[time-nuts] Photodiodes for high frequency OPLL

Attila Kinali attila at kinali.ch
Sun Mar 31 08:43:12 EDT 2013

On Sat, 30 Mar 2013 21:40:47 +0000
lists at lazygranch.com wrote:

> The circuit is something like the instrumentation amplifier.
> The description starts on page 207 with a schematic on page 208.
> I can scan it later, but the circuit is easy to describe. Think of two
> op amps in the classic current multiplication (I to V)  circuit, that is
> positive input to ground and a resistor from output to negative input.
> Now place the diode between the two negative inputs. The current flow
> will cause the outputs of the op amp to move differentially, which can
> then be made single ended with an op amp circuit. But the thing to keep
> in mind is that the voltage potential across the photodiode has been kept
> to zero, so the capacitance doesn't matter. But it is only held to zero
> as long as the amplifier has loop gain, that is create the virtual ground,
> hence you need a high GBP for the capacitance to be neutralized. 

And here comes the problem: At 7GHz the amplifier will be build
from discrete components. And i'm not sure that i can acheive high enough
gain to cancel the capacitance. But then, the differential approach
in itself might be benefitial.

> Since the bootstrap needs to "sample" the virtual ground, it itself can't
> steal any current from that point, so it usually employs a JFET. 

I might be mistaken, but i think 1GHz is about the maximum one can
get out of modern JFETs? At least the discrete ones.

> Linear Technology app notes use the bootstrap often. They use a common
> low noise JFET from NXP. Noise from the bootstrap adds right to the diode,
> so the bootstrap components need to be low noise. 

Yes, i've read a couple of Appnotes, and at least one from Linear describing
something of that kind.

> The fully differential
> circuit doesn't have the bootstrap noise source, but it has two amplifiers
> on the front end, hence two uncorrelated noise sources. 

I wonder, if i build a difference amplifier by hand and a seperate
bias circuit, can get away with less noise? The noise from the bias
circuit should should cancel out in the difference amplifier.
Or am i missing something?

> When you read the photodiode literature, bandwidth is stated into an
> impedance, so I think they just treat the capacitance as the limiting
> factor. Maybe that is real life, or maybe it is oversimplified. At that
> point this is a solid state physics problem and not a circuit design issue.
> (I'd have to crack a book on the physics.)

That's one of my issues: I have not found much usuefull literature
on photodiodes. Most deal with detecting on-off signals with very
low frequency (a couple of kHz at most). There are some good papers
from the telecom industry dealing with signal detection at high frequencies,
but they always use an custom build IC as amplifier and bias stage,
which limits their applicability to my case.

				Attila Kinali
The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
		-- unknown

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