Making a filter that remains the same when loaded

[Ry122]

To make a filter out of a bjt where the poles don't shift when you load it, do you need to have a buffer stage that follows the bjt such as a unity gain mosfet and then put the load after that buffer stage?

 

[NascentOxygen]

To make a filter out of a bjt where the poles don't shift when you load it, do you need to have a buffer stage that follows the bjt such as a unity gain mosfet and then put the load after that buffer stage?

Basically, yes, though it may be useful to have some gain in the buffer stage.

 

[tech99]

To make a filter out of a bjt where the poles don't shift when you load it, do you need to have a buffer stage that follows the bjt such as a unity gain mosfet and then put the load after that buffer stage?

Yes, this is usually the case.

 

[Baluncore]

The form that the output buffer takes will be determined by the signal power, impedance and the characteristics of the load.
You may be able to use an emitter follower that is already part of the filter.

 

[Ry122]

Can't you just use something really simple like this under all circumstances?

 

[meBigGuy]

That will not be linear. Look up "emitter follower" or "source follower". That would be simplest buffer (with no voltage gain). If you need voltage gain, then you essentially need to build an amplifier. The characteristics of the amplifier will depend on what you need to accomplish.

 

[Jeff Rosenbury]

Your circuit seems to be an amplifier rather than a filter. In addition it will give 180º (ish) phase shift. And as meBigGuy pointed out it's non-linear. (The phase shift is 180º for most of its bandwidth, but drifts near the knee frequency.)

What is it you want this stage to do? If it's a filter, what characteristics do you want it to have (bandwidth, etc.). Are you operating at a really high frequency? Is that why you want a BJT?

There are lots of nice op-amp filters online, but they tend to be frequency limited. Push-pull amplifiers are more linear, but have higher output impedances. Current mirrors can provide impedance matching (like an emitter follower) but tend to be the frequency limiting part of the op-amp.

Filter design is an arcane art. There are lots of considerations that mostly don't matter -- except when they do. We need more information about your project.

 

[Baluncore]

Can't you just use something really simple like this under all circumstances?

Every circumstance is different. There is no one solution to all problems.

Please post the schematic of your prototype BJT filter with your required specifications. We can then give you some good solutions to the output impedance problem.

 

[Ry122]

Alright, thanks. Between C1 and R1 is where the output will be coming from.

 

[Ry122]

Your circuit seems to be an amplifier rather than a filter. In addition it will give 180º (ish) phase shift. And as meBigGuy pointed out it's non-linear. (The phase shift is 180º for most of its bandwidth, but drifts near the knee frequency.)

What is it you want this stage to do? If it's a filter, what characteristics do you want it to have (bandwidth, etc.). Are you operating at a really high frequency? Is that why you want a BJT?

There are lots of nice op-amp filters online, but they tend to be frequency limited. Push-pull amplifiers are more linear, but have higher output impedances. Current mirrors can provide impedance matching (like an emitter follower) but tend to be the frequency limiting part of the op-amp.

Filter design is an arcane art. There are lots of considerations that mostly don't matter -- except when they do. We need more information about your project.

The circuit I posted above is a bandpass filter with a pretty reasonable gain in the midband. There's 40db/decade rolloff either side of the midband and the cut off frequencies are at about 100hz and 10khz.

 

[NascentOxygen]

Is this your design for a particular real-world purpose, or is it a circuit you were provided with and asked to investigate as a lab exercise?

 

[Ry122]

real world

 

[Baluncore]

A real world problem.
The Q1 amplifier with primitive bias will be a disaster.
Gain will be signal amplitude dependent.
There will be harmonic generation due to the non-linearity of the Vbe junction.

Attached is the .asc file for those with LTspice.

 

[Ry122]

A real world problem.
The Q1 amplifier with primitive bias will be a disaster.
Gain will be signal amplitude dependent.
There will be harmonic generation due to the non-linearity of the Vbe junction.

Attached is the .asc file for those with LTspice.

You don't need to worry about that. I've ensured that the BJT is staying within the active region throughout the complete cycle of the AC signal by doing some fairly drawn out small signal analysis.

 

[Ry122]

I graphed the voltage drop across the Vce junction in the circuit you uploaded and you can see that it never falls into saturation, so what are your concerns exactly?

 

[Ry122]

Also, I can't see anything wrong with this bode plot.

 

[NascentOxygen]

There are a number of problems with such rudimentary biassing. Okay, you have tweeked it so it works just fine right now, but ...
things will change as temperature drifts, but worse is when that transistor burns out (for some mysterious reason that I cannot always pin down) the transistor you replace it with will be quite different in gain and the circuit will need to be checked and tweeked all over again. That's why a more stable arrangement is always used.

