Documenting additional 2020 LM3900 work, a higher voltage reference was needed for part of the CMA circuit design. The purpose of this reference was to isolate the +15V power supply from being the voltage source from which a voltage drop is made by the op amp across the 374Ω current sensing resistor. So, the voltage reference needed to be lower than +15V, but also be of high enough that compliance from the current source would still allow ≈10Vpp signal output. And, the voltage reference needed to be temperature stable.
Upon thinking more about the +6V ±2% voltage reference, it became apparent that that the nVbe biasing technique from AN-72 Section 3.3 could be adapted further for a higher voltage reference. By looking up Zener diode temperature coefficients and comparing those with n-factors of Vbe temperature coefficients (from the LM3900 -input), various forms of near or total self-cancellation are possible. In this instance the temperature coefficient of a 12V 1N5225B Zener diode is approximately the opposite of 2Vbe. In the nVbe circuit, this results in a temperature stable reference voltage of ≈13V. Note that the circuit design does not eliminate the initial tolerance of the Zener diode (which for B-grade Zeners is ±2%). So, this circuit design relies on precision ratios (±tempcos), but not absolute values. However, having the reference voltage be bounded in value allows other circuit trimming ranges to be set to cover the tolerance.
The engineering notes covering the development of the +13V reference in the context of the CMA are shown below.
Comments
Post a Comment