Hi Ronan,

Impedance is everything in E. If you understand the impedance characteristics of each of the 4 electronic devices: resistors, capacitors, magnetics (inductors, transformers, chokes, etc) and semiconductors (diodes, transistors, triacs, unijunction transistors, etc), then you will have a very good understanding of E.

Fortunately the concept of impedance is pretty simple and is just what it sounds like. It refers to how a device impedes the flow of electrons. If the terminal of a device is high impedance, electrons do not easily flow into or out of it. Low impedance would be electrons can easily flow.

However, the impedance at the terminal of a device depends on the properties of the device and it can also depend on the electrical conditions on other terminals. Let's take a quick run though each of the 4 types of E devices to get a better idea of what I mean:

1) Resistors. Resistors are easy because if the impedance at the other end of the resistor is 0, the impedance of it is just the resistor value.

Consider your 1K into op amp with 10K to ground. When talking about impedance, we're talking about the impedance at a "net". A "net" is some network of devices connected together. So "In" is a net, the intersection between the 1K, op amp + input and 10K is a net. Ground is also a net but it's impedance is assumed to be 0 for basic discussions.

So what is the impedance at net "In"? We there's only one device connected (in practice this would not be true of course!) so its' the impedance of the 1K in series with 10K in parallel with the op amp to ground. For basic discussions the impedance of the op amp is assumed to be infinite and again, ground is 0. So that just leaves 1K in series with 10K or 11K.

So what is the impedance at the intersection between 1K, the op amp + input and 10K? Well there's nothing at the other end of the 1K in your drawing so we have to pretend for now that it's infinite. And the op amp + is infinite. So that just leaves 10K to ground which is 0 which means the impedance is 10K.

However, in a real circuit the impedance on the other end of the 1K would not be infinite and therefore the impedance of the intersection net would not be 10K. But this is commonly overlooked because what we usually want to know about the impedance of a device is what it's "input impedance" is such that if we plugged something into it, would it be loaded or not. And so for this example, if we say were "looking into the input impedance" of this circuit, it would be 11K.

2) Capacitors. Capacitors are slightly more complicated because their impedance depends on frequency. At very low frequency, they are high impedance. In fact, at DC the impedance is infinite because no current flows at all. But at high frequency the impedance is very low. This is because the capacitor is like a really fast battery and it will just sink or source electrons immediately. If you have a capacitor that has no charge and you instantaneously put 1V across it, the initial voltage will be 0V but it will start to rise as it charges up eventually settling on 1V. How quickly it charges up depends on two things, the impedance of the source of current and the size of the capacitor. If you make the source a very low impedance but through a resistor of say 10K and a capacitor of 10nF, then you have a filter more commonly referred to as an RC filter. A high frequency voltage will just be completely absorbed by the capacitor where as low frequencies will pass. Meaning you have a low pass filter.

3) Magnetics. Magnetics can get very complicated with devices like transformers. But the simplest magnetic device is an inductor and an inductor is basically the inverse of a capacitor. When you instantaneously put 1V across it, the initial voltage will be 1V but it will start to drop as it the magnetic field builds up. If you take a 1H inductor and 10K resistor to ground, you again have a low pass filter.

4) Semiconductors. Semiconductors are too complicated to describe in one post but consider a diode. When the voltage across a diode is 0V, the impedance is very high. When it gets to about >= 0.6V (for a silicon diode anyway), the impedance is very low. And a transistor base emitter junction is basically a diode which is to say the impedance of a transistor is non trivial. In a circuit like your helios preamp, the circuit is designed so that the Vbe is very narrow. This is facilitated by something called "feedback". In other circuits like say a fuzz circuit, this may not be true. But if the Vbe junction is operated over a narrow "ohmic" region, then the input impedance of a conventional bipolar transistor is approximately the impedance of the net the emitter is connected to multiplied by the Hfe of the transistor which is usually between 100 and 400. So if the only part connected to the emitter is a 1K resistor for example, then you know the input impedance "looking into" the transistor base is going to be at least 100K-400K. But that's just the base. If there are other parts connected to the base you have to consider those to determine the true impedance of that "net".