Understanding basic class a biasing. (noob)

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hans a

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Feb 17, 2014
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I´m trying to undersand how to think with a simple class a mic preamp. One transistor and 4 resistors.
I was watching this video which i think really explain in a good and intuitive way how current flows and how to bias and different ways of biasing. (https://www.youtube.com/watch?v=YQlbPGNB-ys)

But i still doesnt know how to think with regards of impedances. I know it is a good practice to "bridge" impedance values so that the mic in this case "sees" a high impedance value.  But do i think about the impedances in series or parallell? Im thinking R1 and R2 will dictate most of the impedance but the transistor and, if active, also Rc and Re will also contribute, right?

Is the impedance there for current to not "leave" (or inhibit the current to leave) the circuit and "load" down the microphone?
(Ie; could i see the circuit as stiff with high impedance..? )
If so could i begin with calculating impedances that fit with 1:10 "rule" in regards with a fixed value of lets say 200 Ohm.. (Thats what specified in the tech-sheet of the LL..) for R1 and R2? (200 x5 squared? x10?)

Br Alex
 
Most audio systems are voltage signaling, and current is generally a nuisance. You could construct "conjugate" systems using current signaling, that would result in a sort of reversal of concerns.

High input impedance means low signal loss from loading. This follows the voltage divider formula Vo = Vi Zi/(Zo+Zi). If you let Zi approach infinity the gain will approach unity (Vo -> Vi) regardless of any finite Zo.

In addition to loss of level, lower input impedance as you point out leads to higher signal current. This is commonly associated with increase of distortion.

A third concern is in capacitively coupled output stages. A low input resistance will potentially increase the pole of the RC highpass filter so as to cause loss or phase distortion of intended low frequency signal.


There are ways of achieving very high input impedance, even using bipolar transistors. Douglas Self has a chapter on it in the book Small Signal Audio Design. The keyword is 'bootstrapping', which in this context is the application of unity gain positive feedback to increase the effective impedance of a circuit element dramatically.

In case of the 4 resistor common emitter amplifier circuit, input impedance is essentially determined by the base bias resistors yes. The transistor does contribute some finite impedance in parallel, but it will usually not be significant since it is much larger than a reasonable choice of resistors.
 
hans a said:
I´m trying to undersand how to think with a simple class a mic preamp. One transistor and 4 resistors.
So you mean a circuit like this?

amp5.gif


hans a said:
But i still doesnt know how to think with regards of impedances. I know it is a good practice to "bridge" impedance values so that the mic in this case "sees" a high impedance value.  But do i think about the impedances in series or parallell? Im thinking R1 and R2 will dictate most of the impedance but the transistor and, if active, also Rc and Re will also contribute, right?
First of all, the concept of impedance is everything in electronics. Electronics is actually pretty simple at a high level involving only 4 major components: resistors, capacitors, magnetics (inductors, transformers, etc) and active devices based on semiconductor junctions (transistors, diodes, triacs, etc). Each of these components have rules that boil down to impedances under various conditions. So if you connect together a few components with "nets", you can reason fairly well about a circuit with a good understanding of impedance. Although even a few components can create some complex interactions which is why I almost always just jump right into computer simulation with LTSpice for things that most people would probably think are trivial like your common collector single transistor amplifier. So if you're trying to really understand electronics, you should drop everything you're doing and understand the basic concept of impedance.

What is impedance? In the shortest possible explanation, impedance is to AC as resistance is to DC. Why do we need a different definition for AC vs DC? Because each electrical component (and by inference each combination of components) can have different impedance at different frequency:

[list type=decimal]
[*]Resistors - Actually resistor impedance is generally fixed. That's sort of the point.
[*]Capacitors - Capacitors are higher impedance at low frequencies than at high frequencies. This is because a smaller capacitor will charge up/down faster as current is flowing in and out. A slowly changing current will charge a small capacitor to it's full capacity at which point the capacitor will stop absorbing current. So "slow" means low frequency and "stop absorbing current" means high impedance.
[*]Magnetics - Magnetics, like an inducator, are higher impedance at high frequencies. This is because a large inductor (an inductor with high inductance) will build-up/tear-down a magnetic field slower as current is flowing in and out. With a fast changing current the magnetic field will not have time to build-up/tear down and therefore not absorb current. So "fast" means high frequency and "not absorb current" means high impedance.
[*]Semiconductor junctions - A semiconductor junction is very high impedance when it does not have a voltage across it and low impedance when it is "biased". The actual impedance transition curve is complex. In the case of a bipolor transistor impedance decreases exponentially over a junction voltage of about 0.6V.
[/list]

Note: I'm leaving out some details of course. For example, the impedance of an inductor at low frequencies will not be zero. There is the resistance of the wire which for a large inductor can be many ohms. And it's impedance at very high frequencies is not infinity because of parasitic capacitance. The EEs and experts here will jump in and refine to my simple comments (as they should).

