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FN7330.1

The reason for that is the very small but not zero value serial

inductance of the capacitor.

The capacitor will behave as a capacitor up to its resonance

frequency, above the resonance frequency it will behave as

an inductor.

Just 1nHy inductance serial with 1nF capacitance will have

serial resonance at:

C = 1nF, L = 1nHy, F = 159 MHz

And an other 1nHy is very easy to get together with the

inductance of traces on the pcb, and therefore you could

encounter resonances from ca 50MHz and above anywhere.

So if the amplifier has a bandwidth of a few hundred MHz,

the proper power supply by-pass could become a serious if

not difficult task.

Intuitively, you would use capacitors value 0.1µF parallel

with a few µF tantalum, and to cure the effect of it’s serial

resonance put a smaller one parallel to it.

The result will surprise to you, because you will get even

something worse than without the small capacitor.

What is happening there? Just look what we get:

Above its serial resonance C2* the ideal capacitance of C2 is

a short, the Tantalum capacitor for high frequencies is not

effective, the left over is C1 capacitor and L1 + L2 inductors,

we get a parallel tank circuit, which is at it’s resonance a high

impedance path and do not carry any high frequency

current, it does not work as bypass at all!

The impedance of a parallel tank circuit at resonance is

dependent from it’s Q. High Q high impedance.

The Q of a parallel tank circuit could be reduced by

bypassing it with a resistor, or adding a resistor in serial to

one of the reactive components. Since the bypassing would

short the DC supply we do have to go to add resistor in serial

to the reactive component, we will ad a resistor serial with

the inductor. (See Figure 33.)

The final power supply bypass circuit will look:

FIGURE 31.

Ci

Li

Z

F

F RES

C

L

2

1

F

×

π

=

C1

1n

C3

22

µF

C2

0.1

µF

L2

L1

C1

1n

C3

22

µF

C2

0.1

µF

=

<

FIGURE 32.

C3

0.1

µF

L3

C1

Z

F

F RES

R3 = 3

R3 = 0

R3

2 to 3

Ω

FIGURE 33.

C1

1n

C11

22

µF

R10

3R3

C12

33nF

Vs+

FIGURE 34.

EL5100, EL5101, EL5300