14.1. Current-Source Bias for the Differential Amplifier Stage

The drain-current bias resistor, R_bias, of the basic differential amplifier stage as introduced in Fig. 8.1 will now be replaced by a current source as shown in the circuit of Fig. 14.1. The bias resistor is replaced by two transistors, M₁₀ and M₁₁, and one resistor, R₁. This seems like an unfavorable exchange but it is not. This is because the reference voltage V_GS10 formed by the reference circuit of R₁ and M₁₀ can be used for additional current sources in the complete circuit, thereby eliminating the need for a number of other bias resistors.

Figure 14.1. Differential amplifier stage with bias resistor, R_bias (Fig. 13.1), replaced with a current source. Bias circuit consists of R₁, M₁₀, and M₁₁.