Introduction#
In Chapter Basic amplification: CS stage, we have learned that a properly biased CS stage can provide a large available power gain. We studied the noise behavior, the dynamic behavior and the nonlinear behavior of this basic amplifier stage.
We have seen that the noise addition of the stage can be minimized through optimization of the device geometry and the operating current.
After studying the small-signal dynamic behavior of the stage driven from and loaded with an \(R || C\) network, we found that if the parasitic feedback capacitance \(c_{dg}\) is much smaller than the total input capacitance and the total output capacitance, the product of the poles equals the product of the eigenfrequencies of both \(RC\) networks, while, if the stage has a high low-frequency voltage gain, the parasitic feedback capacitance largely influences the sum of the poles.
The nonlinear behavior of the CS stage is governed by the voltage dependency of the active capacitance, the current dependency of the transconductance factor and the voltage dependency of the output impedance.
Aside from these performance limitations, the CS stage does not show natural two-port behavior because the input port and the output port share one terminal.
In Chapter Balancing techniques, we introduced balancing techniques such as complementary-parallel connection and anti-series connection of CS stages. Application of these techniques yields odd transfer functions with a reduced sensitivity for even order effects, such as, threshold voltage mismatch and drift. In addition, anti-series connected CS stages provide improved port isolation, while complementary-parallel connections provide a current drive capability that exceeds its quiescent operating current. We found that application of balancing techniques does not result in a significant change of the small-signal dynamic behavior, the gain accuracy, the odd nonlinearity and the noise behavior.
Local feedback amplifier stages#
Negative feedback is considered a much more powerful error reduction technique than compensation. Negative feedback uses a controller to minimize the error between actual transfer and the desired transfer of an amplifiers. Through application of negative feedback, the dependency of the source-to-load transfer from the controller properties can theoretically be reduced to zero. This will be the case if the controller is a natural two-port with an infinite available power gain. The nullor is the network concept of the ideal controller.
In this chapter, we will study the design of so-called local feedback amplifier stages. These stages are negative feedback amplifiers of which the controller is implemented with a single transistor CS stage, or its balanced version. Fig. 448 shows the implementation of a nullor with NMOS or PMOS CS stages or with a complementary-parallel CS stage. Fig. 449 shows the implementation of the nullor with anti-series connected NMOS or PMOS CS stages. In both figures, the transistors are assumed to be biased; their bias sources have been omitted.
This chapter#
In this chapter, we will discuss design and the behavior of local feedback amplifier stages. Local feedback amplifier stages can be applied as single-stage amplifiers or as amplifier stages in multiple-stage feedback amplifiers.
Prerequisite knowledge#
The design of negative feedback amplifiers using operational amplifiers as controllers has been described in Chapter Design of feedback amplifier configurations\ to Chapter Frequency compensation.
The synthesis of feedback amplifier configurations
The modeling of feedback circuits
The relation between the controller performance and the amplifier performance
Frequency compensation techniques
The reader is assumed to have a clear understanding of these topics.
This chapter#
In section Direct feedback stages, we will discuss direct feedback stages. In section Indirect feedback stages, we will discuss feedback stages that use indirect feedback techniques. Section Application of balancing will be devoted to the design of amplifier stages using both balancing and feedback.