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large bandwidth amplifier (help needed)

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Proud Mary

Sat Jan 16 2010, 05:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

How about a nice bootstrapped avalanche photodiode running in Geiger mode? That's what I'd do.

Sulaiman

Sat Jan 16 2010, 06:40PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Since you are using a jfet why not try this -
Photodiode cathode to +ve voltage
(higher +V = lower junction capacitance = faster)

Connect the anode to the source of the fet with 100 ohms to 0v.
Gate to 0v.
Drain to +V via say 1 kOhm and a source-follower buffer from the drain
(or emitter-follower)

If you use a dual-gate fet with a fixed bias (e,g, 6V) on gate2 then the bandwidth will be limited mainly by the photodiode.

Proud Mary

Sat Jan 16 2010, 06:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Truly differential TIA using PIN photodiodes, bandwidth around 100MHz.

I suppose this should be merged with my last post, thank you.

1263667258 543 FT82554 Differential Tia Bandwith 100mhz

Steve Conner

Sat Jan 16 2010, 06:53PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'm not sure why the circuit doesn't work, but I can think of a few points that might be relevant.

The BF245A is a small-signal JFET with an Idss of a couple of milliamps. Now, Idss is about the most current you'll ever get from a JFET, so this particular device is possibly not a good choice for an output stage. The BF245C might be better, it's the highest Idss grade. Or try a BJT like the 2N2222, and save the JFETs for the input stage.

You're talking about your simulation as if it was a real circuit that you actually built and tested. But it's not. (I hate it when people do that.) How accurate is your simulator? The performance of RF circuits can be hard to model, even for an experienced engineer running something a lot more hardcore than Electronics Workbench. It's often easier to forget the simulation and build a real prototype.

hboy007

Sat Jan 16 2010, 08:13PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Steve,

>> You're talking about your simulation as if it was a real circuit that you actually built and tested. But it's not. (I hate it when people do that.)

I hate that, too. But for now, I don't have all the parts lying around so I try to figure out the basics before I do the happy ordering thing. I chose the BF245A because I have about 10 pc. on my desk and because I've already built a 27 MHz Oscillator with the BC107 and a 100 MHz test signal generator that employs the BF245. I wouldn't call that experience but at least I have already put something together with these parts.
The reason why I use EWB / Multisim is that I haven't spent enough time with pspice to be able to perform the real simulations. I always wired up the circuit on the breadboard or soldered it together before working my way through the tutorials but currently I don't even have an oscilloscope at hand.

As to the other suggestions:

Avalanche photodiodes are really cool devices and building a differential amplifier was the first thing I attempted. However, these are very special solutions.
Let's clarify what my problem is all about: I am working on a system that tracks the motion / position of fast moving objects and therefore I bought position sensitive photodiodes that are basically photodiode strips with a common cathode plane and two anode contacts at the edges of the p-doped top. The structure forms a PIN diode so I won't even be able to get a proper avalanche effect. The datasheet says that the abs max reverse voltage is 30V (typ. 20V).

The manufacturer of these devices also offers complete modules and evaluation boards but my calculation results prohibit using them because the amplifiers they use are way too slow (0.4MHz bandwidth, they must be kidding...).

I could build something that uses APDs *BUT* ...

how would that look like? Proud Mary's post inspired me to come up with a solution for that: the spatial information can be encoded by the degree of polarisation, the incident beam has to have circular polarisation. In the image plane of the lens, a quartz retarder wedge would be placed (for now let's neglect the error due to the solid angle of the incident light and assume it comes in perpendicular to the quartz wedge), followed by a rectangular aperture and a birefringent polarizer. The emergent beam fractions could then be focussed onto two APDs... wow, that would look cool but I could only afford the lambda/4 waveplate.

however, APDs with multiple contacts would be the thing I'm looking for. I guess Hamamatsu would etch me some if I took a lot of 10^6 and paid in gold

The cascode video amplifier I illustrated works fine without any load and has a good bandwidth, but how do I cascade them, let alone construct a suitable output stage?

Steve Conner

Sat Jan 16 2010, 08:56PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Maybe you should model your 100MHz oscillator in EWB and compare the performance to the real circuit. That would help you verify EWB's model of the BF245.

What's the capacitance of these photodiodes you bought? Plug that into the design equations for transimpedance amps that I linked you earlier. The capacitance can seriously limit the bandwidth, no matter what amplifier topology you use. Maybe the manufacturer used slow amps on his demo boards because there's no point in using fast ones...

Re output stages, you should be able to hang a DC-coupled emitter follower, using a 2N2222, straight off the drain of the top FET in the cascode. It may need a base stopper resistor to stop it from oscillating, in real life if not in the sim.

hboy007

Sat Jan 16 2010, 10:25PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

the diode has 5 .. 6pF when biased with 15V and a rise/fall time of 50 .. 80ns, which sounds promising to me.
Dark current is 4..20nA and current noise is 0.4 .. 1 pA / sqrt(Hz) - typical resp. maximum values.
I'll be back with the calculations tomorrow. Thank you for your help!

Proud Mary

Sat Jan 16 2010, 11:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I was wondering what your application was, Mr Bond, and so allowed my reactions to be coloured by my interest in SPADs and such.

But what about short range radar for your application? Millimetric waves are all the rage at the moment...

Pre-crash Sensing â€“ Functional Evolution based on Short Range Radar Sensor Platform
Rainer Moritz
Robert Bosch Gmbh

hboy007

Wed Jan 20 2010, 12:20AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

ok so now I've read some lines of Gerald Graeme's "optimising op amp performance" which I found at the library and now the noise modelling and other requirements are less clear than when I started. Guess I'll have to borrow an oscilloscope and get back to this project with more hands-on experience. Until then the jfet amplifier will get a little more attention. Following your suggestions I added a dc coupled npn transistor (not the 2N2222, but BFR540 because the datasheet promised better performance. To my amusement I figured out that the BF245 oscillator simulation wouldn't work. Let's see the practical results

hboy007

Tue Mar 30 2010, 12:18PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I just found an interesting opamp, the THS3001 which has an insane bandwidth and costs less than 10 Euro / pc. However, this is a current feedback opamp, a quick introduction can be found at

I haven't fully understood how to use these, but maybe they can be used in conjunction with the cascode circuit suggested by Hobbs.

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