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I am planning to use the BPF-A127+ band-pass filter with the GALI-74+ low-noise amplifier, which are from Mini-Circuits. The datasheets are provided below:

BPF-A127+ Datasheet

GALI-74+ Datasheet

There will be a bias tee at the output port of the LNA. And, there is a question mark on the DC blocking capacitor's value. The minimum capacitance could be set in accordance with the characteristic impedance of the band-pass filter, but finding the optimum value is, if not impossible, tricky as the inductor value just after the input port of the filter is unknown. Is there any practical tips to select the capacitance so that the filter's specified response could be maintained as much as possible, assuming that the load is properly coupled to the output port of the filter?

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  • \$\begingroup\$ Please draw a block diagram of how they are connected together to remove any possibility of ambiguity. Your words initially appear to suggest that the BPF feeds the LNA but, your words about the bias tee suggests otherwise. \$\endgroup\$
    – Andy aka
    Commented Jun 21 at 14:24
  • \$\begingroup\$ @Andyaka The filter comes after the LNA. \$\endgroup\$
    – user296701
    Commented Jun 21 at 14:36

2 Answers 2

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Is there any practical tips to select the capacitance so that the filter's specified response could be maintained as much as possible, assuming that the load is properly coupled to the output port of the filter?

The best way to deal with this is to ensure that the reactance of the capacitor is significantly smaller than 50 Ω at the lowest frequency of interest. Nothing good will come of trying to find an optimum value because it will disrupt the input impedance of the filter (designed to be 50 Ω).

So, ensure that the reactance of the coupling capacitor is less than (say) 10 Ω at 100 MHz. A 150 pF capacitor has a reactance of 11 Ω at 100 MHz and is the smallest value I would choose in this situation.

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  • \$\begingroup\$ Yes, that approach could save the day. But, what about the upper limit on the capacitance? Apart from the form factor, is it limited by the introduced ESR and ESL? \$\endgroup\$
    – user296701
    Commented Jun 22 at 17:50
  • \$\begingroup\$ @Karakoncolos of course there will be limitations but, I bet the ESL won't come into play for a 150 pF C0G/NP0 type until well over a few giga hertz. The ESR of that type of capacitor is in the milliohm range i.e. of no consequence. \$\endgroup\$
    – Andy aka
    Commented Jun 22 at 19:15
  • \$\begingroup\$ Alright, thanks. \$\endgroup\$
    – user296701
    Commented Jun 23 at 9:35
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Mini-Circuits provides S-parameters for both components which makes it easy to find out how both behave when cascaded. Between the amplifier and the bandpass filter I chose to insert a small ~2dB attenuator that also serves as bias-tee for the amplifier. The supply voltage (9V) can be applied at the bottom of the 150 Ohm resistor / 680nH inductor. Power supply decoupling capacitor(s) and amplifier input DC-block are not shown. Overall gain in the airband is 20dB.

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Gain and input return loss:

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Gain and output return loss:

enter image description here

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  • \$\begingroup\$ Thanks for giving an insight. \$\endgroup\$
    – user296701
    Commented Jun 22 at 17:47

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