Negative feedback consideration

337

04:21:31

I was wondering what's wrong with implementing negative feedback with a resistor between collector and base. I like it because it biases the input without any need for DC decoupling. I think I answered my own question: given enough feedback to make it insensitive to forward beta, there's no gain and it's still not all that insensitive.

Anyone want to tell me I missed something? Because this isn't the answer I expected. :)

published 8 years ago

hurz

8 years ago

Gain depends on the ratio of ~ Rcb/Rb. And dont forget an dc decoupling cap at input. E.g. 470kOhm Rcb and 47kOhm gives you ~ a gain of ten and not to much harmonics. High input impedance and high voltage gain by low distortion low output impedance, what else can we expect for such a simple amplifier topology. Negative for this circuit is the frequency response and for mass production you have to range the forward beta. If to low or to high it will fail. But it works in quite a big range. I like it 😉

hurz

8 years ago

http://everycircuit.com/circuit/4665553963974656

crake

8 years ago

http://everycircuit.com/circuit/5734108323119104

crake

8 years ago

Id put a cap at the input in order to better control where your bias current for the BJT goes. For example, approx 25uA is leaking to ground through the AC source. In addition to this, I'd put a resistor at the emitter as well as it is the only reliable way to determine the current at that location.

hurz

8 years ago

Eekee?

eekee

8 years ago

I'm alive, but my health isn't too good. Thanks for your comment, hurz, it's encouraging. :) I made some changes starting with your suggestions. Got a lot of current gain. I can see I've got some more to explore on why it does what it does.

eekee

8 years ago

Thank you too, crake, but I'm not looking for an AC circuit. :)

eekee

8 years ago

Hmm, controlling where the bias current goes. Going to have to keep that in mind. ... Oh, I wonder if that's why some of my circuits only work right when there's a resistor from base to ground too?

hurz

8 years ago

Welcome back. Sure the DC operating point is importante for any BJT circuit.

eekee

8 years ago

Thanks hurz. I'll probably be coming and going for a few weeks.

eekee

8 years ago

I thought about bias current some more, and decided, yes I will use an input capacitor with this type of circuit unless it's a digital circuit. I'm also told that this method of biasing "is more sensitive to variations in temperature and/ or the transistor's gain than the 'conventional' bias system." http://sound.whsites.net/no-opamps.htm#s10 Its input impedance is higher than with conventional bias, but hard to calculate.

hurz

8 years ago

But if you do not block DC with an cap, you do not separate the DC operating from the input source! You have to garantie an operating point for your amplifier, doen't matter whats connected.

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