Do you need this filter to continue to work reliably for any length of time?

 

[Ry122]

This is what real world op-amps consist of, you just aren't accustomed to seeing this type of thing because everyone just buys IC op-amps these days.

 

[Baluncore]

This is what real world op-amps consist of, you just aren't accustomed to seeing this type of thing because everyone just buys IC op-amps these days.

That is incorrect. You are getting advice here from experienced professional electronics design engineers.

Also, I can't see anything wrong with this bode plot.

What is the vertical scale dB/div ?

 

[Ry122]

fixed the graph

 

[meBigGuy]

Ry122:

I agree with the others that your circuit is a disaster. It will have high distortion and the gain will vary with temperature, age, and from transistor to transistor.

It is nothing like what is used inside an opamp.

A typical discrete circuit with stable gain and bias would be

You will find no common emitter 1 transistor amplifier circuit without two bias resistors and an Re

Opamps use current mirrors and much more sophisticated structures.

http://electronicsecg1.blogspot.com/2008/01/op-amp.html

 

[NascentOxygen]

We should give Ry122 due credit for devising a filter that does what he wants, and confirming this by simulation.

It sounds to me that you may have come up with this through a good deal of your own effort, so you have good reason to be proud of it. But the reality is that filters can be precision constructs. Your design looks like it may be accommodating the input impedance of the BJT in setting the response, and this impedance will change a little with temperature, and a lot when/if you need to swap in a replacement device. There are improved circuits that seek to minimize this variation, quite separately to avoiding transistor saturation.

 

[meBigGuy]

Maybe calling it a disaster was a bit harsh. I was over-reacting to his charge of ignorance, which was way out of line:
"This is what real world op-amps consist of, you just aren't accustomed to seeing this type of thing because everyone just buys IC op-amps these days."
not realizing that the people answering him could easily design the internals of those opamps.

 

[Ry122]

How come you should use two bias resistors? Also, isn't having an Re only beneficial if you're going to have a non stable/unclean VCC?

 

[LvW]

How come you should use two bias resistors? Also, isn't having an Re only beneficial if you're going to have a non stable/unclean VCC?

An emitter resistor Re (providing negative feedback) has the following advantages:
* DC bias point more stable against temperature changes and BJT parameter tolerances (beta)
* Lowering of signal gain (advantage?)
* Signal gain less sensitive to BJT parameter tolerances
* Reduction of signal distortions (lower THD)
* Drastic increase of input resistance
* Bandwidth increase

Finally, Re provides negative VOLTAGE feedback - therefore, this feedback scheme works best if the input biasing is realized using a voltage divider that can deliver a "stiff" voltage (as stiff as possible). However, due to some other constraints (power consumption, input resistance) the voltage divider must not be too low-resistive. As acommon trade-off the current through the base divider is set to a value of app. 10*Ibase.

 

[Ry122]

Alright, I changed the circuit again to account for the problems that you mentioned. Do you see any problem with implementing this type of filter near the source like this?

 

[LvW]

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.
( I didn`t check the resistor values!)

Question: The first step for designing a filter is to specify the filter parameters (type, bandwidth, gain, attenuation requirements). What is your specification?

 

[Ry122]

Oh, okay. That's something I copied from the image at the top of this page (posted by meBigGuy).
My requirements are that its a band pass filter with a mid band between 100hz-10khz, and 40db/decade attenuation. Gain in the mid band is 400.
The filter part near AC1 is okay though right? That filters out the high frequencies.

 

[LvW]

Oh, okay. That's something I copied from the image at the top of this page (posted by meBigGuy).

OK - but this circuit represents just a gain stage. Of course, due to coupling capacitors and falling current gain for rising frequencies this gain stage has any bandpass characteristics. But - does it fit to your needs? I don`t think so.
My recommensation (in case you are not allowed to use an opamp): Use to separate passive stages for a first-order lowpass (R-C) and a first-order highpass (C-R) - and a buffer (common-collector) between the 2 stages. For the required gain, use another separate gain stage.

Comment: Sorry - I forgot that you did require a 40dB/dec roll-off. Is this really important?

 

[Ry122]

I reverse google searched that image and it seems to be for a microphone which is what I'm also making my amplifier for. I wonder why they want the AC to bypass the emitter resistor.]

If I remove those emitter capacitors then my gain drops from 60db to -20db in the pass band.

