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Compensated Precision Cascode Source

IP.com Disclosure Number: IPCOM000061491D
Original Publication Date: 1986-Aug-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 46K

Publishing Venue

IBM

Related People

Bond, PW: AUTHOR [+3]

Abstract

This circuit provides compensation for the base current losses in the upper stage of a multiple, variable cascode current source. The compensation is by means of a current loss sense resistor placed in the primary reference device of the cascode current source. Fig. 1 shows a cascode circuit which is an example of the prior art. Q4 and Q2 comprise the cascode current source with Q3 and Q1 as the reference devices, whereby I01 has a defined relationship to I. High gain amplifier A forces Q3 to absorb the input current, I. The base current error of Q3 is eliminated by the feedback action of amplifier A. As long as additional devices match Q3, Q1 and R0, the input current I can be replicated many times, as shown at I01 ... I0n . R0 = R1 ... Rn .

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Compensated Precision Cascode Source

This circuit provides compensation for the base current losses in the upper stage of a multiple, variable cascode current source. The compensation is by means of a current loss sense resistor placed in the primary reference device of the cascode current source. Fig. 1 shows a cascode circuit which is an example of the prior art. Q4 and Q2 comprise the cascode current source with Q3 and Q1 as the reference devices, whereby I01 has a defined relationship to I. High gain amplifier A forces Q3 to absorb the input current, I. The base current error of Q3 is eliminated by the feedback action of amplifier A. As long as additional devices match Q3, Q1 and R0, the input current I can be replicated many times, as shown at I01 ... I0n . R0 = R1 ... Rn . Q4 may be replaced by a complex arrangement such as a current switch in a current digital-to-analog converter or a multiplier cell in a multiplier circuit (not shown). Additional series cascode devices may be employed. The circuit of Fig. 2 eliminates the complex amplifier through the use of device Q5 and resistor RC . RC has an ohmic value of R1, as do R0 ... Rn . Essentially, the resistor RC increases the base voltage, and hence the emitter voltage of Q2 sufficiently to compensate for the base current loss of cascode device Q4 . We may write VE1 = (I + Ib)R, where the base current of Q5 is assumed to be negligibly small. To do the first order, VE2 = (I + Ib)R + VBE1 + IbR - VBE2 = (I +...