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Transient signal amplification from millivolts to few volts using LF356 OP amp

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Nov 17, 2009
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I request you kindly go through the mail attachment also.

I want to amplify a signal transient which does not have a symmetrical shape (e.g. shapes like sine or square e.t.c) and certain portion of that transient varies with temperature sweep. The maximum and minimum amplitude of the signal are +5V and -100 mV, which are constant through out the temperature sweep. In that transient signal I want to observe and amplify a particular window region (highlighted with red color) i.e signal amplitude in the time period of T/4 seconds to T seconds and 5T/4 s to 2T s e.t.c (usually T varies from 1ms to 1 second) . It was found that the variation of the signal in that time period is hardly 50 mV to 150 mV w.r.t to temperature sweep. Now The rise time and fall time of the signal are in the range 100-300 nano seconds. I want to amplify the signal with out degrading the rise and fall times of the signal. Also I want to amplify the signal of the window region to volts (say with gain 10 to 100 ).
1. Kindly suggest me an op amp which is as good as LF356 or better performance (e.g. in terms of High input impedance, High slew rate, low noise).

2. If we amplify the signal (as shown in mail attachment) with gain 10, certain portion of the signal will get saturated to power supply voltages and rest of the portion will be amplified to volts. My doubt is if the Op amp get saturated to power supply voltages i.e. +/-15V, will it take any extra time to recover from the saturated levels and to follow the input signal whose out put is less than power supply voltages or say 10 Volts.

3. What parameter of the op amp defines how fast the op amp recovers from its saturated output.

Kindly reply to the above three point.

With thanks and Regards



  • transient amplification1.pdf
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The first point to consider is sufficient gain-bandwidth product (GBW) to achieve the required bandwidth at the intended gain. Assuming e.g. a necessary bandwidth of 5 MHz, the GBW should be 50 to 500 MHz. LF356 isn't a serious option. The amplifier doesn't necessarily need to be compensated for unity gain, so it's a bit more easy. High speed OP's often have maximum supply voltages below +/- 15V, so you need to consider your requirements.

Overload recovery will always add some signal distortion, but should be acceptable with many OPs without special means. It's mainly a matter of the internal OP design, e.g. compensation scheme. You'll find recovery waveforms in most datasheets.

The OP selection will also depend on your local resources.
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clamping diodes on the input or some other clamping arrangement (possibly in the feedback loop) will help limit the voltage range on the i/p to a suitable op-amp (or 2 in series to get the GBW) and allow you to use op-amps with lower voltage rails. Regards, Orson Cart.

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