is in digital applications, they are also

used in linear amplifier circuits.

There are three basic configurations of single-stage or single-transistor FET amplifiers. These are the

common-source, source-follower, and common-gate configurations. We investigate the characteristics of

each configuration and show how these properties are used in various applications. Since MOSFET

integrated circuit amplifiers normally use MOSFETs as load devices instead of resistors because of their

small size, we introduce the technique of using MOSFET enhancement or depletion devices as loads. These

three configurations form the building blocks for more complex amplifiers, so gaining a good understanding

of these three amplifier circuits is an important goal of this chapter.

In integrated circuit systems, amplifiers are usually connected in series or cascade, forming a multistage

configuration, to increase the overall voltage gain, or to provide a particular combination of voltage gain and

output resistance. We consider a few of the many possible multistage configurations, to introduce the

analysis methods required for such circuits, as well as their properties.

6.1 THE MOSFET AMPLIFIER

In Chapter 4, we discussed the reasons linear amplifiers are necessary in analog electronic systems. In this

chapter, we continue the analysis and design of linear amplifiers that use field-effect transistors as the

amplifying device. The term small signal means that we can linearize the ac equivalent circuit. We will

define what is meant by small signal in the case of MOSFET circuits. The term linear amplifiers means that

we can use superposition so that the dc analysis and ac analysis of the circuits can be performed separately

and the total response is the sum of the two individual responses.

The mechanism with which MOSFET circuits amplify small time-varying signals was introduced in the last

chapter. In this section, we will expand that discussion using the graphical technique, dc load line, and ac

load line. In the process, we will develop the various small-signal parameters of linear circuits and the

corresponding equivalent circuits.

EE 329 Introduction to Electronics 282

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Ch. 6 Basic FET Amplifiers In the last chapter, we described the operation of the FET, in particular the MOSFET, and analyzed and designed the dc response of circuits containing these devices. In this chapter, we emphasize the use of FETs in linear amplifier applications. Although a major use of MOSFETs is in digital applications, they are also used in linear amplifier circuits. There are three basic configurations of single-stage or single-transistor FET amplifiers. These are the common-source, source-follower, and common-gate configurations. We investigate the characteristics of each configuration and show how these properties are used in various applications. Since MOSFET integrated circuit amplifiers normally use MOSFETs as load devices instead of resistors because of their small size, we introduce the technique of using MOSFET enhancement or depletion devices as loads. These three configurations form the building blocks for more complex amplifiers, so gaining a good understanding of these three amplifier circuits is an important goal of this chapter. In integrated circuit systems, amplifiers are usually connected in series or cascade, forming a multistage configuration, to increase the overall voltage gain, or to provide a particular combination of voltage gain and output resistance. We consider a few of the many possible multistage configurations, to introduce the analysis methods required for such circuits, as well as their properties. 6.1 THE MOSFET AMPLIFIER In Chapter 4, we discussed the reasons linear amplifiers are necessary in analog electronic systems. In this chapter, we continue the analysis and design of linear amplifiers that use field-effect transistors as the amplifying device. The term small signal means that we can linearize the ac equivalent circuit. We will define what is meant by small signal in the case of MOSFET circuits. The term linear amplifiers means that we can use superposition so that the dc analysis and ac analysis of the circuits can be performed separately and the total response is the sum of the two individual responses. The mechanism with which MOSFET circuits amplify small time-varying signals was introduced in the last chapter. In this section, we will expand that discussion using the graphical technique, dc load line, and ac load line. In the process, we will develop the various small-signal parameters of linear circuits and the corresponding equivalent circuits.

There are four possible equivalent circuits that can he used. These are listed in Table 4.3 of Chapter 4. The most common equivalent circuit that is used for the FET amplifiers is the transconductance amplifier, in which the input signal is a voltage and the output signal is a current. .

and the instantaneous drain current and drain-to-source voltage must also be confined to the saturation region. Transistor Parameters

source is assumed to be constant, the sinusoidal current produces no sinusoidal voltage component across this element. The equivalent ac impedance is therefore zero, or a short circuit. Consequently, in the ac equivalent circuit, the dc voltage sources are equal to zero. We say that the node connecting RD and VDD is at signal ground. 6.1.2 Small-Signal Equivalent Circuit Now that we have the ac equivalent circuit for the NMOS amplifier circuit, (Figure 6.4), we must develop a small-signal equivalent circuit for the transistor. Initially, we assume that the signal frequency is sufficiently low so that any capacitance at the gate terminal can be neglected. The input to the gate thus appears as an open circuit, or an infinite resistance. Eq. 6. relates the small-signal drain current to the small-signal input voltage and Eq. 6.7 shows that the transconductance is a function of the Q-point. The resulting simplified small-signal equivalent circuit for the NMOS device is shown in Figure 6.5. (The phasor components are in parentheses.)

This small-signal equivalent circuit can also he expanded to take into account the finite output resistance of a MOSFET biased in the saturation region. This effect, discussed in the previous chapter, is a result of the nonzero slope in the iD versus vDS curve. We know that The expanded small-signal equivalent circuit of the n-channel MOSFET is shown in Figure 6.6 in phasor notation.

Comment: Because of the relatively low value of transconductance, MOSFET circuits tend to have a lower small-signal voltage gain than comparable bipolar circuits. Also, the small-signal voltage gain contains a minus sign, which means that the sinusoidal output voltage is 180 degrees out of phase with respect to the input sinusoidal signal Problem-Solving Technique: MOSFET AC Analysis Since we are dealing with linear amplifiers, superposition applies, which means that we can perform the dc and ac analyses separately. The analysis of the MOSFET amplifier proceeds as follows:

Analyze the circuit with only the dc sources present. This solution is the dc or quiescent solution. The transistor must he biased in the saturation region in order to produce a linear amplifier.

Replace each element in the circuit with its small-signal model, which means replacing the transistor by its small-signal equivalent circuit.
Analyze the small-signal equivalent circuit, setting the dc source components equal to zero, to produce the response of the circuit to the time-varying input signals only. The previous discussion was for an n-channel MOSFET amplifier. The same basic analysts and equivalent circuit also applies to the p-channel transistor. Figure 6.8(a) shows a circuit containing a p-channel MOSFET. Note that the power supply voltage is connected to the source. (The subscript DD can be used to indicate that the supply is connected to the drain terminal Here, however, VDD, is simply the usual notation for the power supply voltage in MOSFET circuits.) Also note the change in current directions and voltage polarities compared to the circuit containing the NMOS transistor. Figure 6.8(b) shows the ac equivalent circuit, with the dc voltage sources replaced The final small-signal equivalent circuit of the p-channel MOSFET amplifier is shown in Figure 6.

source vi, is in series with an equivalent source resistance RSi. As we will see, RSi should be much less than the amplifier input resistance, Ri = R 1 || R 2 in order to minimize loading effects. Figure 6.14 shows the resulting small-signal equivalent circuit. The small signal variables, such as the input signal voltage Vi are given in phasor form.

The output voltage is

6.3.2 Common-Source Amplifier with Source Resistor A source resistor RS tends to stabilize the Q-point against variations in transistor parameters (Figure 6.18). If, for example, the value of the conduction parameter varies from one transistor to another, the Q-point will not vary as much if a source resistor is included in the circuit. However, as shown in the following example, a source resistor also reduces the signal gain. This same effect was observed in BJT circuits when an emitter resistor was included. The circuit in Figure 6.18 is an example of a situation in which the body effect (not discussed) should be taken into account. The substrate (not shown) would normally be connected to the -5 V supply, so that the

body and substrate terminals are not at the same potential. However, in the following example, we will neglect this effect.

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