Transimpedance amplifier (TIA) oscillating at above 1 GHz. Need Help

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Hi Tim, just re-reading your post. Noise gain is pretty much entrenched in the PD literature. I don't really like the term either. I know it caused me hours of head banging when I first started to study PD's and TIA's. But let's just stick with it for consistencies sake.

I?m not sure why you are concerned that we equate stability with gain. (But you probably just have a deeper understanding than moi.) So here?s a question. If I take the same opamp out of the TIA and just put resistors around it to set the gain. Then it will oscillate unless the gain is greater than 10. Now if I was to try and roll off the gain at high frequency, by putting a small cap across the feedback R. Would that cause it to oscillate? (I?ve only used minimum gain of 10 opamps a few times... and rolling off the gain was *not* what I wanted.)

George H.

ttdesign.com- Hide quoted text -

Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:

resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MHz.

something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.

perfect sine, even without a signal applied!

up to 10pf.

oscillation is getting worth.

Good question, George!

Yes - if that the capacitive reactance is low enough (Zf/Zi < Amin)*. By inserting a zero** - a resistor in series with this feedback capacitor to keep Zf/Zi > Amin you should be stable, though of course you won't be killing the HF gain as much as you might have wanted. The other classic approach is to add capacitance from the inverting node to ground to increase the noise gain even with the added feedback capacitance. Of course there may be other sources of instability, e.g. capacitive loading.

• -evaluated in the vicinity of f=GBW/|Zf/Zi| to GBW.

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Thanks Frank, I'll see if I can dig up a linear chip with a minimum gain requirement and play in LT spice. So I should be thinking about (at least) another pole in the open loop transfer function. (That looks to really complicate the math.) Hey maybe Julian can run 'his' screaming opamp in the TI spice simulator and post some pics. (You'll have to calculate 'right' C to put accross the feedback R.)

George H.

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For anyone following along at home, here's an LTspice file (with LT1127 opamp) If you change C1 from 10 pF to 1 pF it will stop with the oscillations. I don't know what to make of the 10k ohm * 10 pF = 1e-7 second time constant.

George H.

Version 4 SHEET 1 896 680 WIRE 192 80 176 80 WIRE 304 80 272 80 WIRE 176 144 176 80 WIRE -256 208 -256 176 WIRE -128 208 -128 176 WIRE 304 208 304 80 WIRE 304 208 112 208 WIRE 400 208 304 208 WIRE 304 224 304 208 WIRE 400 256 400 208 WIRE 176 288 176 272 WIRE 112 304 112 208 WIRE 144 304 112 304 WIRE 304 320 304 304 WIRE 304 320 208 320 WIRE 400 320 304 320 WIRE 480 320 400 320 WIRE 576 320 560 320 WIRE -256 336 -256 288 WIRE -128 336 -128 288 WIRE 80 336 -32 336 WIRE 144 336 80 336 WIRE -32 352 -32 336 WIRE 176 368 176 352 WIRE 576 400 576 320 WIRE -32 448 -32 432 FLAG -128 336 0 FLAG 176 272 +V FLAG -128 176 +V FLAG 176 368 -V FLAG -256 336 0 FLAG -256 176 -V FLAG -32 448 0 FLAG 176 144 0 FLAG 80 336 IN+ FLAG 576 400 0 SYMBOL voltage -128 192 R0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -256 192 R0 SYMATTR InstName V2 SYMATTR Value -15 SYMBOL opamps\\LT1127 176 256 R0 SYMATTR InstName U1 SYMBOL res 288 208 R0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL res 288 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 2k SYMBOL res 576 304 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R7 SYMATTR Value 10K SYMBOL voltage -32 336 R0 WINDOW 123 24 124 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value SINE(0 10m 1meg) SYMATTR Value2 AC 10m SYMBOL cap 384 256 R0 SYMATTR InstName C1 SYMATTR Value 10pf TEXT 408 424 Left 2 !.tran 30u

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Okay, so here are my simulation results. At first the configuration that, I am think, I schoul have on my board:

C_in = 7pf R_f = 330Ohm C_f = 4pf

Next with increased C_f (It is still hard to believe, that it is really sta rting to oscillate with increased feedback cap, but it definitively is...).

C_in = 7pf R_f = 330Ohm C_f = 15pf

Than, as suggested, with increased C_in to increase the closed-loop? gain(s till a bit confused about the gain thing...).

