Where is all this phase shift from, that's not in the model ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: December 24, 2022, 02:44:43 pm
content: I'm working towards making a very low power AM transmitter/receiver, and being able to calculate some more stuff along the way. So I'm playing around with AM modulation from a BJT diff pair and tail current BJT.So I made a sub 3MHz BJT model for the circuit, leaving out the bypass caps, and ignoring the different frequencies, just treating it like a superposition. I made some nice 2x2 G-matrix for Ve2 and Vout=Vc3, it just a single ended output, no phasors, just conductances. And it works fine. Later I want to find the ABCD parameters, wrt to either the audio or RF.But looking at the 2N2222 circuit, the current of Rc is about 180deg out of phase with the RF voltage.But in the basic AC model, even with the bypass caps, the Rc current is almost in phase with the RF voltage. The polarity of the Rc's is the same, with top positive.So what am I missing or forgetting, that makes the phase that much different? and that's with just 1kHz and 10kHz "RF"am modu.asc(6.69 kB - downloaded 57 times.)am modu.asc(6.69 kB - downloaded 57 times.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2022, 02:55:20 pm
content: You swapped the side: Q1:B2 and Q2:B3 but Q1:Rc1 vs. B3:Rc.Discarding device capacitances (and a model for fT, which isn't strictly driven by capacitance) and some bias capacitances probably accounts for the remaining couple degrees difference.Tim

author: MathWizard
author_rank: Super Contributor
timestamp: December 24, 2022, 03:05:53 pm
content: [ Specified attachment is not available ]But isn't it right, the smaller Rpi=R15=5836, is the AC grounded 2N2222, with the higher base current.Here's the polarities for the Rpi's and how I did it in the node equations, I just used the Beta*Ib in LTspice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2022, 03:47:45 pm
content: What do you mean it's grounded? It's got the source right there!Are these not equivalent too?C3 - C7R1 - R19Q2 - R15, V5, B3, R13tcc - RcTopology is just backwards, that's all. You get complementary currents through the collectors, Rc1 takes the left side, Rc takes the right side, the differential input is identical (grounded on the left, signal on the right).Tim

author: MathWizard
author_rank: Super Contributor
timestamp: December 24, 2022, 04:48:26 pm
content: Right, I was expecting it to be something like me missing some mistake. Sorry for wasting your time even on Christmas eve. In LTSpice I never labelled it like my calculations, and missed it.My calculations matched the model tho, so that's still ok. Just adding in some complex numbers to all this will make it a lot more hairy.I saw some equation for AM modulation and voltage levels of audio and carrier, and after playing around with this circuit and changing Rc a lot, I can get something like decent modulation with like 10mV RF and maybe 70mV max AF.In the original circuit from some article or lab pdf, that had very similar biasing, but said the Vaf could be up to 2Vp (or rms) but IDK how or why that makes for good modulation, it looks terrible to me, with any Rc, especially with a lot of saturation.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 24, 2022, 05:14:25 pm
content: Nice envelope.You could use a Gilbert cell to get that function block and more.The MC1496 has all nodes to pins, so good for experiment.The SA612 has more internal functions, in smaller 8 pin package.

author: MathWizard
author_rank: Super Contributor
timestamp: December 24, 2022, 05:16:03 pm
content: LOL I thought I had you this time T3sl4co1l, I know the -Vbe_left=Vbe_right, but I was expecting someone to say it was some capacitance thing or because I'm just considering magnitudes of the sines and not using phasors either, or some other frequency mixing thing.Maybe some day mag_therm, but for now I'm just trying to make a crude type, but with basic parts so I can learn some more equations.I need to model some 2 transistor AM circuits, that work. They should be easier and also, IDK how some of them make sound pretty good, yet look so un-linear, I guess the receiver works to make it linear again. And in these circuits, I'm just learning transmission parameters, so I hope they simplify nicer.I made some basic receivers before, and I do have a few tank circuits from radio's. Last week I made the above circuit w/ bc547B, and for a few moments I had AM like in the pic. But not as good. But that was on a BB with green coil inductor's too. That needs more work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2022, 06:35:16 pm
content: Quote from: MathWizard on December 24, 2022, 04:48:26 pmWell, Q3 is just making a current, so, whatever voltage scale it's wired for. A few volts is reasonable there, though with only a 5V supply you don't have much headroom here.So, the order of V(vb1), V(vb3), V(vc3) and V(vcc), and the resistor ratios corresponding to the drops. If ever they would not be in order, one stage saturates and you get a flat/sloped top or bottom.The RF port must be some 10s of mV to be linear, but harmonics aren't usually problematic there, so you can go for higher conversion efficiency by overdriving this (100s mV?).Quote from: MathWizard on December 24, 2022, 05:16:03 pmHeh, no problem. Just a sign difference; well, it's topological too, but for the MF analysis you're doing, it doesn't make much difference. Doing it at RF, with Miller effect in play, will show a bigger difference though.Tim