first stages of a transmitter - Page 1
Forum section: electronics / projects

author: ant17
author_rank: Regular Contributor
timestamp: July 07, 2013, 12:30:33 am
content: hi guys i am new to this forum but i really need help with a project i am working on it going to be transmitter i have built a colpitts oscillator from a "practical colpits oscillator on YouTube" which is giving me a nice 3mhz sinewave at 3vP-P but i am using aPower MOSFETHiPerFETN-Channel Enhancement ModeAvalanche RatedFast Intrinsic DiodeIXFN140N30Pas my last stage high power transistor but in order to drive it correctly i needed to make a common collector to buffer the colpitts which works ok but its the next stage the common emitter amplifier i am having great trouble with this the amplifier need to get the output sine wave back in close proximity to 9vP-P to operate the high powered mosfet correctly but i can not get any gain out of the 2n222a transistor but the output always seems the same as the input which i don't under stand if any one can help i would be grateful i will attach a schematic of the circuit which are giving me trouble

author: xrunner
author_rank: Super Contributor
timestamp: July 07, 2013, 01:07:03 am
content: Well, I'm not the most qualified by any means to help you, but is the 2N2222 the best choice for an RF output? I thought it was mostly used as a switching transistor? I could be wrong though.

author: c4757p
author_rank: Super Contributor
timestamp: July 07, 2013, 01:11:41 am
content: 2N2222 will work just fine at 3 MHz.But... I'm not really sure what's going on around R3 and Q3. Any chance you can draw the schematic a bit more carefully?

author: ant17
author_rank: Regular Contributor
timestamp: July 07, 2013, 01:25:35 am
content: hi i tried to clean it up a bit better so i will repost it also sounds like i am using the right transistors but i still can't workout why the the common emitter in the schematic does not give any amplification

author: c4757p
author_rank: Super Contributor
timestamp: July 07, 2013, 01:30:28 am
content: I just tried the exact same thing (but just the common-emitter section) in LTspice and it amplifies quite well, with a very high gain. I'm not at my bench so I haven't tried it in real life, but I'm fairly certain it will work. Are you sure the oscillator is actually oscillating?Edit: Some of your wire junctions don't have a junction dot. That might mean they're not actually connected - I don't know because I don't recognize the software you're using.Edit: Wait a minute........ Colpitts uses an inductor! Where's your inductor?

author: ant17
author_rank: Regular Contributor
timestamp: July 07, 2013, 01:48:15 am
content: hi i for got to add it but did use it on the bench i am using orcad 16.6 i will ammend my schematic again sorry also i don't know why your one would work but mine not

author: c4757p
author_rank: Super Contributor
timestamp: July 07, 2013, 01:59:19 am
content: 1. Check what I said about the dots.2. Does your inductor have an inductance?Try simulating the circuit one part at a time. Drop the oscillator and just put in a 3 MHz signal source, and make sure the amplifier works.Also - I tried driving a similar MOSFET and needed a pretty beefy driver. How are you going to do it?And why do you need a 9Vp-p signal?

author: ant17
author_rank: Regular Contributor
timestamp: July 07, 2013, 02:32:35 am
content: i haven't simulated it as i don 't really know how to but and as to why i needed a 9vp-p signal was to drive gate of the mosfet as i tried it with just the oscilator and at turn on it stayed the full 9 volts the closer it got to the switch of or the leading edge it almost fell to ground just above so idecided i was not turning it on hard engough to keep it falling away leading edge of the wave

author: c4757p
author_rank: Super Contributor
timestamp: July 07, 2013, 02:40:52 am
content: Jesus... take a breath, would you?Periods aren't just for women, and full stops aren't just for cars...How exactly are you driving the MOSFET, and what is connected to it on the output side?

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2013, 02:53:36 am
content: RC coupled transmitters don't work at all well--perhaps you should have a look at some real transmitter circuits!Also,the 2N2222 is an Emitter Follower or Common Collector stage----it has a Voltage Gain of 1.It does have Power Gain,but I doubt that you are measuring power.By the way,the Licencing Authorities may not be impressed if you do get this thing to work & fire it up on 3MHz.The Radio spectrum is a controlled asset,so without a legal reason to be there,you may be in big trouble.But then,you might want to be Bubba's new special friend!

author: c4757p
author_rank: Super Contributor
timestamp: July 07, 2013, 02:54:28 am
content: Quote from: vk6zgo on July 07, 2013, 02:53:36 amThere are two 2N2222s in that circuit.Otherwise - good points.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2013, 03:08:56 am
content: OOPS!I had the screen expanded for something earlier & forgot to reset it!The comment about "Bubba" still stands,though!

author: SeanB
author_rank: Super Contributor
timestamp: July 07, 2013, 05:43:05 am
content: C7 probably is going to be a pretty good approximation of a resistor at 3MHz, depending on the capacitor used, try using 100n ceramic in place of it.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: July 07, 2013, 06:35:31 am
content: Definitely need a tuned circuit comprising a capacitor and inductor, or better still if you just need a fixed frequency, a crystal.As a starting point I suggest building this Colpitts oscillator circuit with a 3.58 MHz crystal (cheaply available). It should start up right away.When confirmed it's working you can put in a tuned circuit instead of the crystal. This will require a coil wound on a plastic former or toroid and a capacitor to get it to 3 MHz. Roughly 10 uH and 200pF will resonate at around 3.5 MHz but there are online LC calculators that work it out.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 07, 2013, 06:56:38 am
content: Having blown my credibility,I'll make the following comments.Even without the lossy C7,the thing is probably starting to roll off pretty savagely at 3Mhz.Using the old 2.2CsRL formula.with a rough guess of 20pf forCs,the 3dB point is around 1.7MHz.Of course,I might have overestimated the value ofCs!If the OP has made a real one,the obvious thing to do is check the DC conditions of the 2N2222 stage,as he should get something out of it.The OP's comments about the MOSFET are a bit confusing,as it sounds as if he is driving it without any external bias like a self biased vacuum tube in class C mode.

author: ant17
author_rank: Regular Contributor
timestamp: July 07, 2013, 08:50:39 pm
content: ok guys thanks for the comments but i just got a book orcad and pspice which i will read first then i will simulate it and see what i am doing wrong or if it will work and i really appreciate your help

author: ftransform
author_rank: Frequent Contributor
timestamp: July 08, 2013, 02:14:06 am
content: BTW, if you want to make your schematics easier to read use a separate ground and separate VDD for every ground and VDD connection. The extra lines you draw connecting everything to a common ground "a star ground" in the schematic are visually distracting.Once you draw a complicated circuit with many parts you will probably find yourself getting lost in all the lines. I recommend even doing so if the grounds are right next to each other, I.E. no two lines going to the same ground.Some people don't do it but most professional schematics (app notes, etc) that you see will do it.