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Working with Current Feed Back OPAMPS- Strange outputs with THS3001

Shyam
Expert 1960 points
Other Parts Discussed in Thread: THS3001

about my problem: my application is a laser pulse detection, in this process when a laser pulse falls on photo diode it generates current, later stages include a I-V conversion, Gain,difference amplifier and comparator

about my pulse characteristics : pulse width min:10ns max:150ns rise/fall time:2ns-5ns pulse to pulse width:22uS

How i selected the opamp for Gain and difference amplifier stages

key parameters: BW - bandwidth and SR - Slew Rate

from this literature i found my BW can be calculated by formula

BW=0.35/(rise time)=0.35/5ns=70MHz

Now slew rate is to be calculated

SR = 2 * 3.14* BW * Vp = 2 * 3.14* 175MHz *4V = 2198V/us


qualifiying above requirements one such opamp is THS3001, with much better slew rate and BW

Please find the design file attached, which makes uses of THS3001, why am i not able to achieve the desicribed gain, i took the spice model given by TI and simulated it, stil i am not able to view proper output, why so ?

GAIN_THS3001.TSC

  • 0
    TI__Genius 12631 points
    Hello,
    The IN+ bias current is typically 2uA with a my of 10uA. In the typical case, the IB+*330kOhm resistor will results in an input offset of around 660mV. This is then amplified by a factor of (1+1k/100) = 11 which results in an output voltage which is greater than the supply voltage. Thus the outputs get clamped. You will need to reduce your gain or the size of R8 in the circuit.

    Before running transient and ac simulations it is good to run a dc simulation and look at all the nodal voltages in the circuit. If these look fine then you can trust that your transient and ac sims have found a good dc operating point.

    -Samir