Question 10.7: Objective: Design a generalized current mirror to meet a set...
Objective: Design a generalized current mirror to meet a set of specifications.
Specifications: The circuit to be designed has the configuration shown in Figure 10.15. The bias voltages are V^{+} = +5 V and V^{−} = −5 V. Neglect base currents and assume V_{B E} = V_{E B} = 0.6 V. Design the circuit such that I_{O2} = 400 µA. Determine the other currents and find the value for R_{1}.
For I_{O2} = 400 µA, we have
I_{REF} = I_{O1} = I_{O3} = 200 µA and I_{O4} = 600 µA
Resistor R_{1} is
R_{1} = \frac{V^{+} − V_{E B}(Q_{R1}) − V_{B E} (Q_{R2}) − V^{−}}{I_{REF}} = \frac{5 − 0.6 − 0.6 − (−5)}{0.2}
or
R_{1} = 44 k \Omega
Trade-offs: Base currents were neglected in this ideal design. Including the effects of base currents (a finite β) will change the current and resistor values slightly.
Comment: If the load and reference currents are to be within a factor of approximately four of each other, it is more efficient, from an IC point of view, to adjust the B–E areas of the transistors to achieve the specified currents rather than use the Widlar current source with its additional resistors.
Design Pointer: This example demonstrates that a single reference current can be used to induce multiple load currents, which can be used to bias various stages of a complex circuit. We will see specific examples of this technique in Chapter 13 when we consider actual operational amplifier circuits.