If you're relatively new to understanding guitar pedals and electronics in general, you may have encountered the term 'impedance matching' in several different places with various contexts. Before we talk about impedance matching, let's talk about impedance.
This article is for absolute beginners, if you already know what impedance is, you can quickly read the summary, and move on to impedance matching.
Impedance
You may already know that when you have a bunch of resistors, connected together you can identify series/parallel combinations and find the equivalent resistance of the network.
However, in almost any AC circuit, you will find two other components that offer hindrance to current, namely capacitors and inductors. So, in order to account for the total hindrance to current offered by resistors, inductors, and capacitors, we define a new metric called Impedance (denoted by Z).
Basically, impedance is 'resistance' in the AC Realm. Impedance too is measured in Ohms
Z= R + X
where R is the resistance (because of resistors, measured in Ohms)
and X is called reactance (because of capacitors and inductors, measured in ohms)
The reason why we write 'reactance' as a separate term is that capacitors and inductors offer hindrance to current in a very different way.
What a capacitor does, is, it makes the voltage of the source (that goes up and down like a sine wave) lag behind the current(that also goes up and down like a sine wave). And what an inductor does is gives the voltage a headstart when racing with the current. And the resistors don't do anything as such, they allow the voltage and current to stay in the same phase. So, guess what, you can reduce the impedance of a circuit consisting of too many capacitors by adding inductors!
This is because the reactance of capacitors and inductors are a function of frequency.
The way you can calculate equivalent resistance in a resistor network, there's a similar way to do so for a circuit consisting of resistors, inductors, and capacitors. But for this article, let's stick to what's relevant.
How do you measure impedance?
In the context of amplifier/pedal circuits, you may want to measure either the input impedance or the output impedance, either way, the technique remains the same.
While signal generators and oscilloscopes are not the pieces of equipment in an average DIYer's toolbox. The idea of the section is just to explain how it is done practically. In fact, even while you design a pedal, you won't need to do any of this. While designing the circuit, you can choose components that allow you to decide the input and output impedances!
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| Fig 1: Setup to measure the output impedance |
To measure output impedance
1. Hook up the input of the circuit to some AC source which has a fixed amplitude, like a signal generator.
2. Measure the output voltage using either an AC voltmeter or an oscilloscope.
3. Connect a variable resistor (simply use any two adjacent legs of a potentiometer) between output and ground.
4. Adjust the variable resistor/potentiometer until the output voltage becomes half of what you had initially measured.
5. Since the voltage across the variable resistor is half you know that the other half of the voltage drops across the output. So, measure the resistance of the potentiometer using a multimeter (in ohmmeter mode)
and that's the output impedance of the circuit!
6. To verify that your reading is correct, repeat all the steps with a different frequency of the input signal. You should get the same value of output impedance.
To measure the input impedance
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| Fig 2: Setup to measure the input impedance |
1. Hook up the input of the circuit to some AC source which has a fixed amplitude, like a signal generator.
2. Connect a variable resistor (simply use any two adjacent legs of a potentiometer) between circuit signal generator and circuit input. Also, connect a resistor around 1k between output and ground. This will work as a load resistance.
3. Measure the output voltage using either an AC voltmeter or an oscilloscope.
4. Adjust the variable resistor/potentiometer until the output voltage becomes half of what you had initially measured.
5. Since the output across the variable resistor is half, you know that the other half of the voltage drops across the circuit's input. So, measure the resistance of the potentiometer using a multimeter (in ohmmeter mode)
and that's the input impedance of the circuit!
6. To verify that your reading is correct, repeat all the steps with a different frequency of the input signal. You should get the same value of input impedance.
To start with, life is easy when you have a DC circuit consisting of resistors and LEDs, etc. You can easily calculate the resistance. But when you're dealing with AC circuits (such as those involving guitar signals), apart from resistors two other fellows can offer hindrance to the current, namely capacitors and inductors. In a nutshell, the total hindrance offered by all three components in an AC circuit is called impedance. So, like resistance, impedance is also measured in 'Ohms'.
But why call it, impedance and not resistance?
The thing is capacitors and inductors do not exactly resist the current. What a capacitor does, is, it makes the voltage of the source (that goes up and down like a sine wave) lag behind the current(that also goes up and down like a sine wave). And what an inductor does is gives the voltage a headstart when racing with the current. And the fellow resistors don't do anything as such, the allow the voltage and current to stay in the same phase. So, guess what, you can reduce the impedance of a circuit consisting of too many capacitors by adding inductors!
Next page- Impedance Matching
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