Noise gain in TIA opamp

G

George Herold

Guest
I’m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I’ve gotten myself confused. I’m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is ‘blowing up’ (going to zero).

I had this ‘silly’ idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn’t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Spice file follows. (I tried adding the file as an attachement but
couldn't do that via google groups.)

Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub



I’m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I’ve gotten myself confused. I’m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is ‘blowing up’ (going to zero).

I had this ‘silly’ idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn’t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold
 
George Herold a écrit :
I’m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I’ve gotten myself confused. I’m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is ‘blowing up’ (going to zero).

I had this ‘silly’ idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn’t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Spice file follows. (I tried adding the file as an attachement but
couldn't do that via google groups.)

Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub



I’m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I’ve gotten myself confused. I’m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is ‘blowing up’ (going to zero).

I had this ‘silly’ idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn’t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold
Click to expand...
Think about how much phase rotation you have around the loop.
And now, why is noise gain rising up?


--
Thanks,
Fred.
 
On Apr 4, 6:04 pm, Fred Bartoli <" "> wrote:
George Herold a crit :





I m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I ve gotten myself confused.  I m
looking at voltage noise gain in TIA amplifiers.  I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input.  But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them?  Some term
in the denominator is blowing up (going to zero).

I had this silly idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency.  This obviously doesn t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Spice file follows.  (I tried adding the file as an attachement but
couldn't do that via google groups.)

Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub

I m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I ve gotten myself confused.  I m
looking at voltage noise gain in TIA amplifiers.  I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input.  But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them?  Some term
in the denominator is blowing up (going to zero).

I had this silly idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency.  This obviously doesn t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Think about how much phase rotation you have around the loop.
And now, why is noise gain rising up?

--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -
Click to expand...
Hi Fred, Thanks for the response. Hmmm.... With a lot of gain the
phase 'snaps', right across at the gain peak. But why the gain
peak?

I've been writing down the loop equations. The second time
through*, I got that the gain at resonance should be the ratio of two
'times'. the RC time/ GBW 'time'.
or the GBW /RC 'freq'. 10Meg/1kHz in the example I posted ~80dB,
close enough?

(The whole equation looks like Gain= a0*t(RC)/{(t(a0)+t(RC)} Where
a0 is the DC open loop gain and t(a0)/a0 is the GBW time. t(a0) is
the time where the open loop response starts to roll off.

George H.

* the first time through I got the zero frequency open loop resopnse
gain. (But I was sloppy and threw away some factors I should have
kept.)
 
George Herold wrote:
I’m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I’ve gotten myself confused. I’m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is ‘blowing up’ (going to zero).

I had this ‘silly’ idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn’t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Spice file follows. (I tried adding the file as an attachement but
couldn't do that via google groups.)

Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub



I’m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I’ve gotten myself confused. I’m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is ‘blowing up’ (going to zero).

I had this ‘silly’ idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn’t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold
Click to expand...
Attaching your schematic .asc as a test.
 
George Herold a écrit :
On Apr 4, 6:04 pm, Fred Bartoli <" "> wrote:
George Herold a crit :





I m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I ve gotten myself confused. I m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is blowing up (going to zero).
I had this silly idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn t happen, and I did a quick
LTspice.
George (feeling like an idiot) Herold
Spice file follows. (I tried adding the file as an attachement but
couldn't do that via google groups.)
Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub
I m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I ve gotten myself confused. I m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is blowing up (going to zero).
I had this silly idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn t happen, and I did a quick
LTspice.
George (feeling like an idiot) Herold
Think about how much phase rotation you have around the loop.
And now, why is noise gain rising up?

--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -

Hi Fred, Thanks for the response. Hmmm.... With a lot of gain the
phase 'snaps', right across at the gain peak. But why the gain
peak?
Click to expand...
At unity loop gain, you've got almost 90° phase shift from the opamp
plus another almost 90° shift from the RC feedback network, which is
almost 180° phase shift. You're not far from having build an oscillator.
(in real life, additional parasitic poles will ensure you reliable
oscillations)

This phase shift builds positive feedback and gives you heaps of gain
enhancement (resonance) near the unity loop gain, hence your noise gain
peaking...


I've been writing down the loop equations. The second time
through*, I got that the gain at resonance should be the ratio of two
'times'. the RC time/ GBW 'time'.
or the GBW /RC 'freq'. 10Meg/1kHz in the example I posted ~80dB,
close enough?

(The whole equation looks like Gain= a0*t(RC)/{(t(a0)+t(RC)} Where
a0 is the DC open loop gain and t(a0)/a0 is the GBW time. t(a0) is
the time where the open loop response starts to roll off.
Click to expand...
I didn't get through your maths but just drop ao, which is just
obscuring things, and think GBW product. In the region of interest the
opamp is almost always already behaving asymptotically and you can
(often) accurately model it as a single pole at DC.

