Engineering Notebook page for LM334 current source into LM3900
Consolidated Data Table of Rset vs. Vbe+ vs. Iin+
This circuit design was a test to insert a precision current into the LM3900, bypassing the effect of the Vbe normally present at the inputs to the LM3900, which is otherwise affected by voltage-to-current conversion when using input resistors.
By use of a programming resistor Rset across a programmable LM334 current source, a current into the LM3900 +Input can be used to set a voltage output at a specific level, based on the value of the negative feedback resistor tied from the output to the LM3900 -Input. Because of the LM3900 current mirror at the +Input, the feedback current into the -Input is very closely related to that into the +Input. This test verified very good current ratio compliance accross a current range of > 25:1.
The test also showed that the current mirror always archieves a current ratio of very slightly >1.0, and that ratio over temperature never is less than 1.0. This charactertistic is also noted in the LM3900 data sheet. However, the LM334 current was computed from it's datasheet formula, but not directly measured. Whereas the LM3900 feedback current was precisely measured. Considering earlier current and Vbe measurements conducted with the LM3900, the current mirror at the +Input has previously shown actually considerably better than 1.0 ratios, perhaps well under 1% difference. It's likely then that the feedback current into the -Input more correctly represents the LM334 current value into the +Input.
Use of an ultra-precise 0.1% tolerance feedback resistor, combined with voltage measurements from a 6.5 digit DMM enabled very accurate accounting of the feedback current. Providing that the supply voltage is larger than needed for output compliance, the LM334 is otherwise insensitive to the voltage level in producing the output current, due to it's internal band-gap reference voltage.
By use of a programming resistor Rset across a programmable LM334 current source, a current into the LM3900 +Input can be used to set a voltage output at a specific level, based on the value of the negative feedback resistor tied from the output to the LM3900 -Input. Because of the LM3900 current mirror at the +Input, the feedback current into the -Input is very closely related to that into the +Input. This test verified very good current ratio compliance accross a current range of > 25:1.
The test also showed that the current mirror always archieves a current ratio of very slightly >1.0, and that ratio over temperature never is less than 1.0. This charactertistic is also noted in the LM3900 data sheet. However, the LM334 current was computed from it's datasheet formula, but not directly measured. Whereas the LM3900 feedback current was precisely measured. Considering earlier current and Vbe measurements conducted with the LM3900, the current mirror at the +Input has previously shown actually considerably better than 1.0 ratios, perhaps well under 1% difference. It's likely then that the feedback current into the -Input more correctly represents the LM334 current value into the +Input.
Use of an ultra-precise 0.1% tolerance feedback resistor, combined with voltage measurements from a 6.5 digit DMM enabled very accurate accounting of the feedback current. Providing that the supply voltage is larger than needed for output compliance, the LM334 is otherwise insensitive to the voltage level in producing the output current, due to it's internal band-gap reference voltage.
In addition, the temperature coefficient of the LM334 is very nearly opposite that of the Vbe at the LM3900 inputs. So, the effect here is that the LM334 can be programmed to offer a current into the +Input which then becomes an output voltage, and due to canceling temperature coefficients, that output voltage will remain steady without regard to any change in Vbe at the -Input. This effect may prove quite useful, where the LM334 provides the reference current for a LM3900 antilog generator.
The schematic and raw data are shown in the Engineering Notes page, above and a Data Table spreadsheet with "cleaned" data follows that.
The schematic and raw data are shown in the Engineering Notes page, above and a Data Table spreadsheet with "cleaned" data follows that.
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