THE CURRENT REGULATOR IN Figure 1 boasts a very high current stability of around 1.7% within a temperature range that spans -40°C to 125°C. It can be used in precise current and voltage regulators, oscillators, and amplifiers.
For its operation, the device utilizes a unique composition of thermal coefficients of bipolar transistors and Schottky diodes, which efficiently compensate for each other, as well as a very high feedback loop gain employing a JFET.
Zener diode D1 in Figure 1a, as well as resistor R2 in Figure 1b, are just the loads for the very stable current sunk by transistor Q1. These are used for demonstration and simulation. Transistors Q1 and Q2 form a well-known configuration of a current stabilizer. For good matching, Schottky diode D2 is placed in series with the currentsetting resistor R1.
Transistor Q2 employs a p-channel JFET (J1) as a load. This JFET features a very high dynamic resistance, especially in the presence of resistor R4 in its source. Thus, it provides a very high loop gain, which supports output current stability.
Capacitor C1 and resistor R3 phase-compensate the load of Q2, which removes unnecessary bouncing of the output current.
Figure 2a depicts an LTspice simulation of the current regulator within the temperature range from -40°C to 125°C at a 9-V power supply turn on and thereafter. Figure 2b is the simulation at a 16-V power supply. Comparison of these plots shows that the input voltage regulation is significant— around 14.3 µs.
Simulation of the circuit in Figure 1b is plotted in Figure 3. It does not reveal any difference in the load current compared to the case of the Zener diode in Figure 1, although the loads have different voltage drops and impedances.
It is crucial to have a Schottky diode for D2 to achieve maximum output current stability. The model of a diode may affect the output current, but it will keep a high stability anyway.