Gain measurment - Page 1
Forum section: electronics / projects

author: Brians lab
author_rank: Newbie
timestamp: September 22, 2015, 07:27:02 pm
content: Hello to the forum. I have a question. But first let me tell you a bit about my experience. I have been in the electronics field for 25 years. I did bench repair and field service on many types of equipment. This is my first time into design though. I am in the process of building a rf amplifier. I am going to go through stages of design and hope I do it right. This leads me to my question. When you are trying to factor in gain do you compare the input signal from the rf generator unloaded on the scope or do you take a sample from the base of the transistor? I am having trouble coupling stages together without oscillation problems and I think I need to measure the base input and not the rf gen output. I have not did it yet as I am writing this and then I need to run some errands and when I get back I will head out to the shop. Hope to hear from someone on this matter.Thank You,Brian's lab

author: tggzzz
author_rank: Super Contributor
timestamp: September 22, 2015, 09:24:16 pm
content: Everything correctly loaded.

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 22, 2015, 09:24:48 pm
content: Quote from: Brians lab on September 22, 2015, 07:27:02 pmNeither. Typically, an RF amplifier is designed for a specific system impedance. 50 ohms is the most common, so I will use that as an example. The input power is the power you would measure with a power meter with a 50 ohm impedance. Your generator impedance should also be 50 ohms. So the voltage you would measure will be half the unloaded impedance. If you use a scope, then it would be the voltage measured with a 50 ohm input termination. (You would disconnect the amplifier, then connect the scope in it's place).Similarly, the output power is the power measured with a 50 ohm power meter. Or again, you could use a scope with a 50 ohm impedance. The gain is the ratio of output power divided by input power.This all depends on what you mean by RF amplifier. You might just mean something that amplifies RF frequencies. A high frequency op amp would be an example. Here people often just look at voltage gain. So the gain depends on how you define it.

author: Brians lab
author_rank: Newbie
timestamp: September 23, 2015, 12:59:36 am
content: That makes sense to me. I just came in from the shop and checked out the forum. Thank you for the explanation I understand it more clearly. I built the second stage of my amp. The basics are both stages are common emitter circuits. I built a common base amp but had trouble with oscillation but that is the problem I am having now. My first stage gain is not much but when I couple it into the next stage I get plenty of gain. On the order of 20mv in and about 1100mv out. This is of no use because the frequency changes and does not change when I check bandwidth so it must be oscillating I am using a breadboard and could be picking up reflected power or whatever. Has anyone else experienced this problem? Remember I am new to engineering so I have probably bent many rules not meaning to.Thank you for responding,Brian's lab

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 23, 2015, 04:51:43 pm
content: Oscillation is one of the most common RF amp problems.A bit too much to deal with here.You have some positive feedback somewhere. Either in the devices themselves, or in the circuit. Common feedback paths are ground loops and inadequate power supply decoupling.Every wire in your circuit acts like an inductor or transmission line. You have a single ended amplifier that requires every ground point to be at the same potential. So you need a solid ground plane. A copper clad blank pc board is a good ground plane. Keep all the wires short, especially the wires to ground. You might want to google "manhattan circuit construction" to get some ideas.You might also want to try testing each stage separately. If they don't oscillate when not connected together, that could be an indication there is some feedback through the power supply connection. Another cause could be that power is reflected at some point in the circuit, like the input of the second stage, and you have enough gain to amplify this signal bouncing back and forth into an oscillation. You could try putting an attenuator in between the stages to see if this is the problem.So a combination of good construction techniques and some trial and error may be required.

author: Brians lab
author_rank: Newbie
timestamp: September 24, 2015, 12:06:50 am
content: Thanks for the good tips. I have rebuilt the second stage and I used a common base design. I have got control of the gain of the circuit. I have lost all the gain out of the circuit but have regained frequency response. I need to go over the things you mentioned because it makes a lot of sense. This project only has to go to 500 to 700mv into 50 ohms. I should be able to use 3 transistors to get the results I want. I just came from my bench leaving a stable amp and still searching for the elusive gain I want. Let me ask this. When you couple the stages together and you get the out of control gain the amp is most likely oscillating right? I wonder if I should use negative feedback?Thank you for responding,Brian's lab

author: rfeecs
author_rank: Frequent Contributor
timestamp: September 24, 2015, 12:33:18 am
content: Not sure what you mean by out of control gain. But of course an amplifier should have linear gain, so output should be the same frequency as input.If you change the input power by 1dB, the output should change by 1dB. If you are getting non-harmonically related output frequencies then it's probably oscillating. If it's oscillating at a high frequency beyond the range of your scope or analyzer, then it may be harder to tell that it is oscillating.If the output power is constant, but at the right frequency, it might be saturated. In that case, reduce the input power until you get 1dB drop on the output for 1dB drop on the input.Negative feedback might improve stability, but will also reduce your gain.

author: Brians lab
author_rank: Newbie
timestamp: September 24, 2015, 11:46:54 pm
content: I want to thank you for the tips again. When I say out of control gain I would equate that to audio feedback as an example. I used some of your suggestions and I finally got control of the amp. Another way "out of control gain" equates to is when you get wave forms on the scope that exceed the volts/division scale. They also are not even the same frequency. I know it sounds strange but there is no way I was getting 20 v pp out of the circuit. I don't know how to explain this but I eliminated it thru filtering. I also discovered that without proper filtering the circuit would oscillate. I discovered this by removing the input source and viewing the scope output. I applied filtering and I eliminated the oscillation. I am in the process of transfering the circuit to a board and hope it still works! Then it will be on to stage 3. I want to thank you for taking the time to respond that is really nice.Thank you,Brian's lab