So, the max will occur at sqrt(GBW/(2*pi*tau)) frequency and will be
(2*pi*GBW*tau), and this amounts to the sqrt of product and ratio of the
respective frequencies.


--
Thanks,
Fred.
 
On Apr 5, 1:00 am, ehsjr <eh...@nospamverizon.net> wrote:
George Herold wrote:
I�m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But Iďż˝ve gotten myself confused.  Iďż˝m
looking at voltage noise gain in TIA amplifiers.  I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input.  But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them?  Some term
in the denominator is �blowing up� (going to zero).

I had this �silly� idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency.  This obviously doesnďż˝t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Spice file follows.  (I tried adding the file as an attachement but
couldn't do that via google groups.)

Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub

I�m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But Iďż˝ve gotten myself confused.  Iďż˝m
looking at voltage noise gain in TIA amplifiers.  I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input.  But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them?  Some term
in the denominator is �blowing up� (going to zero).

I had this �silly� idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency.  This obviously doesnďż˝t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Attaching your schematic .asc as a test.

[TIA amp.asc1K ]Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub- Hide quoted text -

- Show quoted text -
Click to expand...
OK did you do that via google groups? I mucked about, but couldn't
get it to accept an attachment.

George H.
 
On Apr 5, 7:11 am, Fred Bartoli <" "> wrote:
George Herold a crit :





On Apr 4, 6:04 pm, Fred Bartoli <" "> wrote:
George Herold a crit :

I m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I ve gotten myself confused.  I m
looking at voltage noise gain in TIA amplifiers.  I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input.  But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them?  Some term
in the denominator is blowing up (going to zero).
I had this silly idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency.  This obviously doesn t happen, and I did a quick
LTspice.
George (feeling like an idiot) Herold
Spice file follows.  (I tried adding the file as an attachement but
couldn't do that via google groups.)
Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub
I m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I ve gotten myself confused.  I m
looking at voltage noise gain in TIA amplifiers.  I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input.  But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them?  Some term
in the denominator is blowing up (going to zero).
I had this silly idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency.  This obviously doesn t happen, and I did a quick
LTspice.
George (feeling like an idiot) Herold
Think about how much phase rotation you have around the loop.
And now, why is noise gain rising up?

--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -

Hi Fred, Thanks for the response.  Hmmm.... With a lot of gain the
phase 'snaps', right across at the gain peak.  But why the gain
peak?

At unity loop gain, you've got almost 90 phase shift from the opamp
plus another almost 90 shift from the RC feedback network, which is
almost 180 phase shift. You're not far from having build an oscillator.
(in real life, additional parasitic poles will ensure you reliable
oscillations)

This phase shift builds positive feedback and gives you heaps of gain
enhancement (resonance) near the unity loop gain, hence your noise gain
peaking...

  I've been writing down the loop equations.  The second time
through*, I got that the gain at resonance should be the ratio of two
'times'.  the RC time/ GBW 'time'.
or the GBW /RC 'freq'.  10Meg/1kHz in the example I posted ~80dB,
close enough?

(The whole equation looks like Gain= a0*t(RC)/{(t(a0)+t(RC)}   Where
a0 is the DC open loop gain and t(a0)/a0 is the GBW time.  t(a0) is
the time where the open loop response starts to roll off.

I didn't get through your maths but just drop ao, which is just
obscuring things, and think GBW product. In the region of interest the
opamp is almost always already behaving asymptotically and you can
(often) accurately model it as a single pole at DC.

So, the max will occur at sqrt(GBW/(2*pi*tau)) frequency and will be
(2*pi*GBW*tau), and this amounts to the sqrt of product and ratio of the
respective frequencies.
Click to expand...
Yeah sorry for the math weirdness.... Your '(2*pi*GBW*tau)' is the
gain I get as long as 1/(2*pi*tau) isn't too close to the GBW.

I'm writing an appendix explaining how to choose the parallel cap to
put across the feedback resistor, and ran into the limits of my own
understanding. (As often happens when you try explaining something to
someone else.)

Thanks for the help,

George H.


--
Thanks,
Fred.- Hide quoted text -

- Show quoted text -
Click to expand...
 
On Tue, 5 Apr 2011 06:42:04 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

snip of "Attaching your schematic .asc as a test."

