Design of a Simple and Variable Constant-Current Source and Its Applications
Hiroshi SHIRAHAMA, Kiyonobu KUSANO
This paper proposes a simple and variable constant current source for teaching the concept of negative feedback (NFB) control in technical high schools. In addition, a detailed design of the circuit is described and a detailed analysis of its performance was conducted.
Devices used in the circuit are one Zener diode from which a reference voltage is derived, one bipolar junction transistor connected as an emitter follower to supply constant current higher than that available from an operational amplifier (OP amp) to load, and three OP amps which are a basic amplifier, a difference amplifier in a feedback network, and a voltage follower as a buffer which is used so as not to make a load current flow into a feedback network.
Features of the circuit are as follows: (1) Zero crossing type of Summing for reference signal and feedback signal is adopted, which makes the restriction of Common Mode Input Voltage for basic amplifier free; (2) Circuit configuration of an additional OP amp as a buffer into a feedback network provides a relatively constant current which is approximately independent of an external load resistance. (3) Load current can be set simply by adjusting only reference voltage.
Moreover, the permissible operating range of the circuit was analytically obtained and experimentally tested out, and a measurement of diodes and transistors characteristic was carried out as an application of the constant current source.
Consequently, this study clearly showed that the constant current source could be a valuable material in helping the students understand the concept of NFB control, it also allowed students to cultivate their abilities and apply it in introductory technology education.
Key words :constant current source, design, operational amplifier, negative feedback control, operating characteristics.