# Photodiode operating modes - confusion about wording

The more I read about the operating modes of photodiodes, the more confused I get since it seems that people are using the same wording for different stuff.

Please consider photoconductive (PC) mode versus photovoltaic mode(PV).

• 1, and 2 define the photovoltaic mode to be the mode where a short circuit is enforced across the PD terminals (by the OPAMP).
• 4 defines the PV mode completely different in the sense the zero bias is not interpreted as zero volts across the diode but simply no bias in the sense of "open" circuit (very high impedance) across diode.

Also, 4 uses the definition of the PV mode of 1 and 2 to talk about PC mode with just zero volts reverse bias. In other words, where 1 and 2 talk about PV mode, 4 calls it one special variant of PC mode.

• 6 uses Fig8 to talk about PV mode while NOT shorting the diode as 1 and 2. Instead, it is used similar as in 4, without too much details, but mentioning that the forward voltage is kepts below the turn-on threshold of the diode by loading with a resistor.

I have lots of questions because of the confusion, but to formulate three very specific questions:

1.)How would you guys interpret "zero-bias"? Is it a short circuit or an open circuit? I assume this is a key for further discussion and understanding.

2.)What is the reason that the PV mod as described in 4 is not used in practise? 6 just also mentions that fact but does not explain why: "If the photodiode is open-circuit or loaded only with a very high resistance the anode will be sufficiently positive for the photo current to flow in the diode and the voltage across the photodiode will be proportional to the logarithm of the light intensity. It is possible to measure this behaviour using an electrometer op-amp with very low bias current such as the AD549 (Ib ˜ 40 fA) but photodiodes are rarely used in exactly this way."

3.) PC mode seems to represent 3rd quadrant, what about PV mode? If used open loop I would say 1st quadrant, but 7 suggest it is the 4th quadrant?

• You need to focus on one aspect of the whole number of problems you are considering, to make this question suitable for this site. If you are making links to other documents that confuse you, you should copy and paste a relevant image so that it is available within your question. You should still keep the link to the document. Focus on one problem so that this can be answerable then, if necessary, you can ask a new related question after this has been dealt with. – Andy aka Nov 28 '20 at 11:19
• It doesn't get less confusing if you don't introduce abbr.s (PD, PV, and PC). Photodiode? Photovoltaic? Photoconductive? We are left guessing. – Peter Mortensen Nov 28 '20 at 11:32
• thanks for the comments. I added the content of the external references to the question as well as the hint that PC is for "photoconductive" and PV stands for "photovoltaic" – Junius Nov 28 '20 at 11:45
• An obscure (very old) ap-note:**EG & G ELECTRO-OPTICS, SALEM, MASS. Application note D3011C-3 (October 1979). On silicon photovoltaic (photodiode) detectors and detector/amplifier combinations.** It is scanned on this page: modulatedlight.org/Modulated_Light_DX/modlightrx.html – glen_geek Nov 28 '20 at 15:35

1, and 2 define the photovoltaic mode to be the mode where a short circuit is enforced across the PD terminals (by the OPAMP).

The picture you linked for 2 is describing what the transimpedance amplifier circuit in the picture is doing (holding the voltage across the diode at zero), which causes the photodiode to be in photovoltaic mode. It is not defining anything. The picture in 4 is not of a transimpedance amplifier, and so the circuit does something else.

1.)How would you guys interpret "zero-bias"? Is it a short circuit or an open circuit? I assume this is a key for further discussion and understanding.

Zero bias means that you are not applying a reverse voltage to the diode. It does not imply either short circuit or open circuit operation, just that there is no externally applied voltage. Both of the pictures you presented do not apply an external voltage across the diode, so they're both zero bias.

2.) What is the reason that the PV mod as described in 4 is not used in practise? 6 just also mentions that fact but does not explain why

That quote does explain why. The signal you get is the log of the optical signal and requires very sensitive circuitry to read out. That makes it hard to use.

3.) PC mode seems to represent 3rd quadrant, what about PV mode? If used open loop I would say 1st quadrant, but 7 suggest it is the 4th quadrant?

That is a plot of current vs energy (not voltage). In that picture, quadrant 1 is an LED (energy going in, light coming out). Quadrant 4 is the reverse, light coming in, energy going out).