14.1. Current-Source Bias for the Differential Amplifier Stage
The drain-current bias resistor, Rbias, 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 VGS10 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, Rbias (Fig. 13.1), replaced with a current source. Bias circuit consists of R1, M10, and M11.
Resistors in an integrated circuit cost much more in chip space than do transistors. Also, the output resistance of a current-source transistor, M11, will be much higher than the resistor equivalent, leading to improved amplifier characteristics, as discussed below.
Following the discussion of transistor current sources in Unit 9, the ratio of the reference current and the current-source current is
Equation 14.1
The ratio of the gate widths will thus be selected to satisfy the design of the differential-stage bias. The current ratio will deviate from the simple ratio due to the λn terms, but this is not critical, as the design will not call for an exact value for ID1 ≈ ID2.