RF amplifier does not work - Page 1
Forum section: electronics / rf-microwave

author: Bromsnortor
author_rank: Newbie
timestamp: June 29, 2022, 08:24:21 am
content: Hi, I am trying to build an oscillator with a bigger output amplitude than the oscillator itself creates. I have a collpits oscillator that generates a 200 mVpp voltage but I want my output to be bigger to drive a class E amplifier. I have tried to make a circuit that consists of a common collector then a common base to amplify and then a common collector again. I have tried to make the impedance match between the stages and I know that the last stage is not something I can build. Does anyone know how to amplify a 150 Mhz signal using this, or another circuit?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 29, 2022, 01:37:19 pm
content: Quote from: Bromsnortor on June 29, 2022, 08:24:21 amA RC coupled amplifier will tend to run into problems at frequencies around 150MHz (or even lower), as all the load resistors will be in parallel with circuit stray capacitance, so will deliver less gain than at lower frequencies & largish value coupling capacitors tend to look like series inductors, causing still more loss of gain.Broadband RC amplifiers at VHF need special design, including "peaking" circuits, & novel configurations to be effective.On the other hand, if you just need gain at a single frequency, or quite a narrow range of frequencies at VHF, you would be better served to use classic RF amplifier configurations with LC resonant tuned circuits used as the device load impedances.

author: BigBoss
author_rank: Regular Contributor
timestamp: June 29, 2022, 06:00:43 pm
content: What is your operating frequency ?? 150 MHz more or less ??OrYou would have a strong signal around 150 MHz from an oscillator ??That amplifier is not adequate for amplification. 150 MHz is not a negligible frequency to amplify with simple primitive transistor amplifiers. You should think to build it more seriously.If you need an oscillator that works around 150MHz, I can post my past design here. The output level is roughly 2.6Vpp after filtering.

author: Bromsnortor
author_rank: Newbie
timestamp: June 29, 2022, 09:47:36 pm
content: I would like to see your oscillator yes.

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2022, 12:22:20 am
content: C9 is crushing the output of the first emitter follower if it is connected to anything.

author: BigBoss
author_rank: Regular Contributor
timestamp: June 30, 2022, 02:10:54 am
content: Quote from: Bromsnortor on June 29, 2022, 09:47:36 pmThis oscillator has been designed for FM band as a VCO but it can be easily be modified for up to 300MHz.

author: profdc9
author_rank: Frequent Contributor
timestamp: July 03, 2022, 07:27:12 pm
content: I built a very similar amplifier that works up to 1 GHz. I used the 2SC3356 transistor which has fT=7 GHz. I used a nonresonant RC coupling, it can be done. It requires careful attention to layout and I used 0805 resistors/capacitors as a compromise so that they would large enough to handle, but could be placed close enough to minimize parasitics.I made a board with my various circuits on it. I made it so it can be populated by JLCPCB (though parts may need to be substituted because I put the BOM in so long ago).http://www.github.com/profdc9/RFUtilityKnife/I use Qucs-S to simulate the circuit and to simulate what the effects of various parasitics might be.This is a UHF probe amplifier I designed. For a common-collector front-end, it is important to have a series resistor near the input base. The emitter-base capacitance and the emitter lead inductance can form a series resonant circuit that can oscillate the base voltage, and so adding the resistor dampens this resonance. Basically this is a cascaded common-collector to provide a high impedance input, common-emitter to provide voltage gain, and common-collector to provide a low impedance output with an output pad for a better wideband match.This is an amplifier I designed that is similar to the above but it uses input termination which does increase the noise figure (probably to about 5 dB or so). So this isn't going to be lowest noise LNA if that's what you're looking for, and you will need a passive matching circuit to the high input impedance of the amplifier. I tried a common-base amplifier here too but it can get down to about 3 dB noise figure, but not much lower.I also attached some simple 2N3904 amplifier circuits I simulated for HF that might also give you other ideas. I was doing a study of noise figure of different configurations.You may be able to "dead bug" a 150 MHz amplifier but it may be difficult. I remember in college dead-bugging a 100 MHz amplifier and it worked, so possibly 150 MHz is doable as well.Dan