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currently I'm studying MMIC amps like this one:

MMIC

From what I can understand, these MMIC claims to be wideband, and I guess that the operating frequency is given by the components around the MMIC.

Let's take a look to the recommended application circuit for the MAR-8 (datasheet): MAR-8 recommended application circuit

First of all, the first important component is the bias resistor: is determined by the given supply voltage for the circuit (eg if Vcc is equal to 7 volt, the bias resistor must have a value of 88.7 ohm; if the Vcc is equal to 12 volt, otherwise, the bias resistor must a have a value of 226 ohm). Another thing that is not clear to me are the values of the two capacitors "Cblock" and the value/presence of the inductor, since, on the above datasheet, the inductor is also stated as optional; I guess that the values of Cblock capacitors and of the inductor will determine the working frequency, am I right?

Well, let's take as example this wideband amp kit: RF wideband amplifier which claims to be able to operate from 0.1 Mhz to 2000 Mhz, and the supply voltage can go from 6 to 12 volt. Is not very clear to me: this kit seems provided with fixed values components: so: how is possible that we can apply a supply voltage from 6 to 12 volt, if there is a bias resistor with a fixed (and unknown) value? And is the same for the inductor and Cblock capacitors: if the inductor and these capacitors will determine a specific frequency, how, this kit (which is provided with fixed value components), would be able to achieve a very wide range of frequencies? Or is just a waste of money, since, maybe, wouldn't be able to work as claimed?

Ok: can someone explain me how to choose Cblock capacitors and the inductor (and why the inductor is stated as optional) to have a MMIC able to effectively work in a wide range of frequencies? And maybe can someone explain/show me a practical example with a schematic, eg, to achieve a MMIC circuit which is able to work from 800 to 1000 Mhz?

Please consider that I am a "newbie" about these subject, so I would appreciate a clear and exhaustive answer.

Many thanks.

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  • \$\begingroup\$ You need to know your source and load impedances, and consider the capacitive impedances in that context. \$\endgroup\$ – Brian Drummond Oct 16 '15 at 12:52
  • \$\begingroup\$ What evidence do you have that the "kit" uses the MAR-8 amplifier? Just because something is for sale doesn't mean it's any good (or has been designed properly) and that especially applies to some electronic crap I've seen on ebay. \$\endgroup\$ – Andy aka Oct 16 '15 at 13:38
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    \$\begingroup\$ You are trying to draw conclusions about a likely crappy circuit on ebay by looking at the data sheet of the transistor that has the same physical form factor with zero evidence that the two are connected. \$\endgroup\$ – Andy aka Oct 16 '15 at 13:45
  • \$\begingroup\$ Notice how the resistor/Vcc values all provide the same bias? ~3.8V @ 36mA. The part claims to be internally matched to 50ohm. The external components are selected so they don't tune Zin/Zout away from 50ohms at your operating frequency. \$\endgroup\$ – curtis Oct 21 '15 at 18:31
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The inductor is just an RF choke to block RF fro the power supply. Cblock in both cases is a blocking capacitor to block DC from the input and output pins of the MMIC. At Microwave frequencies, 100pf would work fine.

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  • \$\begingroup\$ Hi Rob, thank you for the simple but intelligible answer. But what about the inductor? In many MMIC's datasheets I can see that the inductor value is related to a specific frequency. Eg, in the HMC481MP86 datasheet - at page 8.7 I can see that to reach 50Mhz I need a 270 nH inductor; to reach 5000Mhz, instead, I need a 6.8 nH inductor. So: how to design an MMIC amplifier which would be suitable for a wide range of frequencies? I have to choose a "middle" value for the inductor? \$\endgroup\$ – Mister D Oct 17 '15 at 16:20
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    \$\begingroup\$ If you want to design for a wider range of frequencies, use the L1value that corresponds to the low frequency end of the range, for the RF choke. This wil still work for the higher frequencies as long as you don't approach the self-resonant frequency of what ever choke you use. If you do, you could just use a larger and a smaller choke in series do that the smaller one will. Still have a high impedance when the larger one is starting to resonate. \$\endgroup\$ – Rob Moore Oct 18 '15 at 21:27

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