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Improved Absolute Value Voltage to Current Converter

IP.com Disclosure Number: IPCOM000060689D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Atallah, FI: AUTHOR [+2]

Abstract

This circuit converts the absolute value of an input voltage to a current. The circuit has many applications such as the input stage for a square-law circuit.

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Improved Absolute Value Voltage to Current Converter

This circuit converts the absolute value of an input voltage to a current. The circuit has many applications such as the input stage for a square-law circuit.

Fig. 1 shows an absolute value circuit which is a typical example of the prior art. When Vin is positive, the amplifier turns on PNP device Q2 in order to maintain the input of the amplifier near ground. If the current gain of Q2 is high, then the current through R, I = Vin/R, passes through Q2 to the output. For negative inputs, Q1 is turned on (Q2 off) by the amplifier, and the current through R, I = Vin/R, passes through Q1 and is reflected by the current mirror B to the output. The circuit suffers from a potential lack of symmetry between positive and negative input signals. The lack of symmetry is ca by the gain error of the current mirror B which can also vary with the current level adding a nonlinearity to the circuit.

The circuit of Fig. 2 eliminates the symmetry problem by eliminating the current mirror. Amplifiers A1 and A2 are high gain amplifiers; Q2 and Q3 are high current gain PNP transistors. For positive Vin, amplifier A2 maintains node B at ground by turning Q4 on (Q3 off). The input voltage Vin is impressed across R, and the current, I = Vin/R, flows through PNP Q4 to the output. For negative Vin, Q2 turns on and Q3 turns on. Vin is impressed across resistor R and the desired current, Vin/R, flows through Q2 to the output. (Note that mat...