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?