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I am using this circuit to measure the voltage of a small solar panel - around 3 or 4 V max (so instead of the photodiode consider there is a solar panel). That means Vin = -3 V. When I first built it, I didn't think about the fact that the op-amp is single supply and what effects that might have. Today, I was thinking about it and after reading some things, came to the conclusion that a single supply op-amp should not accept voltages outside its rail-to-rail range. I have 2 questions:

  1. Why is the anode of the panel connected to ground and not the other way around? (I based this circuit on one I saw online)

  2. Why does this circuit work? The output is exactly what I expected it to be in the first place (-0.65*Vin), but it shouldn't, because it's a single supply op-amp, right?

Thanks

schematic

simulate this circuit – Schematic created using CircuitLab

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The current produced by the photodiode in the presence of light is a reverse current not a forward current. This means current flows from R1 thru D1 to ground.

This means that to keep both input pins of the op-amp at the same voltage (e.g. 0V), the op-amp output has to rise above 0V to feed current into R2 thus keeping -Vin and +Vin at precisely the same potential\$^1\$. As more light hits the photodiode the op-amp output has to rise higher.

If the photodiode were the other way round photo-current would be traveling into R2 and this means the op-amp output has to provide negative voltage and, of course it can do this when it has a negative supply.

I'm assuming you are using rail-to-rail op-amps in this analysis.

Same problem with solar cells - you need the left hand end of R1 to be pulled negative by the solar cell for this to work.

\$^1\$ If you don't understand why -Vin = +Vin then this is another question and is unrelated to photo-diodes or solar cells.

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  • \$\begingroup\$ Thanks for your answer, it was very useful. I guess I was a bit confused because of what I read somewhere else. So to clarify: when someone says that a single supply op-amp can't take negative inputs (if supply is positive), it means the actual input at -Vin, and not an input before R1, for example? \$\endgroup\$ – user43606 May 28 '14 at 11:08
  • \$\begingroup\$ correct. Only the actual inputs need to be "inside" the power rails but there are exceptions. \$\endgroup\$ – Andy aka May 28 '14 at 11:18
  • \$\begingroup\$ What exceptions do you have in mind, by the way? \$\endgroup\$ – Nicolas D May 29 '14 at 7:56
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    \$\begingroup\$ @NicolasD some op-amps will allow inputs to be lower than the most negative rail or higher than the most positive rail. It's usually by only a few hundred milli volts. Also, some op-amps don't like the two inputs to be driven apart by more than 0.7V because of inbuilt protection diodes. Always read the small print!! \$\endgroup\$ – Andy aka May 29 '14 at 8:00
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  1. So a negative voltage will be presented to the input of the opamp.

  2. Since the circuit is an inverting amplifier, its gain is defined as:

            R2
     -Av = ---- = -0.65,
            R1 
    

and with a negative input, then, the output voltage will be:

      Vout = Vin * Av = -3V * -0.65 = 1.95V

This is possible because the opamp's output will slew in whatever direction it has to in order to force the voltage on the inverting (-) input to be the same as the voltage on the non-inverting (+) input.

With the + input at zero volts, then, the voltage divider feeding the - input looks like this:

     -3V Vin
      | 
    [2M0] R1
      |
      +--V2--> 0V TO OPAMP - IN
      |
    [1M3] R2
      |
     Vout

and, since the current into the opamp's - input will be miniscule, we can determine the value of Vout by first finding the current through R1 with Ohm's law:

         Vin - V2    -3V
    I = ---------- = ----- = -1.5 microamperes   
           R1        2MR

or, current into V1.

Similarly, since the current in a series circuit is everywhere equal, we'll have 1.5 microamperes out of Vout, through R1 and R2 and into Vin, and R2 will drop:

    Vout = I R2 = 1.5 microamperes * 1.3 megohms = 1.95 volts, which is exactly what you got.
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