Need BJT amplifier noise help - Page 1
Forum section: electronics / beginners

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 05:34:32 am
content: I'm taking an electronics lab class this semester, and in the last lab we were to construct a common emitter amplifier using a BJT. Its a straight forward design. Vcc=11volts. From Vcc there is one resistor going to the base of 361k, from Vcc to collector there is one resistor 1k. An input cap of 415nf is between input and the base. We did a curve trace on the BJT before starting, and ours had a Beta-dc of 160. 2n2222 npn BJT by the wayUnder DC conditions everything is spot on. No problems at all.However, we were instructed to hook up a 20db attenuator to the HP function generator in the lab, set it to 40kHz, and adjust the amplitude until the output measured at the collector was 2Vpp-rms.The equipment in our lab is ancient, and the signals from both the function gen and the output were just complete noise. There is no way you could even make a barn door estimate from them. It was a total joke. My lab group tried moving to another bench with newer equipment, and the scope wouldn't turn on. I got pissed, and told my group I would just do it at home.I have a digilent analog discovery, and I set it up as the function generator, set it to 40kHz and 4mv. I then hooked up the channel 1 scope to the function gen inputs to monitor the function gen, and scope channel 2 went on the collector.The input looks fine. Its a smidge noisy because its so low, but its a very clear sign wave, no problems at all. The output is complete and total junk. The amplitude of the noise roughly corresponds to the amplitude of the function gen, but there is no discernable pattern to it. There is no way you could see a sign wave in it.I'm pretty much stuck at this point. Is the BJT bad, does a bad one do this? Perhaps something else? Help!I asked the teacher about it, he isn't real interested it seems and just wanted us to guesstimate from what we saw, so I came here.

author: amspire
author_rank: Super Contributor
timestamp: April 02, 2013, 05:42:05 am
content: Two questions:1. What is the DC voltage on the collector of the 2N2222?2. If you connect the channel two input (set to AC coupling) to the 11 V, how much noise do you get?

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 05:47:25 am
content: Am I correct in thinking that you have both resistors running straight to Vcc? Move the base resistor so that it runs to the collector, and drop its value a bit. When you run them both to Vcc, there's nothing keeping the transistor from saturating, so you end up having to nearly starve it of base current (which I see you've done) to make it work. It will be very susceptible to noise of all kinds.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 05:54:31 am
content: Quote from: skysurf76 on April 02, 2013, 05:34:32 amYou were completely unable to get the circuit running and he wasn't interested at all?Pleasetell me you are referring to the overworked and disinterested TA who got stuck with the lab group and not the actual prof.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 05:55:33 am
content: Quote from: amspire on April 02, 2013, 05:42:05 amDC voltage at the collector should be about half Vcc per our design. I just measured it and its 5.614v.I hooked up channel 2 as you requested and its completely silent even on millivolt ranges. Over the holidays my present to myself was an Owon ODP3032 power supply.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 05:56:41 am
content: Quote from: c4757p on April 02, 2013, 05:47:25 amCircuit only has two resistors. One between Vcc and the base, one between Vcc and collector. That was what our lab manual instructed us to construct.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 05:58:35 am
content: Your lab manual has instructed you to build a real bitch of a circuit to get working properly. The noise doesn't surprise me one bit, especially with the very high impedance on the base.Just for the hell of it, try it like I said and see if you get better results.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 05:59:03 am
content: Output that I'm seeing...

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 06:02:45 am
content: Quote from: c4757p on April 02, 2013, 05:58:35 amI don't understand. You want the base resistor going between the base and collector? I'm sorry, I'm not to the point where I can see this stuff in my head yet. =/

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:06:11 am
content: So C1 is the input and C2 is the output? Notice that it's notpurenoise - it does go down a bit as the input goes up, and vice versa, as it should. The way it keeps randomly spiking down suggests to me that the transistor is on the verge of saturation. Try this test - keep the circuit running like that, then put your fingers tightly around the transistor to warm it up. You should get more of those downward spikes.Quote from: skysurf76 on April 02, 2013, 06:02:45 amThat's OK. Takes practice. I attached a schematic.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 06:13:03 am
content: Ok I moved the base resistor from Vcc to the collector. I'm seeing the following....

