Power Amplifier - Page 1
Forum section: electronics / rf-microwave

author: tancheehao
author_rank: Contributor
timestamp: April 08, 2025, 02:51:23 pm
content: I am interested to design a power amplifier from 2 to 4GHz to match 50 ohm of load. 1.8V or 3.3V as supply input to the amplifier.What is the expected output power at those frequency?How do I choose the transistor size as well as the biasing point.

author: CaptDon
author_rank: Super Contributor
timestamp: April 08, 2025, 04:52:35 pm
content: Using 3.3vdc as the raw input won't get you much power at all. The most I could possibly imagine would be around .5 to 1 watt. There are off the shelf circuit boards and internally impedance matched LDMOS hybrid amplifier modules for those frequencies but If you want over a watt of output you are going to need a higher raw D.C. voltage. Why design it yourself? C.O.T.S. solutions exist.

author: tancheehao
author_rank: Contributor
timestamp: April 08, 2025, 10:34:25 pm
content: I have to design it using Cadence.

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2025, 12:29:03 am
content: Is cadence capable of doing the strip line impedance matching stubs and so forth? with 3.3vdc input and a good tank circuit the transistor collector would swing about 6 volts peak to peak, 3 volts peak and about 2 volts RMS. That isn't much power at 50 ohms. You will need an impedance matching network on the collector or drain which would show a load impedance of perhaps one or two ohms while the output impedance was transformed to 50 ohms to get any decent power output. A 2 to 4GHz bandwidth is a full octave. Constant impedance matching across an octave will need some consideration. If this is your college project please don't ask us to design it for you!! I have had too many college grad engineers come to our company with degrees that weren't worth the paper they were printed on because the grad didn't do his/her own design work and it showed.

author: tancheehao
author_rank: Contributor
timestamp: April 09, 2025, 12:54:27 am
content: I am using MOSFET. Do you mind to show the diagram for tank circuit and its steps?

author: CaptDon
author_rank: Super Contributor
timestamp: April 09, 2025, 01:15:08 pm
content: There are many examples in 'Microwave Design Guide' publications online and also examples in the data sheets from RF semiconductor manufacturers. They have already done your work for you. MiniCircuits, MACom and others offer already built 2 to 4 Ghz amplifiers and some of the semiconductor folks show designs in their app notes. None of this will be of educational value in a 'cut and paste' world. If you are going to obtain a job in the world above 1GHz you need to start learning about stripline design because it is the basis of microwave amplifiers, receivers and filters.

author: tancheehao
author_rank: Contributor
timestamp: April 10, 2025, 01:01:46 am
content: I want to design it from scratch.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: April 10, 2025, 02:40:42 am
content: Quote from: tancheehao on April 10, 2025, 01:01:46 amStart scratching.

author: radiolistener
author_rank: Super Contributor
timestamp: April 10, 2025, 05:13:04 am
content: Quote from: tancheehao on April 08, 2025, 02:51:23 pmThe expected output power of a power amplifier depends on several factors, including the efficiency and the required current. Typically, power amplifier efficiency ranges from 30% to 60%. For instance, if you need 10 W of output power, the amplifier would need to consume approximately 30 W of input power, considering typical efficiency.When using a 1.8 V power supply, the required current would be around 30 W / 1.8 V = 17 A. However, designing such a system with this high current could be challenging, as it would result in significant heat dissipation, potentially degrading efficiency further.Using a higher voltage supply would help reduce the current required, making the design more simple and feasible and improving overall efficiency.Quote from: tancheehao on April 08, 2025, 02:51:23 pmThe choice of transistor for a power amplifier is largely dependent on the operating mode (сlass A, AB, or C), which is influenced by factors such as circuit design, supply voltage, current, output power, operating frequencies, and impedance matching. Additionally, the selection of the operating mode and, consequently, the transistor type is heavily influenced by whether linear operation is required or if non-linear distortion can be tolerated. For applications where linearity is crucial, Class A or AB biasing is often preferred, while Class C may be suitable for high-efficiency applications where distortion is less of a concern. Proper impedance matching and thermal management are also critical to ensure optimal performance, efficiency, and stability.Developing a power amplifier is generally not a straightforward task, even for relatively low frequencies (1-150 MHz). Therefore, if you lack experience in this field, jumping directly into designing microwave power amplifiers might be too complex. I advise to start with the design of a shortwave power amplifier, for example, with a power output of around 100W and working frequenciy range 1-30 MHz. An even better starting point would be to begin with an audio power amplifier, where high-frequency and microwave related challenges are not present.

author: tancheehao
author_rank: Contributor
timestamp: April 10, 2025, 07:11:53 am
content: I am interested to design a microwave power amplifier. The output power seems to saturate.

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 07:33:31 am
content: Quote from: ahbushnell on April 10, 2025, 02:40:42 amfind my idea about using fine scribes to scratch RF inductors or antennas instead of masks

author: stretchyman
author_rank: Regular Contributor
timestamp: April 22, 2025, 07:55:53 am
content: Whilst designing a simple amplifier (from scratch) may seem simple and asking a question on here will yield results, it simply(!) will not. Nether will a supply of 3.3V.You will have to do a lot more research and would recommend the use of ADS (other programs are available).From scratch I'd say you're looking at a couple of years work from having zero knowledge to be capable of designing something useful.Something I had to do myself so can speak from experience!

author: tancheehao
author_rank: Contributor
timestamp: April 22, 2025, 03:42:17 pm
content: Wanted to enhance my understanding! Do you mind to share the steps involve?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: April 22, 2025, 03:43:36 pm
content: Get an EE degree.

author: tancheehao
author_rank: Contributor
timestamp: April 22, 2025, 04:12:46 pm
content: Haha. EE degree might not be so practical or hands on.

author: stretchyman
author_rank: Regular Contributor
timestamp: April 28, 2025, 06:50:16 am
content: Spend a couple years in an RF lab and learn to use ADS or something similar.This is not a subject you gain any knowledge of by asking a few vague questions or for that matter watching any YouTube videos.However on my last point I did see some ADS tutorials on there so have a look.RF design is a highly paid and skilled field of knowledge and will not be learnt easily.Probably best to enrol in a degree course at a local university and learn some basics.