The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> 6 X supply voltage seen on RF FET drain
https://designers-guide.org/forum/YaBB.pl?num=1299009405

Message started by daveC on Mar 1^st, 2011, 11:56am

Title: 6 X supply voltage seen on RF FET drain
Post by daveC on Mar 1^st, 2011, 11:56am

Hi I was wondering if someone can help me with an issue I don't understand. On the output drain of an RF FET transmitting at a VHF frequency under exceptional circumstances, i.e. an open circuit I have measured with an oscilloscope 6 X the FET supply voltage pk-pk, it does not die down and is a steady RF oscillation. I understand that poor mismatches can cause a standing reflected wave which can cause higher voltages but I don't understand how it is possible to achieve 6X the FET's supply voltage. Any ideas, thanks in advance

Title: Re: 6 X supply voltage seen on RF FET drain
Post by loose-electron on Mar 2^nd, 2011, 4:06pm

Reactive ringing of an inductance is probably whats happening.

Now, if there is an inductance that is getting given driven, that can be a steady state signal.

Title: Re: 6 X supply voltage seen on RF FET drain
Post by rfcooltools.com on Mar 3^rd, 2011, 12:39pm

If the Fet is oscillating usually it will start to draw more current then normal operation this is one factor you should be aware of since depending on the Q of the load the bias current can run up. Also note that the circulating current in a resonant load (typically called a tank circuit) fed by a transistor in oscillation can be significantly larger than the transistor bias current, especially if the Q is high. It is this circulating current multiplied with the equivalent parallel Resistance that sets the voltage. So the higher the Q the higher the voltage due to higher current and higher parallel equivalent resistance.

http://rfcooltools.com