RF Circuit Design, 2nd Edition

Chapter 13: Speciality of “Zero if” System

13.1 Why Differential Pair?

13.1.1 Superficial Difference between Single-Ended and Differential Pair

In the early years of electronic circuit development, only single-ended stages were studied and implemented. Gradually, it was found that some special features of performance could be achieved only by a differential pair instead of a single-ended stage or a double-balanced mixer. At present, both single-ended and differential pairs have been developed for most RF blocks, such as the LNA (low-noise amplifier), mixer, VCO (voltage-controlled oscillator), filter, and PA (power amplifier). In the recent years, a number of differential circuits have appeared in RF or RFIC designs, especially in “zero IF” or direct conversion communication systems.

Without the input and output impedance matching networks, a single-ended stage is shown in Figure 13.1, and a differential pair is shown in Figure 13.2.

Figure 13.1 Typical single-ended block. (a) Bipolar single-ended stage. (b) MOSFET single-ended block.

Figure 13.2 Typical differential pair. (a) Bipolar differential pair. (b) MOSFET differential pair.

The differential pair appears as two single-ended blocks joined together with a common emitter or source resistor, R_e or R_s, in a bipolar or a MOSFET ...