When a circuit is connected to another circuit we should make sure that the output impedance of the first circuit is as low as possible compared to the input impedance of the second circuit. This makes sure that the voltage drop across the second circuits input impedance is as high as possible.

Nowadays high input impedances and low output impedances are the norm in most circuits.

The input impedance can be represented by a resistor that is connected from the input to ground.

Every circuit that has an input and an output has an input and output impedance.

![[Impedance.png|500]]


The output impedance can be represented as a resistor connected in series with the circuit.

We can also represent these impedances as seperated things:

![[input-output-impedance.png]]



# [[glossary#Input Impedance Z_ in|Input Impedance]]

Input impedance is the impedance seen by anything connected to the input of a circuit. It is the combined effect of all resistance, capacitance and inductance connected to the input side of the circuit.

Usually the input impedance **should be around ten times higher** than the output impedance.

This ensures that the circuit does not overload the source of the signal, and reduce the voltage of the signal by a substantial amount.

```circuitjs
$ 1 0.000005 10.20027730826997 50 5 43 5e-11
R 144 128 48 128 0 0 40 10 0 0 0.5
r 304 128 304 240 0 1000000
g 304 240 304 320 0 0
O 400 128 496 128 1 1
w 304 128 400 128 0
r 144 128 192 128 0 1000
w 192 128 304 128 0
b 251 87 432 374 0
x 252 79 287 82 4 12 Circuit
x 270 189 286 192 4 12 R2
x 162 152 178 155 4 12 R1
o 3 16 0 159746 9.990009990009991 0.0001 0 1
o 0 16 0 159746 10 0.0001 0 2 0 3
38 1 F1 0 1000 1000000 -1 Resistance
```

When we drop the input resistance, the voltage that is provided to our circuit drops.

We can use the voltage divtider equation to calculate the voltage that is available to the circuit:

![[voltage-dividers#Simple Voltage Divider#Equation]]

# [[glossary#Output Impedance Z_ out|Output Impedance]]

The output impedance is the combined effect of all resistors, capacitors and inductors connected to the output inside the circuit.

The output impedance **should be less than a thenth of the input impedance**. 

```circuitjs
$ 1 0.000005 10.20027730826997 50 5 43 5e-11
x 183 158 278 161 4 12 output\simpedance
w 256 128 368 128 2
r 208 128 256 128 0 1
g 368 240 368 320 0 0
r 368 128 368 240 0 1000
R 208 128 112 128 0 0 40 10 0 0 0.5
x 398 190 420 193 4 12 load
b 64 96 287 175 0
x 64 88 95 91 4 12 circuit
38 2 F1 0 1 1000 -1 Resistance
```

If we play with the resistance slider on the right side


# Impedance Matching

Normally the input impedance should be much higher than the output impedance. But in certain cases we want the two to match.

**Example:**

When we connect a Antenna with a $50\ohm$ input impedance to an amplifier circuit. When the input impedance of the antenna is equal to the output impedance of the amplifier we achieve **Maximum Power Transfer**. Power Transfer in this case is $P = VI$.


**Example2:**

![[coax-cable#Impedance Matching in Coax Cable]]