Multiple Current sources Using Current Mirrors
A number of current sources can be obtained from a single reference voltage. If the current is approximately the same as the reference voltage, the simple current source can be used as shown in fig. 4 for Q2 and Q3.
Fig. 4
Notice that Q4 has an emitter resistance, which makes the current source a Widlar current source. Thus the amount of current delivered by this source can be determined by the size of the emitter resistor. This type of circuit is useful in integrated circuit chips as the one reference circuit can be used to develop current sources throughout the chip. When using the Widlar circuit, the currents can be different from the reference current.
The errors in base current, however, do accumulate when multiple outputs are used and the current gain tends to deviate from unity. In these types of circuits, lateral transistors can be used since it is not important that b be large. Lateral transistors usually have a β of approximately 20 which is more than adequate for current sources.
Example -2:
For the circuit shown in fig. 5, determine the emitter current in transistor Q3. Given that β = 100, VBE = 0.715V.
Fig. 5
Solution:
Since all transistor are identical, there VBE voltage drop will be same.
Let IB be the base current of each transistor and IC be the collector current of Q1 and Q2.
Therefore,