hans a said:
Is the impedance there for current to not "leave" (or inhibit the current to leave) the circuit and "load" down the microphone?
(Ie; could i see the circuit as stiff with high impedance..? )
Yes. That's pretty much correct. Consider a speaker as an example. An 8 ohm speaker is low impedance compared to the the amplifier which has an output impedance that is (probably) a fraction of an ohm. So even that is actually "bridged". An LDC mic output is maybe 300 ohms. So you probably want your mic input to be around 1200 ohms usually. Why not make it very high? Because it would pickup voltage noise. But that's another discussion entirely!

hans a said:
If so could i begin with calculating impedances that fit with 1:10 "rule" in regards with a fixed value of lets say 200 Ohm.. (Thats what specified in the tech-sheet of the LL..) for R1 and R2? (200 x5 squared? x10?)
So let's work out your example circuit above. The input impedance of your basic common collector amplifier is going to be the impedance of the capacitor in series with R1 in parallel with R2 in series with the impedance of the semiconductor junction of the transistor which is the beta of transistor times the impedance at the emitter. If we assume the input capacitor is large, it's impedance will be low compared to R1||R2 (the || means in-parallel) and since the beta of a typical transistor is ~150, even a small Re will make the transistor base junction high impedance compared to R1||R2 so that means that the impedance is in fact mostly going to be defined by R1||R2.

However, there is a capacitor across Re in the above schematic. That changes everything! A large capacitor compared to Re will make the impedance seen by the semiconductor junction very low. However, the emitter of a bipolar transistor has some builtin resistance of around 25 ohms or so. So 25 ohms * 150 is 3.75K. So now the base looks like 3.75K to ground which means the impedance becomes more like R1||R2||3.75K. Vaguely.

WARNING: You seem to be at an early stage of learning about electronics. This is dangerous. If you have a regular job, you might want to focus on that instead. Electronics can be very addictive. Before you know it, you will be buying transformers and knobs and all sorts of s**t your woman will not understand. OTOH, if you're a single retiree with pleasant touch of Asperger's, then welcome to the party!
 
"WARNING: You seem to be at an early stage of learning about electronics. This is dangerous. If you have a regular job, you might want to focus on that instead. Electronics can be very addictive. Before you know it, you will be buying transformers and knobs and all sorts of s**t your woman will not understand. OTOH, if you're a single retiree with pleasant touch of Asperger's, then welcome to the party!"

Thanks, haha! Well your right about that (i´m sorting 74xx logic chips for fun this weekend) Having asp. in the dignosis too doesnt help hehe.

I think you made everything very clear, a really good explanation. Thanks very much! Basically there inhibition and reactance depending on electrical fields and different materials.

Yes thats the circuit a was thinking of. I will start with that since it is "simple". And you can always stuff all the vintage capacitors you collected in a drawer until the pledges and so on hehe.
 
"WARNING: You seem to be at an early stage of learning about electronics. This is dangerous. If you have a regular job, you might want to focus on that instead. Electronics can be very addictive. Before you know it, you will be buying transformers and knobs and all sorts of s**t your woman will not understand. OTOH, if you're a single retiree with pleasant touch of Asperger's, then welcome to the party!"

Squarewave FTW!!

I can warmly recommend the N.E:E.Ts ( https://www.fcctests.com/neets/Neets.htm )

The reason being that sometimes it can be hard to find material that doesn't overload  on the math when starting out.  If you are a Math person this can be No problem but if you spent the last 25 years forgetting  everything learned about Derivatives and X/Y like me, then the N.E.E.Ts are EXACTLY what you want. In a matter of time you will read up on the Math too, but by the you know what its for!


S
 
Thanks. That was a great link. Seems like it covers a whole lot of useful topics. I have to read up on virtual earth and positive ground (germanium pre-amp). Maybe post the questions and try to find a better understanding on the link.

Virtual earth. Negative feed-back with gain. The opamp is tricked to create potential by 'sinking' virtual earth to and beyond ground reference while resistance is added?

Q2 im going to try to hook up a couple of 2 old germanium preamps. Model Philips EL3660.

While i have a hunch about how to order and recap them i'm a bit worried about how to ground them correctly. Chassi ground is 24v. What if i want to connect a condenser with external Si phantom power. Im gonna try reading on the forum to see if i find the answer.


 
hans a said:
I have to use ground lift right?
safety grounds are for human protection...

24v is relatively low voltage for a chassis, but high voltage for an audio ground (not same as safety ground), that said chassis are generally bonded to safety ground (also bonded to mains neutral) so that needs to be resolved. Safety ground lifts are not safe or good practice.

JR
 
JohnRoberts said:
safety grounds are for human protection...

24v is relatively low voltage for a chassis, but high voltage for an audio ground (not same as safety ground), that said chassis are generally bonded to safety ground (also bonded to mains neutral) so that needs to be resolved. Safety ground lifts are not safe or good practice.

JR

Ok. I guess i have to think over the usefullness of these preamps. I was a bit aware of the netrual not being a good place for this practice. But i'm not sure about if the preamps really have to be connected between chassis and audio ground. I think maybe im wrong there.  I´f theres a special pin named "chassis" maybe this is supposed to be connected to safety-ground. Which there is..
Thanks / H
 

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