Amplifier Differential from Web

Section G3: Differential Amplifiers

The differential amplifier may be implemented using BJTs or FETs and is a commonly used building block in analog IC design. We are going to be concentrating on the BJT implementation of the differential pair as emitter-coupled, common-emitter (or emitter-resistor) amplifiers. In addition to providing the input stage of the operational amplifier, this configuration is the foundation for emitter-coupled logic (ECL), a very high speed, universally employed family of logic circuits. The simplest form of the differential amplifier is shown to the right (a modified version of Figure 9.1 in your text). This circuit is formed using two matched transistors (Q1 and Q2), in the CE configuration whose emitters have been tied together. The differential pair has two inputs (v1 and v2), and three possible outputs (vo1, vo2, and vout). A couple of notes about this circuit: The output voltage, vout, is the difference between vo1 and vo2, or v out = v o1 − v o2 . The notation of the emitter resistor RE has been changed to REE since is common to both amplifiers. As we will see shortly, this resistor may be the equivalent resistance of a current source as discussed in the previous section, both to save chip space and reduce fabrication complexity (transistors are smaller than resistors and it’s easier to make a whole bunch of the same thing). However it is created, it is necessary that REE have a large value to keep the voltage drop across it nearly constant with reasonable changes in current. The resistors RC in the circuit above may be replaced with other transistors in some applications (once again, to remove the requirement of resistive components). It is absolutely imperative however, that whatever is in the collector circuit ensures that Q1 and Q2 never enter saturation. To solve for the dc transfer characteristics of the differential amplifier, we begin by using the expression for base-emitter voltage derived from

Equation 4.10. In terms...