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Droop/backswing in 3-stage amplifier

I'm attempting to build an amplifier for a photomultiplier to measure transient pulses of light with a characteristic time constant on the order of a few microseconds and rising edge of a few nanoseconds. The anode is terminated into 50 ohms (for speed reasons), and the voltage across that resistor is amplified in three stages by three noninverting AD797s, each with a gain of 10. The configuration is the same at each stage: it's nearly the same as shown in Figure 39 of the AD797 datasheet, with R1=33.2 ohms, R2=300 ohms, Cl=5 pF. There is no resistor RL to ground, although I have a 300 ohm resistor followed by a protection diode to ground (Vbias=0.5 V) in between Vout and Vin for two successive stages. I do not have bypass capacitors, although I am using an Acopian +/-15 V linear regulated supply.

Now for the problem: Amplification after one stage works fine, with little to no distortion and a gain of 10 as expected. After two stages, however, I see a significant droop/backswing in the pulse. I've attached an Excel file with an example waveform (note, the photomultiplier produces a negative voltage across the terminating resistor, and its peak was clipped by the oscilloscope since it was off-scale). The third stage does not work at all. Even with no light input, the output at this stage oscillates between +/-2 V.

In regards to the former problem, there are two things I can think of trying: lower the gain per stage or adding a small load resistor to ground between each stage. The first is just based on intuition, but in the second case I wonder if the added resistor could serve to discharge any input capacitance present int the AD797 inputs that may be charging up during the pulse.

For the second problem, I can only guess that I would need to lay out my board better. I've attempted to keep leads as short as possible, with the largest ones being ~1 cm. Note that all elements are soldered into a breadboard.

Are these ideas worth pursuing, or would I be better off looking in another direction?

ad797_two_stage3.xlsx
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  • Walter,

      As Jon pointed out, the Vos of the first stage gets multiplied by the overall gain.

    Second, The AD797 has extremely high gbw and is very hard to use.  People call in all the

    time with oscillations;  I'm amazed your board works at all.  It usually takes a four layer board

    with two bypass caps per supply very close to the part.

    I would consider using "tamer" parts, such as AD8597, AD8671, or ADA4084-1.

    Harry


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  • Walter,

      As Jon pointed out, the Vos of the first stage gets multiplied by the overall gain.

    Second, The AD797 has extremely high gbw and is very hard to use.  People call in all the

    time with oscillations;  I'm amazed your board works at all.  It usually takes a four layer board

    with two bypass caps per supply very close to the part.

    I would consider using "tamer" parts, such as AD8597, AD8671, or ADA4084-1.

    Harry


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