Browse Prior Art Database

Precision Current Source Circuit

IP.com Disclosure Number: IPCOM000085065D
Original Publication Date: 1976-Feb-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Sanford, JL: AUTHOR

Abstract

Precision current sources are used in digital-to-analog conversion circuits. In high-speed, digital-to-analog conversion circuits high-frequency power supply rejection may be important.

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Precision Current Source Circuit

Precision current sources are used in digital-to-analog conversion circuits. In high-speed, digital-to-analog conversion circuits high-frequency power supply rejection may be important.

Fig. 1 shows a current source using discrete components. Resistor R1 provides bias current for the zener diode D1 and base current for the transistor Q1. The voltage across the zener diode D1 forward biases the base-emitter junction of Q1 and creates an emitter current equal to: I(EQ1) = V(Z) - V(BEQ1) over R2.

If the base-collector junction of transistor Q1 is reversed biased, then the collector current I of Q1 is: I = I(CQ1) = (V(2) - V(BEQ1) B(Q1) over R2 (B(Q1) +
1).

To generate a precision current source by this circuit technique would require high-performance components. The components would be expensive. This circuit will exhibit fair low-frequency power supply rejection. A capacitor across the zener diode is required for high-frequency supply rejection.

Fig. 2 shows a more precise current source circuit. If feedback is assumed then the potential at node A is considered to be at a 'virtual ground' or approximately zero volts. Because of high-input impedance characteristics of most operational amplifiers, the collector current of Q1 is: I(CQ1) = I(REF) + V(REF) over R(REF).

Assuming R1 = R2, B(Q1) = QB(Q2), V(BE1) = V(BE2) then the current source I is equal to the collector current of transistor Q1. Feedback is created by connecting the bases of Q1 and Q2 to the...