the following:• Design, analyze, and evaluate different types of
MOSFET constant-current sources for biasing
MOSFET amplifiers.• Design, analyze, and evaluate different types of BJT
constant-current sources for biasing BJT amplifiers.• Describe and analyze the characteristics of differential amplifiers and their DC and small-signal
characteristics.• Identify the parameters influencing the differential and
the common-mode gains of differential amplifiers.• Analyze and evaluate cascode-connected transistors
in order to obtain higher differential voltage gains.
Symbols and Their Meanings
Symbol Meaning
Ad, Ac Small-signal differential and commonmode voltage gains
CMRR Common-mode rejection ratio of an amplifier
vid, vic Small-signal differential and commonmode signal
vo1, vo2 Small-signal output voltages due to input
voltages at inverting and noninverting
terminals
ro1, ro2 Small-signal output resistances of transistors
VM, VA MOSFET channel modulation and BJT Early voltages
Vt, Vp Threshold voltage of enhancement MOSFET and pinch-off voltage of
depletion MOSFET
Content:
+IntroductionVM, VA MOSFET channel modulation and BJT Early voltages
Vt, Vp Threshold voltage of enhancement MOSFET and pinch-off voltage of
depletion MOSFET
Content:
+Internal Structure of Differential Amplifiers
+MOSFET Current Sources
+MOS Differential Amplifiers
+Depletion MOS Differential Amplifiers
+BJT Current Sources
+BJT Differential Amplifiers
+BiCMOS Differential Amplifiers
+Frequency Response of Differential Amplifiers
+
Design of Differential Amplifiers
Detail:
1/Introduction:
Differential amplifiers are commonly used as an input stage in various types of analog ICs, such as operational amplifiers, voltage comparators, voltage regulators, video amplifiers, power amplifiers, and balanced modulators and demodulators
This chapter covers the operation, analysis, and characteristics of differential amplifiers using BJTs and
MOSFETs. It also covers active current sources and voltage sources.
This chapter covers the operation, analysis, and characteristics of differential amplifiers using BJTs and
MOSFETs. It also covers active current sources and voltage sources.
2/Internal Structure of Differential Amplifiers :
A differential amplifier acts as an input stage; its output voltage is proportional to the difference between
its two input voltages v1 and v2. It has a high voltage gain and is directly DC coupled to the input voltages and the load. As we will see later in this chapter, the voltage gain of a differential amplifier depends
directly on the output resistance of the current source acting as an active load.
b/Internal Structure of Differential Amplifiers
its two input voltages v1 and v2. It has a high voltage gain and is directly DC coupled to the input voltages and the load. As we will see later in this chapter, the voltage gain of a differential amplifier depends
directly on the output resistance of the current source acting as an active load.
a/Characteristics of Differential Amplifiers
The differential stage can be represented by an equivalent amplifier, as shown in Fig. 9.1(a). If the two input
voltages are equal, a differential amplifier gives an output voltage of almost zero. Its voltage gain is very
large, so the input voltage is low, typically less than 50 mV. Thus, we can consider the input voltages as
small signals with zero DC components. That is, vG1 = vg1 and vG2 = vg2
voltages are equal, a differential amplifier gives an output voltage of almost zero. Its voltage gain is very
large, so the input voltage is low, typically less than 50 mV. Thus, we can consider the input voltages as
small signals with zero DC components. That is, vG1 = vg1 and vG2 = vg2
Note:
■ A differential amplifier consists of an active biasing circuit, an active load, and a differential transistor pair.■ The performance of a differential amplifier is measured by a differential gain Ad that occurs in response
to a differential voltage between two input terminals, a common-mode gain Ac that occurs in response
to a voltage common to both input terminals, and a common-mode rejection ratio CMRR.■ The CMRR is the ratio of the differential gain to the common-mode gain, and it is a measure of the
ability of an amplifier to amplify the differential signal and reject common-mode signals.
to a differential voltage between two input terminals, a common-mode gain Ac that occurs in response
to a voltage common to both input terminals, and a common-mode rejection ratio CMRR.■ The CMRR is the ratio of the differential gain to the common-mode gain, and it is a measure of the
ability of an amplifier to amplify the differential signal and reject common-mode signals.
3/MOSFET Current Sources
we saw the effects of active current sources in increasing the voltage gain of an
amplifier. Transistor current sources are widely used in analog ICs both as biasing elements and as loads
for amplifying stages. Current sources are less sensitive to variations in DC power supply and temperature.
