Using DC Feedback to Bias and Drive a BJT

Hi guys, some of you might remember seeing my previous post in the forums about this circuit:

Thanks to some helpful pointers, I was able to get the automatic gain control working properly. But it's time to take the next step. The amplification is currently represented by a voltage-controlled voltage source but it's time to design the actual circuitry for amplification.

I'm currently attempting to do this with a BJT, for no other reason honestly than that's the chapter I've reached in my textbook. My idea is to use the rectified DC feedback which is currently serving as the bias on my diode dr as both the Base and Collector voltage to drive the amplification.

However, I quickly ran into the problem that the DC voltage which actually reaches the Base of the BJT is only ~446 mV, not enough to actually bias the transistor! I believe I need at least 600 mV or potentially even more. Problem is that I don't know how I can increase the DC voltage. I tried lowering the value of the two resistors Rc and R2 (which was undesirable from a design standpoint) as well as increasing the gain of the voltage controlled voltage source considerably, but neither caused more than a dozen mV change.

I'm just getting started on this puzzle but I would love any input from the community about what would be the best way to get a DC voltage from my rectifier at the base of the BJT at the desired voltage. Of course, being as minimalistic as possible.

I will post some of my ideas as I go. Thanks!

by ConnorBecz
December 29, 2020

I am not sure about what you are trying to do here.

Here what I can get: Amplificatin of a signal is generally about a very weak signal, such as one coming from an antena were electrons got weakly excited by a radio wave. The amplification generally implies transistor AND a stable DC source (or tubes and DC source, or other variation on that principle). Using the signal itself to create the (stable) DC source which will amplify the signal, ... while respecting the law of conservation of energy, that is, without pumping energy from another reservoir ( E= mc2, or thermal to electrical energy, etc... ) is possible if the signal can support it: the electrical domestic distribution network, supported by an hydro electric power plant (or nuclear plant, ...) does exactly that when I plug an oscilloscope to it and probe the alimentation signal with a probe of that said oscilloscope. Is it what you want to do, more or less? Then, ONE key point is the stability of the "DC" power used to supply the transistor, because if it has ripples, your transistor, even analyzing a constant voltage signal, will unfortunately have ... ripples (from the DC ripples) which will be taken as coming from the analyzed signal ... while the said signal was constant (as assumed). In other words, your AC to DC conversion (to support the amplification through the use of a transistor) has to be ultra-stable, EVEN if it can deliver a close to an infinite amount of energy ( or at least, a huge amount of energy).

That is looking at your circuit from hundred of kilometers from it, I agree, and the details are not visible from that distance, but sometimes, it may be useful to look at the problem with another light, or in a different view. I don't know if that help, if not, no harm done, I hope.

by vanderghast
December 30, 2020

Hi vanderghast, thanks for your remarks!

Yes, you understand my objective exactly. What I've done so far is create a rectifier which receives the output signal of the voltage-controlled voltage source and I've used that to create a dynamic resistance in my diode "dr" that results in AGC.

What I want to do now is take the design further, and replace the voltage-controlled voltage source with actual circuitry. I'm attempting to use a BJT for this.

Now, my first thought has been to use the rectifier which I already have as my stable DC source for the BJT. I figure I can send the DC signal into both the Base and the Collector terminals of the BJT and kill two birds with one stone. That's what I'm working on right now

I believe that ripple voltage will be one challenge, like you say.

I'm also dealing with the challenge that my DC voltage on the rectifier is too weak. After passing through my 100k resistor, the voltage drops to 446 mV. Too low to forward bias the base-emitter junction, I think I need at least 600 mV to create a forward bias strong enough to create significant amplification between the collector and the emitter.

Unfortunately, all I could think to do so far is increase the gain of the voltage-controlled voltage source to try and increase the voltage on the rectifier. But alas, after passing through the 100k resistor the voltage will still drop to ~450 mV even if I raise the voltage in the rectifier as high as 9 V.

What I'm trying to figure out is what I can do, if anything, to get my DC signal to enter the base node at the right voltage. At least 600 mV or more is what I'm looking for.

Thanks for your help and hopefully my explanation made sense

by ConnorBecz
December 30, 2020

If the input voltage is some kind of AC, could you use a voltage multiplier sub-circuit to increase the voltage, feed this rectified voltage to the collector of the transistor and between the collector and the base, insert your 100k resistor?

Remember that if you have two resistors in series, and one is of 100k, then the other resistor of r ohm would only see a voltage of V * r / ( r + 100k). Not a suprise that you get only 450mV across it. Someone can assume that whatever is there is like a resistor of about r = 5100 ohm.

by vanderghast
January 01, 2021
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