C_in = 7pf R_f = 330Ohm C_f = 15pf

I will try to increase the C_in on my board tomorrow and report if it is he lping. I can not believe, that my current C_in should be bigger than 10pf and so t here is a difference between simulation, where oscillation starts at around 15pf, and reality. So maybe there is also something else wrong...

Cheers, Julian

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tarting to oscillate with increased feedback cap, but it definitively is... ).

(still a bit confused about the gain thing...).

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helping.

there is a difference between simulation, where oscillation starts at arou nd 15pf, and reality. So maybe there is also something else wrong...

Cool, Everyone should do their own measurements.

So I don't recommend the 10pF as a permanent solution... just to see if it makes the oscillations stop.

I'd try the opamp as X10 buffer, or get a bigger photodiode, or find a different opamp, There's a fast Fet one from AD.

George H.

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Huh? The 'thing' oscillates at about 23MHz, that's like a 7-8 ns time constant, 2k and 10pF is only 20ns. You can change the 2k to 4k ohm and it doens't change much. I guess it's some other pole in the opamp transfer function, but I'm not sure how to think about it.

George H.

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Look, I gave you an alternate to your sim to remove the obvious mess it was generating. what I got as a results was nothing more than the normal phase margin error between input and output..

I didn't see any oscillations after that with the sim you supplied ?

Maybe we're using version XYZ from planet mars!

Jamie

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Oh sorry, you should have said that the file didn't work. It was just an lt1127 with gain of 6, non-inverting, and 10pF in parallel with the 10k ohm feedback resistor.

George H.

feedback resistor of 330Ohm and a feedback cap of 4pf and a signal of

something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.

with a perfect sine, even without a signal applied!

form 1pf up to 10pf.

the oscillation is getting worth.

issue.

the

be approximated by a single pole response?

Not really. Normally there are two or three dominant poles and often enough one zero in the open loop response.

?-)

Am Dienstag, 14. Mai 2013 10:25:29 UTC+2 schrieb Julian Arnold:

resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MHz.

something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.

perfect sine, even without a signal applied!

up to 10pf.

oscillation is getting worth.

Hey everyone,

sorry for the late reply but I had to first gain some more theoretical knowledge and proceed with my project.

I do now understand, that my TIA is running out of gain but as I am not able to equip the very small capacitor needed, I simply live with the resulting oscillation. This oscillation is very small in amplitude and thus, the applied signal is easily dominating. I am now able to transmit data at rates above 100 Mbps. However, a new problem has shown up. It seems, that for some particular input power a second, even worse, oscillation arises. I am not quiet sure if it is some saturation problem or something completely different like reflections in front of the receiver diode. The following images show the received signal at different input power levels. With very low power there is only the 'normal' high frequency oscillation.

When increasing the input power (by moving the receiver optics deeper into the beam) the additional oscillation appears.

When further increasing the optical power, the 'down' parts of the signal calm down (Maybe some saturation ?).

I do not think, that it is an transient effect because the following image shows that the oscillation appears nearly randomly when operating near the 'critical' power.

The yellow curve is the signal applied to the transmitter. I am thankful for any suggestion!

Cheers, Julian

eedback resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MHz.

something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.

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calm down (Maybe some saturation ?).

e shows that the oscillation appears nearly randomly when operating near th e 'critical' power.http://img692.imageshack.us/img692/1231/increasingonesze rososc1.png

Maybe you could repost your schematic... as it is now. Maybe it's a power supply 'thing'. (any sign of the oscillation on the power line?) The original schematic showed the photodiode tied directly to the power rail... maybe a little series resistor and capacitor to ground as RC isolation.

I'd be worried about the continued HF oscillations. (If you build more than one it's guaranteed that to be worse in some.)

George H.

feedback resistor of 330Ohm and a feedback cap of 4pf and a signal of 10MH z.

o something above 20pf. In my opinion it should be stable for higher caps a nd only start oscillating if the cap is to small.

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able to equip the very small capacitor needed, I simply live with the resu lting oscillation.

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levels.

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al calm down (Maybe some saturation ?).

age shows that the oscillation appears nearly randomly when operating near the 'critical' power.http://img692.imageshack.us/img692/1231/increasingones zerososc1.png

Hey George,

here is the link to the actual schematic:

On the board, the diode is next to the power decoupling caps of the TIA. Is it OK? I have not measured the power supply yet but I will do it on Friday . Do you think this could be related to the optical input power?

