Gate bias - RF Cafe Forums

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SDO
 Post subject: Gate bias
Posted: Fri Oct 22, 2010 5:59 am 
 
Captain
 

Joined: Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Hi,
Can any please explain to me how do we select the value of gate bias resistor in combination with inductor in the design of RF power amplifier. Inductor is frequency dependent but why do we use varios values of resistor apart from stability.
Thanks.
SDO
 
   
 
IR
 Post subject: Re: Gate bias
Posted: Mon Oct 25, 2010 2:00 pm 
 
Site Admin
 

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi,

Don't you mean a voltage divider?
if you mean a single bias resistor, can you show an example of such circuit?

_________________
Best regards,

- IR
 
   
 
SDO
 Post subject: Re: Gate bias
Posted: Thu Oct 28, 2010 2:48 am 
 
Captain
 

Joined: Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Hi,
Thanks for your reply. I am attaching JPeg of Schematic showing 27R resistor (R39). Please explain how do we select this select resistor. This PA has been designed for 60MHz.
I cannot attach/insert Image, anyhow this resistor is from gate to a series inductor and after that are decoupling caps on the gate bias. SO it is like DC--->decoupling caps--->inductor--->resistor--->gate. It is definitely not a voltage divider but used to present low impedance at low frequencies.
 
   
 
IR
 Post subject: Re: Gate bias
Posted: Thu Oct 28, 2010 4:33 am 
 
Site Admin
 

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
I guess that this is a recommendation by the transistor manufacturer.
Have you tried to simulate the circuit with and without the resistor?

This resistor is most probably used for stability and it works to stabilize the amplifier by reducing the gain in low frequencies (Where the gain is extremely high). The resistor works for DC to very low frequencies in which the reactance of the inductor has almost no influence.

_________________
Best regards,

- IR




Posted  11/12/2012