out of band gain power amplifier
I am designing an RF power amplifier operating at C band. I am simulating some gain (spikes) at low frequencies up to 4 GHz. The transistor itself shows obviously a very high gain at low frequency and it appears difficult to me to lower this gain with the matching network. The power amplifier is design using an hybrid approach and SMD passive components.
Do you have any idea for a micro-strip topology which could help in this way? How to get a bandpass response type for the PA?
Thanks for your contribution
To minimize the gain at low frequencies, better than using a band-pass topology is to use a high-pass topology.
Unfortunately high-pass response cannot be done using only microstrip components, because the circuit needs at least a series capacitor. There is possible to get some end-coupled capacitors in microstrip, but with very low values.
While you measure your amplifier with VNA and if you see some unpredictible spikes ,your amplifier probably oscillates.
Check the oscillations with a SA up to 50GHz..
If you have gain margin, you can use limiter, such as RLM-63-2W+.
@ vfone :
I have a series capacitor in my input matching network, this is a DC block capacitor (SMD). How to you do usually to get a such high pass circuit?
@ BigBoss :
I am designing the amplifier, I did not built it yet. Actually I am trying to avoid getting an oscillator. :) that s why I would need to find a way to kill low frequency gain
@ tony_lth :
Thank you for the information, however this component is not qualified for the targeted application...
Series cap and a shunt inductor is the simplest high-pass filter. Adding more series/shunt components (poles) you get better rejection at low frequencies.
The high-pass filter could be part of the matching network, tuned to get better stability at low frequencies.