Wideband LNA with cutoff in low frequency
I've to use a wideband LNA (like the ABA-53563 from AVAGO, it's a "internally 50-ohm matched silicon monolithic amplifier that offers excellent gain and flat broadband response from DC to 3.5 GHz").
My concern is that I've to pass the low frequencies as well, typically 50kHz. I calculated that I've to choose about 100nF as a capacitor. The problem is that this kind of capacitor has a weak behavior in the high frequency region (become a inductor).
What can I do ? Putting 100nF in // with 1nF for example? Somebody has a trick?
Yes, placing a small value capacitor in parallel with the big one might do the job. Is possible to need an extra inductor to compensate the broadband matching.
The bad thing is that Avago gives S-parameters only down to 50MHZ, and you cannot simulate the circuit below this frequency...but only doing real measurements.
By the way...sometimes this DC starting point of broadband MMIC amplifiers is just a marketing terminology.
somebody actually makes a chip capacitor that is two capacitors bonded together for that purpose. The manufacturer escapes me at the moment.
You should be aware that a lot of amplifiers with these very large bandwidths do not actually go all the way down to DC. They stop measuring at something like 50 MHz, because below that the gain goes to hell! The reason is that the amplifier is made up of two transistors with feedback (cascode or darlington), and the feedback kills the gain at lower frequencies. Make sure the amp you are using actually does have "DC" gain!
Hello,
Thanks for your answers.
I think that we where wrong with the capacitor resonant frequency.
I made the equivalent circuit of a real capacitor (R-L-C with parameters C=100nF, ESR=0.08ohm and fc=10MHz) and I put this circuit at the input of a 50 ohm impedance system (LNA or whatever) and the result is quite good: the upper cutoff frequency depends only of the series inductor and the input impedance (I found fcut =R/(2*pi*L)=+-3GHz). Nice, isn't it? No more small capacitor required! That confirms what I've seen also on some high-speed development kit. Sometimes they put such big capacitor (order of 100nF) on their transceiver lines (2Gb/s) and I was wondering how this stuff worked.
I've attached the schematics and bode plot + a matcad calculation.
What is very important for me is that I've to be sure that a given amplifier can do its job (as stated in the datasheet) with the low frequencies. Is someone has some references of good IC? (Gain: 20dB, 50kHz->2GHz)
Hello,
Any news ?