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I am setting up a high sensitivity light detection circuit and came across this thread just a bit ago:

Phototransistor transimpedance amplifier

Within it was a response with this image: enter image description here

My question is this: Should the phototransistor in the diagram above be connected to the true ground coming from the power supply or to the virtual ground?

Thank you and I know this may seem like a newbie question and perhaps similarly asked in other threads, but it doesnt seem to have been explaind in my opinion.

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First of all the phototransistor seems to be drawn upside down, the emitter should be at the negative side.

In your circuit, the inverting input is held at the virtual ground potential by the feedback loop. In order for the output of the opamp to be made to go positive, current must be drawn from the circuit node to which the inverting input is connected. This will cause the inverting input to tend to go negative and the output voltage will rise, causing current to flow (to the left) through the feedback components thereby restoring the voltage at the inverting input terminal. In order for this to happen the phototransistor must be connected to a more negative potential than the virtual earth. The "true ground" would be a suitable point. If it were returned to the virtual ground, then there would be no potential difference across the transistor and the circuit would not work.

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It has to be tied to your true ground, not virtual ground. The virtual ground is used by your opamp

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COMPLETE REWRITE:

I kinda cheated last time by using floating voltage sources as the opamp inputs.

Here's something a little closer to the truth, using the phototransistors in optocouplers.

Both ways work, (grounded and pseudogrounded) but the grounded version has a lot more gain as evidenced by the difference in the LED ballast resistors.

enter image description here

Version 4
SHEET 1 3416 680
WIRE 1696 -176 1584 -176
WIRE 1920 -176 1760 -176
WIRE 2672 -176 2560 -176
WIRE 2896 -176 2736 -176
WIRE 1584 -64 1584 -176
WIRE 1696 -64 1584 -64
WIRE 1920 -64 1920 -176
WIRE 1920 -64 1776 -64
WIRE 2560 -64 2560 -176
WIRE 2672 -64 2560 -64
WIRE 2896 -64 2896 -176
WIRE 2896 -64 2752 -64
WIRE 1712 80 1680 80
WIRE 1824 80 1824 64
WIRE 1824 80 1792 80
WIRE 2688 80 2656 80
WIRE 2800 80 2800 64
WIRE 2800 80 2768 80
WIRE 1824 128 1824 80
WIRE 2800 128 2800 80
WIRE 1232 144 1168 144
WIRE 1360 144 1312 144
WIRE 1584 144 1584 -64
WIRE 1584 144 1552 144
WIRE 1792 144 1584 144
WIRE 2208 144 2144 144
WIRE 2336 144 2288 144
WIRE 2560 144 2560 -64
WIRE 2560 144 2528 144
WIRE 2768 144 2560 144
WIRE 1920 160 1920 -64
WIRE 1920 160 1856 160
WIRE 1952 160 1920 160
WIRE 2896 160 2896 -64
WIRE 2896 160 2832 160
WIRE 2928 160 2896 160
WIRE 1680 176 1680 80
WIRE 1792 176 1680 176
WIRE 2656 176 2656 80
WIRE 2768 176 2656 176
WIRE 1360 240 1328 240
WIRE 1584 240 1552 240
WIRE 1680 240 1680 176
WIRE 1712 240 1680 240
WIRE 1824 240 1824 192
WIRE 1824 240 1792 240
WIRE 2336 240 2304 240
WIRE 2656 240 2656 176
WIRE 2656 240 2528 240
WIRE 2688 240 2656 240
WIRE 2800 240 2800 192
WIRE 2800 240 2768 240
WIRE 1008 320 1008 304
WIRE 1168 320 1168 144
WIRE 2144 336 2144 144
WIRE 1008 448 1008 400
WIRE 1168 448 1168 400
WIRE 1168 448 1008 448
WIRE 1328 448 1328 240
WIRE 1328 448 1168 448
WIRE 1584 448 1584 240
WIRE 1584 448 1328 448
WIRE 1824 448 1824 240
WIRE 1824 448 1584 448
WIRE 2144 448 2144 416
WIRE 2144 448 1824 448
WIRE 2304 448 2304 240
WIRE 2304 448 2144 448
WIRE 2800 448 2800 240
WIRE 2800 448 2304 448
WIRE 1008 512 1008 448
FLAG 1008 304 Vcc
FLAG 1008 512 0
FLAG 2800 64 Vcc
FLAG 2928 160 OUT2
FLAG 1824 64 Vcc
FLAG 1952 160 OUT1
SYMBOL voltage 1008 304 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 12
SYMBOL res 2784 64 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 1000
SYMBOL res 2784 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 1000
SYMBOL res 2768 -80 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 1meg
SYMBOL cap 2736 -192 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 22p
SYMBOL Opamps\\LT1055 1824 96 R0
SYMATTR InstName U4
SYMBOL res 1808 64 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R13
SYMATTR Value 10k
SYMBOL res 1808 224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R14
SYMATTR Value 10k
SYMBOL res 1792 -80 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R15
SYMATTR Value 1meg
SYMBOL cap 1760 -192 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 22p
SYMBOL voltage 2144 320 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PULSE(0 5 4 1 1 1u 0 1)
SYMATTR InstName V4
SYMBOL Optos\\PC817A 2432 192 R0
SYMATTR InstName U5
SYMBOL res 2304 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R8
SYMATTR Value 75
SYMBOL voltage 1168 304 R0
WINDOW 3 24 96 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PULSE(0 5 1 1 1 1u 0 1)
SYMATTR InstName V3
SYMBOL Optos\\PC817A 1456 192 R0
SYMATTR InstName U6
SYMBOL res 1328 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R16
SYMATTR Value 500k
SYMBOL Opamps\\LT1001 2800 96 R0
SYMATTR InstName U1
TEXT 1048 472 Left 2 !.tran 7
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  • \$\begingroup\$ How does this work when the phototransistor is referenced to virtual earth? The only way I can see that working is if the phototransistor exhibits significant photovoltaic effect. Is this the mechanism you are thinking of for this circuit configuration? \$\endgroup\$ – user1582568 Feb 24 '16 at 16:45
  • \$\begingroup\$ No. All I was pointing out was that if a voltage source is connected between an opamp input (- in this case) and either a ground or a pseudoground related to the opamp, there will exist a potential difference across the opamp inputs which the opamp's output will servo to cancel out and thereby provide an output. \$\endgroup\$ – EM Fields Feb 24 '16 at 20:32
  • \$\begingroup\$ Yes, if it is a voltage source that's true, but this is a phototransistor \$\endgroup\$ – user1582568 Feb 24 '16 at 20:34
  • \$\begingroup\$ Yes, but it's acting like a voltage-variable resistor in a voltage divider, (with a potential across it) the other resistance being the feedback resistor from the opamp's output to its inveting input. Check out my edited/new answer. :) \$\endgroup\$ – EM Fields Feb 24 '16 at 23:12
  • \$\begingroup\$ Ah, but it would not work if all the components were ideal. I suspect the tiny amount of response that you are getting in your simulation is due to the opamp bias current providing a little offset and so biasing the phototransistor. In the ideal circuit the opamp output would be at the virtual earth potential and there would not be any voltage across the feedback resistor. \$\endgroup\$ – user1582568 Mar 1 '16 at 13:28

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