why is the VCC tied to rf out in LNA application circuit RF2472G?

LNA Output pin.This pin is an open-collector output. It must be biased to
VCC through a choke or matching inductor. This pin is typically matched to
50Ω with a shunt bias/matching inductor and series blocking/matching
capacitor. Refer to application schematics.

Can someone tell me what the functionality of the "RF choke circuit" on pin 5 of the LNA? So pin 5 has both VCC (DC) and RF (AC)? Not sure what the description listed above really means.

please help.

The impedance of an ideal inductor is

Z = 2*pi*f*L

so at DC the impedance of the inductor is very low (just a small series resistance), at high frequency the inductor will look like a large impedance. So dc current will flow right through the inductor, but the RF signal will not.

pstuckey said:

The impedance of an ideal inductor is

Z = 2*pi*f*L

so at DC the impedance of the inductor is very low (just a small series resistance), at high frequency the inductor will look like a large impedance. So dc current will flow right through the inductor, but the RF signal will not.

Click to expand...

Dont you want the RF to come out of the rf pin #5 instead of DC? Why block the rf? and what is 1k doing?

Due to the internal MMIC structure you should follow its data sheet operation instruction to obtain the specified parameters. Your "Why?" can be answered by MMIC designers but the schematic is typical for most MMIC LNAs.
Like in any amplifier, the transistor collector or drain must be fed a DC current to amplify. On the same line the amplified RF output appears. You must feed the DC current, so use the choke as indicated, and the capacitor allows only the amplified RF to output. The 1K resistor flattens the choke impedance so no resonance peak occurs.

Learn about DC bias tee, this is the circuit composed of the choke and capacitor at MMIC output.

robismyname said:

Dont you want the RF to come out of the rf pin #5 instead of DC? Why block the rf? and what is 1k doing?

Click to expand...

You would really have to look at the S-Parameters on a smith chart, but I will guess that
the 10nH in // w/the 1K is doing a few things.
1) It's High impedance but allows DC
2) the 1k will dampen any Ocilations (just to be safe)
3) the Amps S22 + (1K//10nH) // (5.6nH +22pF) produces a very good noise match .

P.S. what is the Freq of Operation ? At 1GHz the 10nH ≈ 70Ω and the 5.6nH ≈ 30Ω

Element_115 said:

P.S. what is the Freq of Operation ? At 1GHz the 10nH ≈ 70Ω and the 5.6nH ≈ 30Ω

Click to expand...

2.4GHz......rf2472g

the Amps S22 + (1K//10nH) // (5.6nH +22pF) produces a very good noise match .

Click to expand...

I guess this match should be power output match.
Noise is not important at the amp output.

According to the data sheet of this part the base is connected to VCC and then in the notes for each pin they talk about pin 5 being the LNA Output pin.This pin is an open-collector output. It must be biased to
VCC through a choke or matching inductor. This pin is typically matched to
50 ohm with a shunt bias/matching inductor and series blocking/matching
capacitor. Refer to application schematics.

So to your point the VCC is on the base so again why place VCC on the RF OUT PIN other than just to do so because the data sheet said to do it. What does this have to do with open collector output? I thought open collector open was used when you wanted to connect multiple signals on a bus or wire. The only signal I have is RF so why do I need to apply VCC on the RF OUT?

---------- Post added at 23:44 ---------- Previous post was at 23:32 ----------

jiripolivka said:

Like in any amplifier, the transistor collector or drain must be fed a DC current to amplify.

Click to expand...

According to the data sheet of this part the base is connected to VCC and then in the notes for each pin they talk about pin 5 being the LNA Output pin.This pin is an open-collector output. It must be biased to
VCC through a choke or matching inductor. This pin is typically matched to
50 ohm with a shunt bias/matching inductor and series blocking/matching
capacitor. Refer to application schematics.

So to your point the VCC is on the base so again why place VCC on the RF OUT PIN other than just to do so because the data sheet said to do it. What does this have to do with open collector output? I thought open collector open was used when you wanted to connect multiple signals on a bus or wire. The only signal I have is RF so why do I need to apply VCC on the RF OUT?

robismyname said:

So to your point the VCC is on the base so again why place VCC on the RF OUT PIN other than just to do so because the data sheet said to do it. What does this have to do with open collector output? I thought open collector open was used when you wanted to connect multiple signals on a bus or wire. The only signal I have is RF so why do I need to apply VCC on the RF OUT?

Click to expand...

You need to provide Vcc on the RF output pin so that is can tie to the drain of the amplifier's transistor. In order to have a nice class A (linear) amplifier (which is what LNA's are), you also need to set the amp's bias point near the middle of it's operating range. The Vcc supplied to pin 1 goes through a bias circuit (see table on sheet 3 of spec) to set the bias point for the amplifier.

Class A amp schematic

You might want to go back and look at amplifier basics to get a firm understanding of the need for biasing, the various bias points for different classes of amplifiers, and why you need a voltage on the drain of a transistor in order to make it work.

enjunear said:

You might want to go back and look at amplifier basics to get a firm understanding of the need for biasing, the various bias points for different classes of amplifiers, and why you need a voltage on the drain of a transistor in order to make it work.

Click to expand...

will do. thanks for all the info, all of you.