100 W class AB amplifier using BLF647P

Aparna Bhuvanagiri

Hi,
I'm designing a 100 W amplifier using BLF647P for the frequency range of 512- 1250 MHZ. I'm using ADS for my design.
According to the datasheet, The device is suitable for HF-1500 MHZ frequencies. I've simulated and plotted stability factor and stability measurement for the frequency range of 100 MHz to 3 GHz and found out that the device is unstable at frequencies around 2 - 2.5 GHz which is beyond the required range and device range.
Does this instability affect my amplifier and oscillate it even though it's not in my required frequency band?
Do I need to take any measures while designing matching network?

View Answer

Osman Ceylan

Hi Aparna,
That transistor is conditionally stable. It can oscillate. Therefore you have to consider it during matching network design. Since the gain of the device is not very high at that region, it is not difficult to make your amplifier almost unconditionally stable.
You need to check the stability factor with input and output matching network.
You can see this article: https://www.researchgate.net/publication/325101221_Refine_biasing_networks_for_high_PA_low-frequency_stability
Osman

Aparna Bhuvanagiri

Hi Osman,
I've seen the article. It's about stabilizing at low frequencies by carefully biasing. Can I apply this concept at higher frequencies? Here for my case at 2 GHz?

Aparna Bhuvanagiri

↪Osman Ceylan

Hi Osman,
I've seen the article. It's about stabilizing at low frequencies by carefully biasing. Can I apply this concept at higher frequencies? Here for my case at 2 GHz?

Osman Ceylan

For higher frequency, you can use R//C type network. I attached an example. You need to find suitable values which is not degrading the gain depending on your operating frequency.

Aparna Bhuvanagiri

↪Osman Ceylan

Thank you. I'll try and simulate for my frequencies.

Aparna Bhuvanagiri

↪Osman Ceylan

Also for push pull (balanced) type amplifier is stability analysis wrt one Fet enough?

Osman Ceylan

This is a kind of initial check for the transistor. If you do load-pull simulations, it is enough. After designing matching networks, it is necessary to check the stability of all circuit. Matching networks can create undesired resonances at out of operation band which can cause oscillation.

Aparna Bhuvanagiri

↪Osman Ceylan

thank you. I'll follow your suggestion.

Aparna Bhuvanagiri

↪Osman Ceylan

I'm replying after a year. I have followed your suggestion. Used conical inductor for DC feed (since wide band application) and used series R and parallel R-C network for making the device unconditionally stable. This have solved my stability problem and Amplifier is working as expected. Thank you so much for your suggestion and insight. :smile:

100 W class AB amplifier using BLF647P

Welcome!

Categories

More Discussions