**Constant Current Source in Differential Amplifier:**

An ideal current source provides a constant current regardless of the load connected to it. There are many applications in electronics for a circuit that provides a constant current with a very high impedance. Constant Current Source in Differential Amplifier can be built using FET devices, bipolar devices, and a combination of these components. There are circuits used in discrete form and others more suitable for operation in ICs. Some forms of both types will be considered here.

**JFET Constant Current Source:**

Figure 20.16(a) depicts a simple JFET current source. With V_{GS} set to zero volt, the drain current I_{D} is fixed at I_{DSS} = 10 mA. The device operates like a current source of magnitude 10 mA.

**BJT Constant Current Source:**

There are different methods of connecting BJTs in circuit that acts as a constant current source. Figure 20.16(b) depicts a circuit using an NPN transistor and a few resistors for operation as a constant current source in differential amplifier.

Constant current l_{C} is given by the following equation

where

assuming the base input impedance much larger than R_{1} or R_{2}.

**Transistor/Zener Constant Current Source:**

If resistor R_{2} is replaced with a zener diode in the circuit depicted in Fig. 20.16(b), an improved constant current source is obtained in Fig. 20.17.

The zener diode results in a constant current I given by the equation

The noteworthy point is that the constant current delivered by the circuit shown in Fig. 20.18 depends on the zener diode voltage, which remains quite constant and the emitter resistor R_{E}. The voltage supply V_{EE} has no effect on the magnitude of current I delivered by the circuit.