Making a filter that remains the same when loaded

[Ry122]

Yeah, but what about the arguments made in the first 2 pages of this thread about Re? Would you implement that in the same as I have above, or would you implement it in the way that its done in mebigguy's diagram.

 

[LvW]

There is only one way to "implement" Re - as a resistor between the emitter node and ground. I suppose, you know the circuit called "emitter follower"?

But I forgot to mention the relevant time constants: R1C1=R2C2=11.3µsec and C3R3=C4R4=2.3msec.

 

[Ry122]

yes, but in mebigguy's diagram it's bypassed by a capacitor. In mine only 50ohms is present at AC due to the other 450ohms being bypassed by a capacitor. The question is, what are the trade offs between these. Is 50ohms enough or too little? Should I increase it, and use more transistors to make up for the loss in gain?

 

[LvW]

To me, this is a secondary problem - and it depends on your accuracy requirements regarding the gain value.
Bypassing the emitter resistor in the gain stage increases the gain value but reduces its sensitivity to temperature and tolerance parameters.
Of course, a trade-off (bypassing a portion of Re only) makes sense and is used quite often.
But - why not first concentrate on the filter sections and the decoupling stage (emitter follower)?

 

[Ry122]

the finite input resistance of the first stage (after the highpass section) must be taken into account in parallel to the highpass resistor R4

Why not just throw another unity gain mosfet buffer between the 1st gain stage and the filter stages so it doesn't even see the impedance of R4?

 

[LvW]

Up to now - I was of the opinion that you want to use BJT`s only. Of course, with FET`s you have other options.

 

[Jeff Rosenbury]

yes, but in mebigguy's diagram it's bypassed by a capacitor. In mine only 50ohms is present at AC due to the other 450ohms being bypassed by a capacitor. The question is, what are the trade offs between these. Is 50ohms enough or too little? Should I increase it, and use more transistors to make up for the loss in gain?

Your design is more advanced and better, at least for learning purposes. Someone, (meBigGuy?) pointed to this emitter configuration earlier. (Sorry, I'm on a satellite connection and reloading pages is a pain, so I can't check the thread.)

The low value resistor gives gain stability, while the high value, bypassed resistor gives DC stability.

I haven't run the numbers, but copy pasta stages rarely match impedances for common emitter configurations. A low impedance feeding a high impedance is sometimes acceptable for signal work, but the opposite is not good. As a learning lesson, you need to match them. This becomes important in power work where loss of power gain is heat and smoke. Of course changing the resistor values means changing the caps, means recalculating the filters... Good practice! And also a good motivation for using op-amps.

Op-amps are frequency limited, so this sort of work goes on all the time at higher frequencies. It's not a wasted effort to learn various configurations of discrete transistors.

 

[LvW]

Your design is more advanced and better, at least for learning purposes.

May I ask (for clarification purposes only): ...better than what?

 

[Jeff Rosenbury]

Quote: "Where the confusion might arise is between power and voltage. A first order filter (1 stage) has 6dB per octave or 20 dB per decade. But I think that's in terms of power. In terms of voltage I think it's half that (power is proportional to the square of the voltage, which in dB is a factor of 2.)"

Just to avoid confusion: No - 20db/dec for a first-order filter is in terms of voltage..

Sorry. Oddly, my dyslexia keeps me from remembering which way the voltage/power 10dB vs. 20dB thing goes. It's great for recognizing symmetries; not so good for remembering which side I'm on though.

 

[LvW]

No problem - we have two different definitions for power and voltage (factor 10*log resp. 20*log).

 

[Jeff Rosenbury]

May I ask (for clarification purposes only): ...better than what?

Better than other configurations. No emitter resistor is somewhat unstable. A simple resistor costs gain. Bypassing it makes the gain dependent on the ß. But bypassing most of it gives the best of all worlds.

Remember there's a cost to each added component. I've seen professional designs with a simple base bias resistor (plus the collector resistor). These have lots of drawbacks, but they are cheap. Sometimes cheap matters more than other considerations.

But for learning purposes, extra parts are free and doing the calculations is good practice.

