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# Active Voltage Divider

IP.com Disclosure Number: IPCOM000089455D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 21K

IBM

## Related People

Gersbach, JE: AUTHOR [+1]

## Abstract

The tolerance of the output voltage of a resistor divider can be improved by inserting diodes in series with the resistors according to the ratio of resistors. This technique has in the past been limited to integer ratios. The circuit shown above is capable of noninteger ratios. The following equations define the output voltage and bias current of this circuit. Vout=Vn + [(1 + R1/R2) (1 + R4/R3) - (N + R1/R2)]Vbe over 1 + R4/R3 i = Vn-(N + R1/R2)Vbe over R3 + R4

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Active Voltage Divider

The tolerance of the output voltage of a resistor divider can be improved by inserting diodes in series with the resistors according to the ratio of resistors. This technique has in the past been limited to integer ratios. The circuit shown above is capable of noninteger ratios. The following equations define the output voltage and bias current of this circuit. Vout=Vn + [(1 + R1/R2) (1 + R4/R3) - (N + R1/R2)]Vbe over 1 + R4/R3 i = Vn-(N + R1/R2)Vbe over R3 + R4

To make the output voltage independent of Vbe, the base-to-emitter voltage of transistor Tl, the term multiplied by Vbe is set equal to zero. This leads to the design equation: R1 = R2[R3/R4(N - 1) - 1] R3 and R4 are added to provide control of the bias current. They must be kept as small as possible but large enough to limit the max-to-min bias current ratio to a reasonable value. Their ratio must be the same as the voltage division ratio: R4 = R3(Vn/Vout-1)

To prevent tolerance degradation as a function of the current in the transistor, R5 can be added and made equal to R2 x (N-1) so that the current in the diodes remains the same as that in the transistor. Diodes may be added in series with R3 to limit the ratio of R1 to R2 to less than one. In some cases it would be desirable to interchange the positions of the transistor and R1 and R2 with one of the diodes below R4. Output voltage tolerance as a function of circuit component variations is improved by a factor of about 2.

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