Re: Update on laser distance meter project (master thesis) and question

> OK, let's assume the total Cin = 12pF, Rf = 68k, corrected, with
> a single opa657 opamp having f_T = 1600MHz, right?
>

yes that's correct.

> fc = sqrt (f_T / Rf Cin 2pi) = 17.7MHz predicted bandwidth, plus
> up to perhaps an extra 35% or so if your effective Rc is carefully
> selected to take advantage of the two-pole response peaking effect.
>
yes.

> Sorry, read it and weep. What are you seeking, just a little bit
> faster, perhaps a 2x improvement. You're almost there but need a
> little more?

I want to improve the most whats possible in gain and bandwidth
without too much peaking because of the rapid phase change
(dphase/dfrequency) and with lowest noise possible. my goal is +-80k
transimpedance with a bandwidth of 30Mhz with a single amplifier. I
get this in simulation with using a feedback tee network (22K,270ohm
and 100ohm) and a OPA846 (bipolar).

Let's see, you can place the PD right on the opamp,
> with the connection wired in air, to reduce Cin a little, but that
> might only improve the bandwidth by 20%.

yes, i will do this at the end but for now i have to try to get the
max out of it with a little bit more Cin due the longer nodes.

You could reduce your 68k,
> and add back the lost gain later, and suffer a little more Johnson
> noise (resistor Johnson noise may not be dominate anyway, see below).

Yes but the problem is really the S/N wich is too low for the higher
frequenties. for my maximum distance of 10m i get the signals between
10 and 20Mhz quite good, but after 20Mhz rapid fall off soo at 30Mhz i
have a bad S/N ratio to have good resolution. For the closer distances
my 30Mhz signal is still +-100mv , this is good enough for 1 cm
accuracy but at 10m this is only 20-30mv.I don't think another
amplifier is needed because it will add noise and because of the
dynamid range of 60db of my phase detector (from 0dbm to -60dbm) i can
detect very low signals soo it's really up to increase the S/N ratio.

> You could increase the loop gain (composite configuration)...
>

yes i only tried it in simulation and this gave good results but i
first want to try the more simple tee network in practice because i
have a feeling that the composite amplifier will give me also
problems(stability) wich i dont have with the simulation.

> Or you could consider another opamp with lower Cin. JFET's have to
> be large for low en, but 5pF is a pretty high value in your scene.
>
> BJT opamps have input-current shot noise, sqrt (2q I). Comparing
> to resistor Johnson noise, sqrt (4kT/R), we get Ib < 50mV/Rf, so
> we have to look for opamps whose Ib is less than 50mV/68k = 0.7uA.
> But you also want a wide low-frequency extrapolated bandwidth, even
> greater than 1600MHz (which means BJT opamps with input transistors
> running at high currents), so finding the ideal part may be tricky.
>

Yes i made a mathcad file with the signal noise calculations and the
OPA846 gave the best results for a feedback resistor of 68K and the
other parameters like i described them.


> BTW, what were your R-C-R values?
>

Rf=80K,Cf=0.2p,R2=160,R3=53,C2=100pf


> > Any old small low-capacitance type will work. The transconductance
> is always, g_m = Ie/Vt, so it's Ie that determines Zin = re.
>

Yes ofcourse

> For Cin = say 5pF total, you want re less than 530 ohms for a say
> 60MHz bandwidth, so Ie > Vt/re = 25mV/530 = 47uA. Call it 50uA,
> which you can get from your +/-5V supplies. We see the BJT bias
> resistors are larger than your chosen 68k feedback resistor, so
> their deleterious impact on noise won't be too severe, or you can
> get the bias current from say +/-15V to reduce the resistor noise.
>
> . +5
> . |
> . 100k ,- 68k ---,
> . | | __ |
> . ----+-- e c --+--+--|- \ |
> . | b | >--+---
> . 90k | ,--|+_/
> . | gnd |
> . -5V gnd
>
> But as you say, this won't help much, because it's merely trading an
> already-low APD capacitance for the transistor's Cob capacitance.
>
> We haven't talked about w-en-Cin noise, which is always a killer at
> high frequencies. en is 4.8nV for the opa657, which is pretty good,
> but a selected low-noise BJT operating at 50uA might be a little bit
> better. Also, many wideband bipolar opamps have less than 5nV noise.

Yes i understand the Eni noise , due the input capacitance this will
give noise peaking in the higher frequency region (till a fall off due
the GBP).i understand it like a 1+Zf/Zin amplification of the noise.
With the OPA657 this is indeed the dominant noise factor but with the
opa846 it's the bias current shot noise of 13pa/rootHz, the Eni there
is 1.3nv/rootHz.

I still cant figure out why the tee network described in greames book
doesnt work in practice for me.
.

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