# RF chock DC block cancels my small signal totally in LTSPICE

#### yef smith

Joined Aug 2, 2020
488
Hello, I am building a Common source amplifier as shown bellow.
As you can see bellow The Vsource signal is good sinus.
but after the capacitor the sinus is totally canceled and i got only the DC bias.
I am supposed to see the sinous with DC offset after the C2 capacitor.
Where did i go wrong?
Thanks.

#### ericgibbs

Joined Jan 29, 2010
17,410
Hi,
Work out the impedance of a 1pF cap at 10kHz.
E

#### BobTPH

Joined Jun 5, 2013
6,508
Your 1nH coil is basically a wire holding the gate at the voltage of the bias supply.

#### BobTPH

Joined Jun 5, 2013
6,508
Oh, and RF chokes do not block DC, they block AC at high frequencies.

#### Bordodynov

Joined May 20, 2015
3,059
L1 is very small and it ruins the signal. Put an inductance value of 1 mH. In general you can use a resistor instead of inductance in that circuit. Use a 100 kOhm resistor. Also increase the capacitor value. Inductance and capacitance form a voltage divider. Calculate their inductance and capacitor values at 10000 Hz.

#### yef smith

Joined Aug 2, 2020
488
Hello ,my formal work frequency is 100Mhz, so i want my AC signal to pass .
I will use C=1.59*10^-9 to get |Zc|=1ohm
For RF choke i put so RF will tacke high resistance and DC will tack no resistance. 1mH Z_L=2*100*10^6*1mH=6.28*10^5Ohm

What real world values do you recomment for RF CHOKE and DC BLOCK

#### BobTPH

Joined Jun 5, 2013
6,508
Hello ,my formal work frequency is 100Mhz, so i want my AC signal to pass .
It would have helped if you told us that initially, instead of leading us to believe it was 10KHz.

#### yef smith

Joined Aug 2, 2020
488
Sorry

#### BobTPH

Joined Jun 5, 2013
6,508
You original values had a cutoff frequency of 5GHz.