Current Sources and Voltage References

Modified current sources and example applications

Using BJTs to Create Current Sources

117

increases the capacitor’s voltage to the level set by V

REF

. Thus, it works like a voltage

regulator or an AGC circuit.

Transistors Q

3

and Q

4

are part of the Q

1

and Q

2

mirror, but are configured as a cur-

rent doubler (times two), because the two currents I

M2

and I

M3

are combined into

I

M4

, which flows through transistors Q

3

and Q

4

. The two currents are required for

symmetrical operation on AC signals. The circuit is probably best built using two

quad arrays for the current mirrors and two dual op amps. Amplifiers A

1

and A

3

should both be quality precision types, while ...