Some times zener diode may be used in place of diodes and resistance as shown in fig. 3. Zeners are available over a wide range of voltages and can have matching temperature coefficient
The voltage at the base of transistor QB is
Fig. 3
The value of R2 is selected so that I2 » 1.2 IZ(min) where IZ is the minimum current required to cause the zener diode to conduct in the reverse region, that is to block the rated voltage VZ.
Current Mirror:
The circuit in which the output current is forced to equal the input current is said to be a current mirror circuit. Thus in a current mirror circuit, the output current is a mirror image of the input current. The current mirror circuit is shown in fig. 4.
Fig. 4
Once the current I2 is set up, the current IC3 is automatically established to be nearly equal to I2. The current mirror is a special case of constant current bias and the current mirror bias requires of constant current bias and therefore can be used to set up currents in differential amplifier stages. The current mirror bias requires fewer components than constant current bias circuits.
Since Q3 and Q4 are identical transistors the current and voltage are approximately same
For satisfactory operation two identical transistors are necessary.