A current source can be designed by using either MOSFETs or BJTs
amplifier. Transistor current sources are widely used in analog ICs both as biasing elements and as loads
for amplifying stages. Current sources are less sensitive to variations in DC power supply and temperature.
MOSFET current sources are analogous to BJT current sources
a/Basic Current Source
b/Cascode Current Source
c/Wilson Current Source
d/Design of Active Current Sources
The specifications for designing a current source will include the output current IQ, the output resistanceRo, and the DC supply voltage VDD. The design sequence is as follows:Step 1. Determine the design specifications: output current and output resistance.Step 2. Decide on the type of device to use—either BJTs or MOSFETs.Step 3. Choose the circuit topology best suited to the specifications. Use simple transistor models
for hand analysis to find the circuit-level solution, including component values and specifications of
BJTs or MOSFETs.Step 4. Use the standard values of components—for example, R1 5.6 M 5% instead of 5.72 M ,R2 30 k 5% instead of 29.3 k , and R3 27 k 5% instead of 27.5 k . Evaluate your
design and modify the values, if necessary.Step 5. Use PSpice/SPICE verification, employing complex circuit models to calculate the worstcase results due to component and parameter variations. Modify your design, if necessary
The specifications for designing a current source will include the output current IQ, the output resistanceRo, and the DC supply voltage VDD. The design sequence is as follows:Step 1. Determine the design specifications: output current and output resistance.Step 2. Decide on the type of device to use—either BJTs or MOSFETs.Step 3. Choose the circuit topology best suited to the specifications. Use simple transistor models
for hand analysis to find the circuit-level solution, including component values and specifications of
BJTs or MOSFETs.Step 4. Use the standard values of components—for example, R1 5.6 M 5% instead of 5.72 M ,R2 30 k 5% instead of 29.3 k , and R3 27 k 5% instead of 27.5 k . Evaluate your
design and modify the values, if necessary.Step 5. Use PSpice/SPICE verification, employing complex circuit models to calculate the worstcase results due to component and parameter variations. Modify your design, if necessary
Note:
■ For the same gate voltage, the drain current depends on the W⁄L ratio; thus, a low current can be
obtained by selecting an appropriate W⁄L ratio.■ For the same drain current, drain gate–shorted MOSFETs—for example, M3 and M4 in Fig. 9.6(a)—can
be used as a voltage divider network to generate biasing voltages of different magnitudes.■ The output resistances for different MOSFET sources are summarized as follows: output resistance of
MOSFET M1, ro1; basic source, ro1; multiple source, ro1; cascode source,
Wilson source, .■ Since MOSFETs do not draw any gate current, there is no need for base current compensation as there
is with BJTs. BJT sources have some advantages over MOSFET sources, such as a wider compliance
range and a higher output resistance. However, a higher output resistance can be obtained by cascodelike connections of MOSFETs.
■ For the same gate voltage, the drain current depends on the W⁄L ratio; thus, a low current can be
obtained by selecting an appropriate W⁄L ratio.■ For the same drain current, drain gate–shorted MOSFETs—for example, M3 and M4 in Fig. 9.6(a)—can
be used as a voltage divider network to generate biasing voltages of different magnitudes.■ The output resistances for different MOSFET sources are summarized as follows: output resistance of
MOSFET M1, ro1; basic source, ro1; multiple source, ro1; cascode source,
Wilson source, .■ Since MOSFETs do not draw any gate current, there is no need for base current compensation as there
is with BJTs. BJT sources have some advantages over MOSFET sources, such as a wider compliance
range and a higher output resistance. However, a higher output resistance can be obtained by cascodelike connections of MOSFETs.
4/MOS Differential Amplifiers
a/NMOS Differential Pair
a/NMOS Differential Pair
b/MOS Differential Pair with Active Load
c/Cascoded MOS Differential Amplifier
Note:
■ A MOS amplifier exhibits a linear DC characteristic and has a very high input resistance, tending
to infinity.■ It is relatively easy to connect MOS transistors in cascode form in order to control the drain current
and give high output resistance. A high-voltage gain can be obtained with a cascode connection.
to infinity.■ It is relatively easy to connect MOS transistors in cascode form in order to control the drain current
and give high output resistance. A high-voltage gain can be obtained with a cascode connection.
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