Also, why should you put a buffer between the two passive stages? So that you don't lose a substantial amount of the signal due to voltage drop between stages?

 

[Ry122]

Comment: Sorry - I forgot that you did require a 40dB/dec roll-off. Is this really important?

I don't see how this changes things? You can just use two RC filters cascaded and each will provide 20dB/dec. In total there would be 4 RC filters.

 

[LvW]

Yes - of course, It is not a huge problem. I have just forgotten to mention that the passive sections are each of 2nd-order.
But it is imopratnt to decouple low and high pass section in order to avoid interactions between the stages.

 

[Ry122]

interactions like Q-point and harmonics?
edit: nevermind, that's only for LC circuits. I'm not sure what interactions you mean then.

 

[Jeff Rosenbury]

Does your client/boss insist on BJTs? I know some audiophiles have strange ideas about what should and shouldn't be in an amplifier.

Anyway, here's a quick and dirty design process:

Do you care about phase shift? It wasn't in the specs, so I'll assume not. If you do, you need to study the many different filter types. They mostly give similar frequency performance, but there are subtleties I don't fully understand having to do with phase shifts and the like. If these characteristics are important to you or you just want to explore the math, there are lots of filter types. For example if you want a flat frequency response in the pass band, look at the Sallen-Key topology (which isn't the one I found, BTW).

Op amps usually contain several transconductance amplifiers usually in a push-pull configuration which give very linear response except very close to the rail voltages. These are followed by a low impedance output stage, usually some sort of current mirror. Because of all the stages, they tend to have a fair amount of parasitic capacitance which limits their frequency response to the Gain-Bandwidth product. This is specified by the datasheet. The gain times the top bandwidth needs to be less than the gain-bandwith product (GBP). Of course this could be a problem if you want your roll off to be exactly 40dB/decade rather than simply greater than 40dB/decade. But that seems unlikely.

A 40dB filter is usually 2 poles. But I was taught good design practice is to limit gain to 10dB/stage and you want 26dB across midband. So typically you want three stages, two filters and a gain stage to round it out.

I'm not sure where you stand on economics. If this is a mass produced unit, keeping to a lower number of cheap parts may be very important, in which case I would stretch the 10dB requirement and build two 13dB stages. I'll go with this assumption. (Higher quality products require more parts and more work.)

So we need an op-amp with; 20 (13dB) times 10,000 = 200,000 GBP. We want two circuits on the chip for low chip count. Digi-Key has a nice database display, so we go there and find a part. I'll choose GPB of 1,000,000 to be conservative. (This isn't an unreasonable number and shouldn't add much cost/complexity.) There are hundreds of possible op-amps. Pick one with your design in mind (voltage levels, price, etc.) Perhaps an AD8542ARUZ-REELCT-ND? (BTW, I would find another chip supplier for mass produced stuff. Digi-Key is great on customer service, but often weak on price.)

Google "op-amp filters" to find a basic circuit layout. I got: http://www.electronics-tutorials.ws/filter/filter_7.html

Next, get the data sheets on your parts. Read them. Read them again. When you realize they don't work, rinse and repeat this procedure until you find what you need. Select your caps and resistors (remembering your input and output impedances).

Next enter all your parts in the BOM (bill of materials) and your cad software. Draw your schematic and lay out your board. Generate your gerber files and send them to the board fab. Wait for the mail to bring your boards. Populate them. Test them. Rework until it works.

Congratulations, you now have a working pre-prototype. Your client/boss will no doubt send it to some third world hell hole where illiterate peasants will change all your parts selections to something cheaper that may or may not work, but that's business, not engineering.

 

[LvW]

May I give some general remarks?

"Do you care about phase shift? It wasn't in the specs, so I'll assume not. If you do, you need to study the many different filter types. They mostly give similar frequency performance, but there are subtleties I don't fully understand having to do with phase shifts and the like. If these characteristics are important to you or you just want to explore the math, there are lots of filter types. For example if you want a flat frequency response in the pass band, look at the Sallen-Key topology (which isn't the one I found, BTW). "

Phase shift is closely related to the amplitude response and has nothing to do with the topology of the circuits. Flat response means "Butterworth" characteristic and a passband response with ripples belongs to a Chebyshev response.

"A 40dB filter is usually 2 poles. But I was taught good design practice is to limit gain to 10dB/stage and you want 26dB across midband. So typically you want three stages, two filters and a gain stage to round it out. "

I am afraid, here is a confusion between a filters slope of 40dB/dec and the required midband filter gain.