Thanks, i remember years ago having an opamp with input above the common mode input range, and its output just latched up high and stayed high.....even when the input fell back inside the common mode input range of the opamp.
As such, do you agree that any operation whatsoever, with an input outside the common mode input range is totally unacceptable?
By the way , do you agree that the photodiode is an extremely high value resistor when no light is incident on it, but is a 0.4v Vf diode when light shines on it?
Hi,
I have two types but unfortunately i dont think i have the part numbers for either type.
The first type is high speed, and will put out about 0.6v or so when light shines on it, but can also be used in series mode where little current is passed until light hits it. This might be able to be classified as a photo voltaic 'diode' of which all silicone diodes are to some extent, but it was sold as a pin photo diode. Diode unit price can range from maybe 50 cents to over 100 dollars (USD) and signals range up to 10GHz.
The second type will not put out any voltage at all. It will just turn into a lower value resistor when light is directed at it. This means that if you put it in series with a voltage source it will allow current to flow when light is directed on it.
For example in the circuit:
+5v o---10k---+---|>|---o Ground
you will see +5v at the anode to ground (across the diode) until you shine light on the diode and then it will go down to a lower level where the level of voltage then depends on the intensity of light.
For the first type above (photovoltaic type) for this circuit:
Meter o---|>|---o Ground
there is no resistor, no voltage source, but when you shine light on the diode the meter reads maybe 0.6v or so.
Alternately to keep noise down, you could place a 5k resistor across that diode above and then read the voltage with a scope and when you direct an IR remote control at the diode you'll see the protocol pulses. You can also see the carrier if you set the scope right and back the IR remote away from the diode a little while pressing a button.
You can not use the non photovoltaic type like this, but you could also use this first type with a series resistor as before while looking across the resistor instead of the diode.
The difference between types could be due to the intrinsic layer thickness, but i never looked into this.
Here is a scope pic of the second type with 10k in series, and using a low voltage power source just above 2vdc. The scope is looking across the diode, and the pulses are from a typical IR remote control. The protocol pattern is seen as the burst pulses, and the carrier is shown as the higher frequency component in each pulse although it alternates faster than the scope presents on that time base setting. The protocol pattern is inverted because the scope is across the diode not the resistor.
This does show that the series resistance of the diode must be much lower than 10k.