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Bandgap Vernier Trim

IP.com Disclosure Number: IPCOM000042303D
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 15K

Publishing Venue

IBM

Related People

Rae, JW: AUTHOR [+2]

Abstract

The figure shows the schematic diagram of the physical implementation of the bandgap vernier trim scheme. The bandgap voltage reference is shown on the left side of the figure. It is made up of transistors T1, T2, T3 and T4, resistors R1, R2, R3 and R3', and amplifier AMP1. Resistor R1 is chosen to select the magnitude of the nominally equal currents i1 and i2. Resistor R2 is then chosen so that the relation (R1/R2) = 7.64 holds. Resistors R3 and R3' are of equal value and are used to sense the currents i1 and i2. The voltages across R3 and R3' are proportional to i1 and i2, respectively. Amplifier AMP1 senses the voltage difference across R3 and R3', which is proportional to the current difference between i1 and i2. AMP1 therefore forces the voltage at the base of T1, T2, T3, and T4 to a voltage where i1 is equal to i2.

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Bandgap Vernier Trim

The figure shows the schematic diagram of the physical implementation of the bandgap vernier trim scheme. The bandgap voltage reference is shown on the left side of the figure. It is made up of transistors T1, T2, T3 and T4, resistors R1, R2, R3 and R3', and amplifier AMP1. Resistor R1 is chosen to select the magnitude of the nominally equal currents i1 and i2. Resistor R2 is then chosen so that the relation (R1/R2) = 7.64 holds. Resistors R3 and R3' are of equal value and are used to sense the currents i1 and i2. The voltages across R3 and R3' are proportional to i1 and i2, respectively. Amplifier AMP1 senses the voltage difference across R3 and R3', which is proportional to the current difference between i1 and i2. AMP1 therefore forces the voltage at the base of T1, T2, T3, and T4 to a voltage where i1 is equal to i2. With i1=i2, R1/R2 = 7.64, and this circuit configuration, the voltage produced at the base of T1, T2, T3, and T4 is the bandgap voltage of silicon (Vgo). This voltage is temperature independent at 300 degrees Kelvin or room temperature, and there is only a small increase in the temperature coefficient as we stray from room temperature. This bandgap voltage (Vgo) is produced at the center of a resistive divider made up of resistors R4 and RT1. Resistor R4 is an integrated circuit resistor produced by diffusion or ion-implantation. Resistor RT1 is a resistor external to the integrated circuit but is fabricated on the substrate on which the integrated circuit chip is mounted and is a part of the final module assembly. Resistor RT1 is made up of either a thick film resistor material screened on the substrate or a thin film resistor material deposited on the substrate. The value of resistor RT1 can be altered by using a laser beam to cut away part of the resistor material. As more resistor material is cut away, the value of the resistor is increased. The resistance of the resistor can be made infinite by cutting all of the resistor material away with the laser. Reliability and repeatability concerns dictate that a certain amount of the resistor material must be left, and therefor the resistor value may be increased only a certain amount. The active trimmable resistor has a finite trim range. The minimum value of the resistor is obtained before any trimming takes place, and the maximum value is obtained after the maximum allowable amount of resistor material is cut away. Resistor RT1 is trimmed to a resistance value half way between the minimum and maximum obtainable values for the resistor. It will be noted that trimming resistor RT1 does not affect the value of the voltage at the bases of transistors T1, T2, T3, and T4. It does, however, change the voltage Vout at the output of amplifier AMP1. Note also that since resistors R4 and RT1 are of different composition and are generally even at different temperatures from each other, they have different temperature coefficients (i.e., they do not track...