Temperature compensated logarithmic circuit

A balanced log ratio circuit comprising:

• 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.

The method of developing a temperature-independent signal corresponding to the logarithm of an input signal, comprising:

• 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.