OK did you do that via google groups? I mucked about, but couldn't
get it to accept an attachment.
Click to expand...
This is what ASC.EXE did for your schematic, George:

: C2
: || 0p
: ,---||---,
: | || |
: | |
: | R5 |
: +--/\/\--+
: | 100K |
: | |
: | |
: C1| |+ |
: || 1.| |\ U1|
: ,--||---+---|-\ |
: | || | >-+--
: | ,--|+/
: | | |/
: | --- |-
: | - V1
: | ---
: | - AC 1.
: | |
: | |
: | |
: | gnd
: gnd .ac oct 25 1k 10Meg
:
: .include opamp.sub
Click to expand...
Sadly, C1 was bunched up against a wire so the 1.6n was
truncated, but I would have just then edited that text by
hand or else gone back to the LTspice schematic and moved C1
slightly. With slight adjustments, your schematic becomes:

: C2
: || 0p
: ,---||---,
: | || |
: | | .ac oct 25 1k 10Meg
: | R5 |
: +--/\/\--+ .include opamp.sub
: | 100K |
: | |
: | |
: C1 | |+ |
: || 1.6n| |\ U1|
: ,-||-----+---|-\ |
: | || | >-+--
: | ,--|+/
: | | |/ aol=10k
: gnd --- |- GBW=10Meg
: V1 -
: ---
: AC 1. -
: |
: |
: gnd
Click to expand...
That was also automatically generated, doesn't require an
attachment or some series of ASCII commands that need to be
copied, pasted, saved, and then loaded up to see. And it is
produced directly from the schematic, so any changes made to
it are automatically hauled in, rigorously.

Your schematic would have been easy to do by hand, I suppose.
Winfield Hill routinely did a good job of it when posting,
all by hand.

Anyway, there it is.

Jon
 
George Herold wrote:
On Apr 5, 1:00 am, ehsjr <eh...@nospamverizon.net> wrote:

George Herold wrote:

I�m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I�ve gotten myself confused. I�m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is �blowing up� (going to zero).

I had this �silly� idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn�t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Spice file follows. (I tried adding the file as an attachement but
couldn't do that via google groups.)

Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub

I�m a bit embarrassed to be asking this question, (I thought I
understood this circuit.) But I�ve gotten myself confused. I�m
looking at voltage noise gain in TIA amplifiers. I understand the
cause of the gain increase as the frequency goes up, the capacitance
to ground at the inverting input. But why do I get a huge peak right
where the voltage gain crosses the open loop gain curve of the opamp?
Do I need to write down the loop equations and solve them? Some term
in the denominator is �blowing up� (going to zero).

I had this �silly� idea that the noise gain is rising at 10dB/dec and
the opamp open loop gain is decreasing by the same amount so that once
they meet the gain would then stay constant as a function of
frequency. This obviously doesn�t happen, and I did a quick
LTspice.

George (feeling like an idiot) Herold

Attaching your schematic .asc as a test.

[TIA amp.asc1K ]Version 4
SHEET 1 5945676 13421556
WIRE 1440 -96 1392 -96
WIRE 1536 -96 1504 -96
WIRE 1392 -32 1392 -96
WIRE 1424 -32 1392 -32
WIRE 1536 -32 1536 -96
WIRE 1536 -32 1504 -32
WIRE 1296 64 1264 64
WIRE 1392 64 1392 -32
WIRE 1392 64 1360 64
WIRE 1440 64 1392 64
WIRE 1536 80 1536 -32
WIRE 1536 80 1504 80
WIRE 1568 80 1536 80
WIRE 1440 96 1408 96
WIRE 1408 112 1408 96
WIRE 1408 224 1408 192
WIRE 1264 240 1264 64
FLAG 1264 240 0
FLAG 1408 224 0
SYMBOL voltage 1408 96 R0
WINDOW 0 44 47 Left 0
WINDOW 123 44 90 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value ""
SYMATTR Value2 AC 1.
SYMBOL cap 1360 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1.6n
SYMBOL OPAMPS\\OPAMP 1472 16 R0
SYMATTR InstName U1
SYMATTR SpiceLine Aol=10K
SYMBOL res 1520 -48 R90
WINDOW 0 56 92 VBottom 0
WINDOW 3 34 27 VTop 0
SYMATTR InstName R5
SYMATTR Value 100K
SYMBOL cap 1504 -112 R90
WINDOW 0 -2 69 VBottom 0
WINDOW 3 -27 -19 VTop 0
SYMATTR InstName C2
SYMATTR Value 0p
TEXT 1888 256 Left 0 !.ac oct 25 1k 10Meg
TEXT 1888 296 Left 0 !.include opamp.sub- Hide quoted text -

- Show quoted text -


OK did you do that via google groups? I mucked about, but couldn't
get it to accept an attachment.

George H.
Click to expand...
No, not Google Groups. And the test failed - the attachment does
not open in LT spice when you double click it. It still can be
saved to file and then opened with LT spice, I think, but I
wanted to see if it would open directly.

The cause of the failure has nothing to do with your schematic - I
just used yours to kill two birds with one stone: provide an
attachment that others could use without the need for cut and paste,
and test whether my attachment of the schematic could be launched directly.

Ed
 
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