Temperature compensated logarithmic circuit
First Claim
1. A log circuit for producing an output signal proportional to the logarithm of an input signal and free from temperature induced variations, comprising:
- input means responsive to said input signal and including means to develop a first signal proportional to the product of (1) the logarithm of said input signal and (2) absolute temperature;
circuit means including a PTAT (proportional-to-absolute-temperature) source producing a second signal and coupled to said input means;
said circuit means further including means for modulating the magnitude of said second signal in accordance with said logarithm while effecting cancellation of the temperature-related factors of said two signals; and
output means for developing an output signal corresponding to said modulation of said second signal.
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Abstract
A log-amp or log-ratio circuit for producing a temperature-independent output signal corresponding to the logarithm of the ratio of a pair of input currents. The basic logarithm function is generated by a pair of opposed P-N junctions through which the respective input currents flow. Temperature compensation is effected by a circuit including a second pair of opposed P-N junctions which receive a PTAT current split between the junctions in accordance with a modulation factor proportional to the desired logarithmic function. The temperature-induced signal variations produced by the PTAT current source are equal and opposite to the temperature-induced signal variations produced in the first pair of P-N junctions, and a temperature-independent output signal is developed in accordance with the modulation factor applied to the PTAT current through the second pair of P-N junctions.
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Citations
18 Claims
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1. A log circuit for producing an output signal proportional to the logarithm of an input signal and free from temperature induced variations, comprising:
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input means responsive to said input signal and including means to develop a first signal proportional to the product of (1) the logarithm of said input signal and (2) absolute temperature; circuit means including a PTAT (proportional-to-absolute-temperature) source producing a second signal and coupled to said input means; said circuit means further including means for modulating the magnitude of said second signal in accordance with said logarithm while effecting cancellation of the temperature-related factors of said two signals; and output means for developing an output signal corresponding to said modulation of said second signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A balanced log ratio circuit comprising:
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A first pair of matched transistors with common emitters; the collectors of said transistors being connected to respective input terminals to receive input currents; an amplifier having its input coupled to one of said input terminals and its output connected to said common emitters; a pair of resistors each connected at one end to a respective base of said pair of transistors; the other ends of said resistors being connected together to a reference potential; a second pair of matched transistors having their collectors connected respectively to the bases of said first pair of transistors and their emitters connected together; a source of PTAT (proportional-to-absolute-temperature) current connected to said second pair of emitters to produce therethrough and through said respective resistors a flow of current modulated in accordance with the logarithm of the ratio of said input currents; and output means to develop an output signal in accordance with said modulation of said PTAT current. - View Dependent Claims (13, 14, 18)
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15. The method of developing a temperature-independent signal corresponding to the logarithm of an input signal, comprising:
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developing a first signal representing the product of (1) the logarithm of said input signal and (2) absolute temperature; developing a second signal from a PTAT (proportional-to-absolute-temperature) current source; combining said first and second signals; setting the magnitude of said second signal to provide for cancellation of the temperature-dependent factors of said two signals; modulating the magnitude of said second signal in accordance with the logarithm of the ratio between said input signal and a reference signal; and developing an output signal in accordance with the modulation of said second signal. - View Dependent Claims (16, 17)
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Specification