Help, I still can't bias a BJT for the life of me... - Page 1
Forum section: electronics / beginners

author: TimNJ
author_rank: Super Contributor
timestamp: February 18, 2016, 05:53:12 am
content: Hi everyone,This is something that I don't like to admit, especially to myself... I am a 3rd year EE student and still cannot correctly bias a BJT. I thought I'd be an electronics master by this point in my education, but as it turns out, there's a lot more to EE than electronics alone.I bring this up because I'm working on a project for my RF communications course. I have to create a 250KHz LC (Colpitts) oscillator that can produce a 2Vp-p signal into a 1K load. Seems easy.So I watched the w2aew video on hFE and biasing schemes and tried to design the circuit in a similar way. I got decent results but I still get some clipping/distortion on the bottom of the sine wave and the sine wave is not symetrical around 0V. It swings from about -2.0V to +1.6V (in simulation). Either way this was still greater than 2Vp-p, so I tried breadboarding it. To my dismay, I got a significant reduction in output swing. It was always less than 1Vp-p, and if I tried to increase Ic, I introduced (even) more distortion. Is the DC resistance of my tank circuit too low?My problem likely lies in the fact that I don't know how the amplifier is reacting with the resonant tank circuit and how the feedback works. However, it also doesn't help that I still struggle with biasing BJTs.Can anyone with more experience recognize where I went awry? Thanks.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 18, 2016, 06:35:43 am
content: TimNJThere's a heck of a lot of hands-on knowledge that is required above and beyond a purely academic education. It only comes with fiddling around and building things and then trying to figure out why they don't work!Your circuit requires the transistor to be operated in the common base configuration. That means that you have to connect the 0.1uF cap from the base to gnd. The bottom of the tank circuit should be connected to gnd also. The feedback path is then from collector to emitter. The lower cap value of the tank determines the degree of feedback. It should be reduced in size until oscillation stops and then increased a little till oscillation starts again and starts reliably. Also, ensure that you supply is bypassed. At 250KHz you'll probably require some small electrolytics.There are other configurations that will also work so there's more than one way of configuring it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 18, 2016, 06:59:27 am
content: Don't worry, you've done a fine job of biasing!Now for what to worry about: see how the waveform average level, kind of slides up, with an exponential time constant? That's the bias adjusting itself -- again. (SPICE does the first adjustment itself, by assuming everything is stable and DC -- that's what gets you the initial state, which quickly begins to oscillate. Which is also good, you've done that right as well: oscillators can be difficult beasts in the simulation world, and this is definitely working.)Why would that happen? Well, same reason your waveform is distorted, in fact: you've set up a linear amplifier with a RHP pole, which causes its output to grow exponentially over time; but no real circuit is LTI, and its output does not grow forever. The amplifier's output voltage and current are limited; the base and collector act to rectify the signal, both clamping its amplitude and drawing some DC from the wave. (A frequency-domain view of this is, the nonlinearities act to mix the signal with itself: thus producing sum (harmonics) and difference (DC) components.)How to fix that? Well, if you can control the gain so that, as amplitude rises (and not just instantaneous voltage, but average magnitude over many cycles), the gain is reduced (perhaps by reducing bias current), you can effectively cause the RHP poles to shift left, so they lie on the imaginary axis in the steady state condition (i.e., stable, constant amplitude oscillation).(Such an AGC (automatic gain control) method still exhibits the frequency mixing effect of a flat-out oscillator, but it's constrained to frequencies within the bandwidth of the AGC loop, which is made to be a much lower bandwidth than the oscillator frequency. Thus, the oscillation frequency doesn't mix with itself, and DC error and harmonic distortion is reduced. For error signals within the AGC bandwidth, amplitude modulation or frequency locking occurs.)You can reduce the loop gain to the minimum necessary, but this will be difficult I think; the C2/C1 ratio is already quite large (~20), and further increasing C2 may simply stop oscillation altogether. With a very precise adjustment, you could get something sort of stable in the simulator, but it wouldn't be replicable in real life because of tolerances, variations and noise.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: February 19, 2016, 09:10:24 am
content: Quote from: Richard Head on February 18, 2016, 06:35:43 amThank you. Looking back on my circuit, I seem to have multiple feedback loops (?). As you suggested, I am now working on a circuit based off an emitter follower design. I'm not quite sure how the tank circuit provides gain on its own, but I'm looking into it.Thanks for your words of wisdom.

author: TimNJ
author_rank: Super Contributor
timestamp: February 19, 2016, 09:20:30 am
content: Quote from: T3sl4co1l on February 18, 2016, 06:59:27 amThanks Tim. You always have wonderful replies. It's late in the evening and my head isn't thinking too straight, but I atleast wanted to thank you.

author: Richard Head
author_rank: Frequent Contributor
timestamp: February 19, 2016, 11:32:01 am
content: TimNJThe purpose of the tapped capacitor tank is simply to provide a low level low impedance drive to the emitter of the transistor. Think of the oscillator as a common base amplifier. The input impedance of the emitter is very low (Zin = 26/Ic, where Ic = mA) so you can't connect it to the top of the tank where the impedance is very high or you would load it too much. As you move down the coil you load the tank less and less (for a given load resistance).