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:15:50 am
content: FYI, I just threw together your circuit and it works fine. It is highly susceptible to variations in well, just about everything, but it works. No noise.That output is absolutely horrible. Make sure the transistor is connected the right way around (that looks kind of like reverse breakdown), and just to be sure, make sure there's nothing wrong with your scope (switch channels).

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 06:18:34 am
content: Quote from: c4757p on April 02, 2013, 06:15:50 amOk. I've been wiggling some of the wires and the output is jumping all over the place. I'm starting to think maybe I have a bad connection. Kind of hard to tell on a breadboard. I think Im gunna switch breadboards real quick.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:20:19 am
content: If you have a shit breadboard (like mine!) and thin component leads, that is highly likely. One trick you can use in a pinch is to stick both leads in the one hole.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 06:33:04 am
content: Output looks better, but it doesn't seem like the input signal is affecting the output. I do have crappy wires and a crappy breadboard. I'm starting to think that may be the problem.Yep. I just found one wire and switched it to a different hole. The lesson I have just learned is that cheap breadboards and wires are not to be trusted. Gahhh.....

author: amspire
author_rank: Super Contributor
timestamp: April 02, 2013, 06:34:27 am
content: Quote from: skysurf76 on April 02, 2013, 06:02:45 amI think the original circuit was correct - at least as far as the course was concerned. You had previously measured the DC gain, and with a gain of 160, it should be biased correctly.It is not a design that would ever be used in practice, but for a teaching lab, it makes sense. You get the actual transistor voltage gain without any possible negative feedback that you get when the resistor is base-collector.As long as the collector is about 5V DC, the circuit should work.You have to work through all the possibilities - like perhaps channel two of the oscilloscope has a problem when set to 5V/div. It is hard to see how that waveform you are seeing is actually the waveform on the transistor collector.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 06:35:07 am
content: I have a couple roles of hookup wire. I think I'm gunna cut some leads and solder tin them. Ive been using the little wires you get from Ebay with the little rubber gromets on them. I guess they are just too small to get good contact in a breadboard.Thanks so much for the help everyone, especially you c47.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:46:39 am
content: Quote from: skysurf76 on April 02, 2013, 06:35:07 amWelcome - but amspire knows his way around a transistor better than I do! I am going to bed so listen to him. He'll accomplish in a sentence or two what would probably take me many posts.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 06:51:01 am
content: Thanks again. I kind of feel bad now that I used up your guy's time over a bad connection. This is the first time I've had a breadboard be the culprit of a non functioning circuit, and seeing as how I'm no BJT pro I figured something was going on that I didn't understand. Yet again I learn the lesson that you should always check the simple stuff first.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:51:31 am
content: And I'm with him in suspecting the scope.I think it may actually be a turd with BNCs stuck in the side. Make sure you can trust it before you actually do.

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 06:53:59 am
content: Quote from: skysurf76 on April 02, 2013, 06:51:01 amI'm one of the least patient people here. If you wasted my time, you'd know it.Happy to help.

author: skysurf76
author_rank: Regular Contributor
timestamp: April 02, 2013, 08:30:17 am
content: The moral of the story.....buy some solid core hookup wire and make your own breadboard jumper wires. All my ebay jumpers are now in the trash can.The results.......

author: AlfBaz
author_rank: Super Contributor
timestamp: April 02, 2013, 10:35:37 am
content: I like threads with happy endings

author: c4757p
author_rank: Super Contributor
timestamp: April 02, 2013, 04:49:19 pm
content: Quote from: skysurf76 on April 02, 2013, 08:30:17 am