Differential Amplifiers

Differential amplifiers are a fundamental configuration in electronics. Every op-amp has a differential amplifier as its core. Differential amplifiers also form the backbone of many communication circuits such as mixers and modulators. The main characteristics that make differential amplifiers so useful are their large gain, ability to reject noise, and the fact that they amplify the difference between two signals. Below, we first show how one can DC bias a diff-amp, and then demonstrate its AC voltage gain. An example configuration for a differential amplifier is shown in Fig. 5.1. In the figure, Q1 and Q2 form the core of the amplifier, while Q3 acts as a DC current source to bias the circuit. The input can be into the base of either Q1 or Q2, or both, with the small signal output taken either from the collector of Q1, Q2, or both. One important thing to notice is that since the bias is accomplished with Q3, the input signal does not have to be AC coupled through a capacitor like was necessary in the previous transistor voltage amplifier circuits we studied.

  

Figure 5.1: Differential Amplifier