ECEN $2714/2011-$ Laboratory

Semester Project

how changing the current source, I, proportionally varies the op$-$amp output voltage through $R_F.$ Since the op$-$amp has no output resistance, we can connect any load to it without changing the behavior!

The Goal: Our goal is to create a $0-5V$ TIA that operates on an input range of $I=[4$:$20]mA.$ In other words, if we set $I=4mA,$ then $v0=0V,$ and if we set $I=20mA,$ then $v0=5V.$ But the circuit above doesn't quite do that. If you analyze this circuit correctly, you'll find that when $I=20mA,$ then $v0=5V.$ But when $I=4mA,$ then $v0=1V,$ the same problem as the simple resistor.

We really have two problems. First, the circuit above isn't scaled correctly. Its full output voltage range is $4$ V $.$ We want an output range of $5$ V $.$ The second problem is the offset. In the circuit above, for $v0=0,$ the input current must also be zero. But we want a non$-$zero input of $I=4mA$ to produce a zero output of $v0=0.$ All of this means we need to make a circuit that corrects both scale and offset.

Your assignment is to modify the TIA shown above to fix the scaling$/$offset problems. You are permitted to:

Add$/$change$/$delete resistors,

Add other op$-$amps,

Change the topology,

Change the polarity of the current source.

In reality, circuit designs are also limited by economics and manufacturability. Therefore, you are not permitted to:

Change the power to the op$-$amp $($it must remain $+12$ V and $-12$ V $).$

Use any other power sources. You can only use the $+-12V$ voltage sources. Adding more voltages and sources gets expensive!

Use the $6$ V source on your lab bench as an auxiliary source for offsets $($for example$).$ See previous point.

We will use the $6$ V output of our benchtop power supply to simulate a sensor$,$ providing $I=[4$:$20]mA.$ The TA will provide instructions on how to use it this way. The circuit should be scaled for an input of $[4$:$20]$ mA$,$ so when you actually vary I from $[4$:$20]$ mA$,$ you should obtain a proportional output voltage $v0=[0$:$5]V.$ This should work equally for any reasonable load attached to your op$-$amp $($let$\text{'}$s say $1k-100k).$

Design and build your circuit using the components in your kit and then verify that its operation is correct. Create an accurate, fully labeled, neat, and legible schematic diagram $-$ it can be hand drawn.

$2$