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?
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:
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[*]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.
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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!