I=92m a bit embarrassed to be asking this question, (I thought I understood this circuit.) But I=92ve gotten myself confused. I=92m 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 =91blowing up=92 (going to zero).
I had this =91silly=92 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=92t 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=3D10K 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=92m a bit embarrassed to be asking this question, (I thought I understood this circuit.) But I=92ve gotten myself confused. I=92m 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 =91blowing up=92 (going to zero).
I had this =91silly=92 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=92t happen, and I did a quick LTspice.
George (feeling like an idiot) Herold