 

[donpacino]

(for some mysterious reason that I cannot always pin down)

story of my life

 

[LvW]

Better than other configurations. No emitter resistor is somewhat unstable. A simple resistor costs gain. Bypassing it makes the gain dependent on the ß.
....
.

Yes - that`s what I also have tried to explain in my posts#50,#52 and#54.
However, I doubt if Ry122`s five.transistors circuit (5 emitterstages in series) is really "better than other configurations".
Example: Where is the bandpass circuitry?

 

[Jeff Rosenbury]

Yes - that`s what I also have tried to explain in my posts#50,#52 and#54.
However, I doubt if Ry122`s five.transistors circuit (5 emitterstages in series) is really "better than other configurations".
Example: Where is the bandpass circuitry?

It's not a contest. He is a student needing encouragement.

His coupling capacitors will act as high pass filters. He had some low pass filters at the beginning. As I said, I didn't run the numbers and have no idea if they are the right values. I suspect I could find lots more wrong with it as well. (Why put the filter where the signal strength is lowest, for example?) But this is a teaching moment. Besides, most of the "wrong" things aren't wrong except in specific contexts. In many (most?) designs the filter is fine at the beginning of the circuit. But there are some cases where that would bite him.

If he wanted a working circuit, he would have used the op-amps. He wants to learn. I'm all for that.

I do have to agree that if I were his boss and he handed me this five transistor stage design for a two pole, 26 dB gain filter, I'd want an explanation.

 

[LvW]

It's not a contest. He is a student needing encouragement.
His coupling capacitors will act as high pass filters. He had some low pass filters at the beginning. As I said, I didn't run the numbers and have no idea if they are the right values. I suspect I could find lots more wrong with it as well. (Why put the filter where the signal strength is lowest, for example?) But this is a teaching moment. .

OK - I got it and I agree with you. It is - perhaps - a teaching event.
And from my own teaching experinece I know that, of course, there are always different teaching approaches.
However, in particular, because of this background I have recommended to the questioner a SYSTEMATIC approach: Starting with the desired filtering action (bandpass) - and shifting the design of the gain stages to the end of the design process. For my opinion, this is the best way to solve the problem.
To be honest - do you really think it would be possible to design the various coupling capacitors of the five stages so that the meet the highpass requirements?
More than that, also the by pass capacitors in the emitter legs exhibit a highpass function!
I really have severe doubts that this approach is a good one.

In short: I think, the best/most simple/most systematic approach would be a series combination of
* A passive 4-pole RC bandpass - decoupled with an emitter follower, followed by
* a two-transistor gain stage (emitter follower and common-emittere stage).

 

[Jeff Rosenbury]

I agree that his approach isn't the one I would use. But we have pointed that out to him repeatedly.

He doesn't seem to want to learn proper design methodology. Nor should he unless he's planning on working as an engineer. Only he knows what he wants to learn from this. He has repeatedly stated he wanted to use common emitter BJT transistors. I have to accept that.

Since this is a Physic Forums, I assume he's a physicist and knows what he wants. So I'll try to help him as I can.

Scientists don't like learning engineering. It's a point of pride with some. It seems to interfere with their ability to ignore reality. Had Einstein been an engineer, he probably would have learned Newtonian mechanics so well he never would have found relativity.

Of course I wouldn't want to drive over a bridge built by a scientist. To each cat his own rat.

 

[Ry122]

Because you have ac-shorted the emitter resistance in both stages the input resustance at the base node as well as the signal gain depends considerably on transistor parameters. Bad design.

Contrary to what you said, I found this at allaboutcircuits"The fact that we have to introduce negative feedback into a common-emitter amplifier to avoid thermal runaway is an unsatisfying solution. Is it possibe to avoid thermal runaway without having to suppress the amplifier's inherently high voltage gain? A best-of-both-worlds solution to this dilemma is available to us if we closely examine the problem: the voltage gain that we have to minimize in order to avoid thermal runaway is the DC voltage gain, not the AC voltage gain. After all, it isn't the AC input signal that fuels thermal runaway: its the DC bias voltage required for a certain class of operation: that quiescent DC signal that we use to “trick” the transistor (fundamentally a DC device) into amplifying an AC signal. We can suppress DC voltage gain in a common-emitter amplifier circuit without suppressing AC voltage gain if we figure out a way to make the negative feedback only function with DC. That is, if we only feed back an inverted DC signal from output to input, but not an inverted AC signal.

