# EMF Difference Amplification and Resistance

Discussion in 'General Electronics Chat' started by Tupinambis, Apr 29, 2014.

1. ### Tupinambis Thread Starter New Member

Jan 23, 2013
15
0
I would like to understand the effects, if any, of a solenoid's DC resistance on a difference amplifier circuit like the one shown below.

A greater number of turns on the solenoid will produce a larger EMF given the same applied magnetic field which in turn produces a greater output voltage. An apparent consequence of having more turns on the solenoid is a greater DC resistance. From how I understand this circuit to work, the DC resistance really shouldn't affect the circuit's behavior because the primary driver is the induced EMF across the coil. That is to say, the increase in DC resistance won't somehow counteract the advantage of having an increased EMF.

I feel that even if the current through the solenoid is affected by the increased resistance it shouldn't impede the output voltage as the output voltage should only be proportional to the induced EMF and the amplifier gain (10x in this case).

If this an accurate analysis of this situation or is there a point where I would have diminishing returns from a great DC resistance of the solenoid?

Thank you for your consideration!

2. ### crutschow Expert

Mar 14, 2008
13,508
3,385
The EMF is the open circuit output voltage but the solenoid resistance would reduce this open circuit voltage by the ratio of the 1kΩ amplifier input resistance to the solenoid resistance (use a Thevenin equivalent of the solenoid to see this effect). So you'd have to determine whether the net gain from increased turns is offset by the increase in solenoid resistance.

You can reduce this resistance effect by increasing the input resistance of the amplifier.

Why are you using a difference amplifier? Can't you just ground the solenoid and use a standard op amp amplifier circuit? A non-inverting op amp amplifier would have a very high input resistance, greatly reducing the effect of the solenoid resistance.

Tupinambis likes this.
3. ### Tupinambis Thread Starter New Member

Jan 23, 2013
15
0
I will test out the increase in amplifier input resistance for a start.

The reason I am considering a difference amplifier is due to the need for a non-zero DC output voltage. I guess if I wanted to go with a non-inverting configuration but still need a non-zero DC output I could do something like this.

Would similar issues arise due to the solenoid's connection with R4 and R3? I presume the small amount of input current through the solenoid might cause an offset at Vout but would this be remedied by adding a resistance of similar magnitude as the solenoid feeding into the inverting terminal of the op amp?

4. ### crutschow Expert

Mar 14, 2008
13,508
3,385
The op amp has a high input impedance so essentially no current will flow through the solenoid. This current is unaffected by the values of resistors R3 and R4 so they will have negligible effect on the solenoid voltage.