I only need to get that one somehow working for my project ;)

a feedback resistor of 330Ohm and a feedback cap of 4pf and a signal of 10 MHz.

to something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.

with a perfect sine, even without a signal applied!

form 1pf up to 10pf.

the oscillation is getting worth.

er issue.

l knowledge

ot able to equip the very small capacitor needed, I simply live with the re sulting oscillation.

nal is easily dominating.

oscillation arises.

pletely different like reflections in front of the receiver diode.

r levels.

tion.http://imageshack.us/a/img62/1231/increasingoneszerososc1.png

into the beam) the additional oscillation appears.http://imageshack.us/a/img23/1231/increasingoneszerososc1.png

gnal calm down (Maybe some saturation ?).

image shows that the oscillation appears nearly randomly when operating nea r the 'critical' power.http://img692.imageshack.us/img692/1231/increasingon eszerososc1.png

/7702/schematicj.png

A much nicer schematic. Is the 0.4pF Cf the estimated stray capacitance?

Is it OK? I would think so, but I've rarely tread above 100MHz. So I'm not much of a HF 'expert'

I have not measured the power supply yet but I will do it on Friday.

Well more power means more output signal.. more current. Maybe a little decoupling would help, like this...

V(+)----RRR-+---|to opamp(-) | (PD) C | GND

So perhaps 10 ohms for R and 0.1uF for C.

Well is the Professor going to see the oscillations? And stopping the HF oscillations may also take care of this other issue.

George H.

- Hide quoted text -

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or a feedback resistor of 330Ohm and a feedback cap of 4pf and a signal of

ap to something above 20pf. In my opinion it should be stable for higher ca ps and only start oscillating if the cap is to small.

Hz with a perfect sine, even without a signal applied!

ed form 1pf up to 10pf.

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ther issue.

cal knowledge

not able to equip the very small capacitor needed, I simply live with the resulting oscillation.

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, oscillation arises.

ompletely different like reflections in front of the receiver diode.

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lation.http://imageshack.us/a/img62/1231/increasingoneszerososc1.png

er into the beam) the additional oscillation appears.http://imageshack.us/a/img23/1231/increasingoneszerososc1.png

signal calm down (Maybe some saturation ?).

g image shows that the oscillation appears nearly randomly when operating n ear the 'critical' power.http://img692.imageshack.us/img692/1231/increasing oneszerososc1.png

. Is it OK?

Hey,

yes, the professor is going to see them but I do not have the time to make new circuit boards.

Your suggestion sounds good! Maybe I will have the time left to apply it. But I think, I will first measure if there is any noise on the power supply .

Cheers, Julian

circuit boards.

I bet the feedback path is from the opamp Vcc pin to the photodiode and thence back into the opamp, so one needs to ensure that the RC filter is between the Vcc pin and the diode, with R connected between Vcc pin and diode, and C from diode to ground.

What also often helps is a ferrite bead on the opamp input line.

I would expand the circuit diagram to show these details, so the possible (unintended) feedback paths are apparent.

Joe Gwinn

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for a feedback resistor of 330Ohm and a feedback cap of 4pf and a signal o f 10MHz.

cap to something above 20pf. In my opinion it should be stable for higher caps and only start oscillating if the cap is to small.

GHz with a perfect sine, even without a signal applied!

ried form 1pf up to 10pf.

tor) the oscillation is getting worth.

other issue.

tical knowledge

am not able to equip the very small capacitor needed, I simply live with th e resulting oscillation.

signal is easily dominating.

se, oscillation arises.

completely different like reflections in front of the receiver diode.

power levels.

illation.http://imageshack.us/a/img62/1231/increasingoneszerososc1.png

eper into the beam) the additional oscillation appears.http://imageshack.us/a/img23/1231/increasingoneszerososc1.png

e signal calm down (Maybe some saturation ?).

ing image shows that the oscillation appears nearly randomly when operating near the 'critical' power.http://img692.imageshack.us/img692/1231/increasi ngoneszerososc1.png

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IA. Is it OK?

e new circuit boards.

ly.

Julian, I was walking around after work today. And I think I know how to fix both your problems. You should attach the PD lead tied to the supply volatge to 'some other' node in your circuit. This will both increase the capacitance seen by the inverting input, and eliminate the PS feedback path. You get to figure out which node.

George H.