The Rfeedback emitter resistor provides negative feedback by dropping a voltage proportional to load current. In other words, negative feedback is accomplished by inserting an impedance into the emitter current path. If we want to feed back DC but not AC, we need an impedance that is high for DC but low for AC. What kind of circuit presents a high impedance to DC but a low impedance to AC? A high-pass filter, of course!"

That high pass filter mentioned above is the bypass capacitor posted in mebigguy's image

Or were you referring to the AC gain being dependent on the beta value and not that thermal runaway is still occurring?

 

[meBigGuy]

He is referring to the AC gain depending on the transistor characteristics. Either you can live with that or not, depending on your application's requirements.

One solution is to get as much gain as you can in a multistage amplifier and then stabilize with negative feedback. This both controls the gain AND reduces distortion. It really is required in high quality audio circuits using discrete devices.

Look at google for "discrete negative feedback audio amplifier design"

http://www.allaboutcircuits.com/vol_6/chpt_5/13.html
http://circuit-diagram.hqew.net/Feedback-Amplifier-Using-Transistors_2749.html

http://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Self_Douglas/Small_Signal_Audio_Design.pdf Do you understand open loop gain in an opamp? Why it is important, and how it decreases distortion? You can accomplish some of that in a well designed multi-stage negative feedback amplifier.

 

[LvW]

Contrary to what you said, I found this at allaboutcircuits

Ry122 - where is the contradiction? I spoke about the DEPENDENCE of the signal gain on transistor parameters - not about the VALUE of the gain.
Of course, the gain drops without the capacitor because you now have not only dc but also ac feedback. But this is a desired effect!
All good amplifier stages work with signal feedback - BJT, FET, Opamp.
(remember: Opamps do not work at all without signal feedback).

 

[Jeff Rosenbury]

To put it another way, maximum gain is dependent on the ß of the transistor. But since each transistor has a different ß it is hard to know just what that maximum gain will be.

Adding a small AC emitter resistance lowers the gain to a fixed and therefore predictable level.

If your goal is maximum gain, hand pack your transistors and ditch the resistor.

All of these variations have uses in differing applications. The key is to choose the one you want for your particular application. Knowing how to do that is why EEs get paid the big bucks. After all, anyone can look up a few schematics online.

 

[LvW]

To put it another way, maximum gain is dependent on the ß of the transistor. But since each transistor has a different ß it is hard to know just what that maximum gain will be.

However, I must state - for the sake of accuracy - that two gain stages with different ß values and the same dc quiescent current Ic have the same signal gain.
This gain does NOT depend on ß but on the transconductance gm only (that is the slope of the Ic=f(Vbe) curve).
What differs is the signal input resistance at the base node only (smaller base current for higher ß values).
(It is a common misconception that higher current gain would give also higher voltage gain; perhaps, this false conclusion is caused by the - false - assumption that the BJT would be a current-controlled device).

 

[Ry122]

hopefully there's some other benefit in addition to it making the gain stage's gain being less dependent on the inherent beta value of each transistor, because hand selecting a couple of transistors so they have the same beta value is extremely doable for me.

 

[LvW]

hopefully there's some other benefit in addition to it making the gain stage's gain being less dependent on the inherent beta value of each transistor, because hand selecting a couple of transistors so they have the same beta value is extremely doable for me.

Ry122 - in my post#25 I have listed all the benefits in case of ac feedback.

 

[Ry122]

only 1 of those is applicable though right, since all the others can be obtained by only having an Re for DC

 

[Baluncore]

@ Ry122. What is the application for this amplifier ?

 

[Ry122]

microphone amplifier

 

[LvW]

only 1 of those is applicable though right, since all the others can be obtained by only having an Re for DC

Sorry - but that`s not true. If Re is bypassed using a capacitor you only have one single effect: Stabilization of the dc opereating point.
All other benefits connected with ac feedback are lost.