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Input impedance into amplifier - mismatch

Discussion in 'Electronic Design' started by [email protected], Aug 3, 2005.

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  1. Guest

    Apologies for what may seem to be a very obvious question, but I've
    been struggling to understand exactly what is going on.

    I have an output from a photodiode, picking up a beat signal between
    two lasers of the order of 200Mhz. This signal is to be fed into a
    chain of RF electronics, all Minicircuits components at 50ohm
    impedance. Before this, I need to amplify the relatively weak signal
    from the photodiode(~-34dBm) to around -5dBm, and to this end I have
    purchased a Minicircuits ZFL series amplifier which should fit my gain

    Question is, assuming the load of the amplifier is 50 ohms (the other
    RF electronics) much effect does the input impedance have on the
    eventual signal?

    Impedances up to 5k from the photodiode circuit have been used
    (improves sensitivity to certain frequencies), but I suspect that its a
    pipedream to imagine this working with the amplifier.

    Any help greatly appreciated!

  2. wrote...
    You'll pay a considerable Johnson-noise penalty if you stick with
    low 50-ohm current-to-voltage signal impedances. Your 200MHz laser
    application may be a good fit for Phil Hobbs' common-base amplifier
    approach. Explore his web pages,

    Properly implemented, a common-base amplifier allows you to present
    a low or 50-ohm impedance to high-frequency PD signals, while still
    developing the signal output voltage across higher resistance values,
    reducing the effect of Johnson noise, which goes as sqrt(4kT/R).
  3. tlbs

    tlbs Guest

    I worked on a very similar project 2 years ago. We had an array of
    heterodyne detectors measuring the beat frequency of a LASER and it's
    time-delayed light. We were sampling the heterodyne signal at 2 GHz.

    Each of the 16 detectors had their own individual preamplifier, the
    inputs of which were matched to the detectors (high impedance). The
    outputs of the preamplifiers were 50 Ohms, so then the rest of the
    system was 50 Ohms. The gain of the preamplifiers was very high (on
    the order of 1000) and the noise figure was impressive for room-temp

    Unfortunately I don't recall the manufacturer of the preamps, and I no
    longer have access to that info.
  4. dalits

    dalits Guest

    Try Transimpedance amplifiers.
    That is what is used to match photo diodes to first amps, current to voltage
    conversion mostly.
  5. Phil Hobbs

    Phil Hobbs Guest

    It all depends on the photocurrent and the noise figure of the amplifiers,
    but generally speaking, you're right that 50-ohm amps are a poor choice.

    What is the optical power level at the photodiodes?


    Phil Hobbs
  6. Guest

    I have 3mW optical power at the minute, leading to photocurrents of
    around 1-2mA.

    I have researched into transimpedance amplifiers for the photodiode,
    but the problem is finding op-amps which are relatively cheap,
    non-surface mount (I would prefer to avoid the cost/inconvinience of
    producing PCBs) and of sufficient bandwidth.

    At the minute, careful photodiode selection has meant noise is fairly
    minimal (I'm quite surprised for what is essentially a simple resistor,
    current/voltage conversion) and should not infringe too much on my
    intended application.


  7. KoKlust

    KoKlust Guest

    At the minute, careful photodiode selection has meant noise is fairly
    Can you tell us what is actually the problem with your current solution?

    If you want the best signal to noise ratio, you will need to use a
    transimpedance amp since you can design its feedback network to match
    optimally to the photodiode impedance.

    Beware that if you want to do anything serious at 200MHz frequencies, you
    MUST design a pcb to keep the parasitic capacitances and inductances low.

    By the way you are lucky to receive so much light. :)
  8. Phil Hobbs

    Phil Hobbs Guest

    A very useful rule is that shot noise dominates thermal noise when
    2*e*I_dc > 4kT/R, i.e. when V_dc == I_dc*R > 2kT/e, 50 mV at room
    temperature. Thus with a room-temperature resistive load, or in your
    case an RF amp with a 3-dB noise figure, a photocurrent larger than 1 mA
    will put you in the shot noise limit.

    Noise temperature in RF amps is a very useful parameter when the source
    is not impedance-matched, as photodiodes are not. Unlike the situation
    in op amp TIAs, the noise temperature is not closely related to the
    temperature of the resistor itself. You can get RF amps with noise
    temperatures down to 35K at room temperature, or 10K at nitrogen
    temperature (77K). Cooled varactor parametric amplifiers can go lower
    still. This noise temperature is what you plug into the formula above,
    so with a cooled amplifier, you could be in the shot noise limit with
    only (2k/eR)*10K = 85 uA of photocurrent, even at 50 ohms.

    Thus if your RF amps are ordinary common-or-garden ones with NF~3dB,
    you're golden.


    Phil Hobbs
  9. Phil Hobbs wrote...
    Unless his photodetector is unhappy with so much light, and his 1mA
    exceeds the recommended maximum current-density for a spot of light.
    Just something that should be checked at high light levels.
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