Electronics > Beginners

Need assistance in calculating resistors in a differential amplifier

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T3sl4co1l:
The transistor is always on, I fail to see what should happen here? ???

The traditional method to measure internal resistance is to apply an AC current, and measure the resulting AC voltage.  The circuit looks very different.  A typical implementation will be simultaneously more complicated (more components required), and simpler (consisting of only a current source and a voltmeter).  The source can be an oscillator and buffer amplifier or switch, and the voltmeter can be an AC coupled amplifier and precision rectifier, or a synchronous rectifier (analog switch), or...

Note that a SPICE "battery" is actually an ideal voltage source, i.e., Rs = 0 by definition.  You will need to find a model that describes a battery, to be able to test your circuit with something realistic.

Note also that, in general, the internal resistance varies with frequency, typically in a sqrt(f) manner (this is due to ionic diffusion within the battery).  This gives an impedance which is equal parts capacitive and resistive, so that it is difficult to speak in terms of resistance as such, even at very long time scales (which implies a large change in charge state, for which we of course expect a change in terminal voltage).  At shorter time scales (~kHz), it is capacitive (the capacitance of the battery plates themselves), and at still shorter times (~MHz), inductive, and so on (wiring stray inductance, radiation losses for the wiring as random antennas, etc.).

Tim

mitrynicolae:
Regarding the schematic, you can completely ignore the transistor and the battery. In fact let me add another simplified schematic. Now on this schematic, considering that the load resistor is disconnected and that the other resistors are not perfect (let's say 5% tolerance) where should I put the potentiometer in order fine tune the circuit and to obtain 2 volts on the output. I have tried several configurations (with the potentiometer in series with R3 or R1 but the output is not proportional with the input anymore.

rstofer:
That schematic looks a lot like a non-inverting amplifier.  Does this calculator help?

https://www.allaboutcircuits.com/tools/non-inverting-op-amp-resistor-calculator/

Without checking the datasheet, I don't know that the op amp can get within 2 volts of ground.

mitrynicolae:
Thanks for your help, but also the online calculator showed that the R1 should exactly match the R2 (according to their schematic). Basically if the resistors are not perfect then weird voltages will appear on the opamp output.

Zero999:
Don't use 5% tolerance resistors, when 1% is widely available and isn't expensive.

The only way to trim the gain, without adding another op-amp or affecting the offset is to set the gain slightly higher than required and add a potential divider to the input.

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