Analog Electronics with LabVIEW®

9.2. Current Source with Source Degeneration

It is normally desirable to have the output resistance of a current source as large as possible. It can be improved over the circuit of Fig. 9.1 by adding resistance in the source branch, as shown in Fig. 9.2, to establish source degeneration (negative feedback). In the discussion of the signal circuit, it is necessary to represent the diode-connected transistor with its linear equivalent, which is just a resistance of magnitude, 1/g_m. This can be seen by inspection of the circuit of Fig. 9.3. The voltage, V_d, is applied directly to the gate such that V_gs = V_d and the drain current is I_d = g_mV_d. Thus the resistance of the diode is just V_d/I_d = 1/g_m. The result is the same for the PMOS and NMOS as ...