# DC equivalent circuit for selective amplifier

#### georgefrenk

Joined Dec 24, 2023
54
I have LC selective amplifier which has real inductor with some DC resistance in series with inductor - see attached file RLC.png
If I have inductor before me and measure it's resistance with multimeter, should I put it's resistance
as a collector resistor inside DC equivalent circuit? Inductors for higher frequencies are usually physically smaller, so the
default DC resistance is small also - in this case no need to put such resistor with smaller value as collector resistor in DC equivalent circuit.
But what about otherwise? Please see my equations of first attached photo (which has only L and C in parallel). Such resistor has fixed value, I don't see where should I use it's voltage or resistance, in which equation should I put it and how - I am talking about equations on attached photo. Can I ignore DC resistance of real inductor for DC equivalent circuit in any case?

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#### Alec_t

Joined Sep 17, 2013
14,381
Can I ignore DC resistance of real inductor for DC equivalent circuit in any case?
It's ok to ignore it when calculating your bias resistor values (bearing in mind resistor tolerances and transistor beta tolerance), but it will affect the Q value of your tuned circuit.

#### crutschow

Joined Mar 14, 2008
34,693
Inductors for higher frequencies are usually physically smaller, so the
default DC resistance is small also
There is no particular correlation between the physical size of an inductor and its resistance.
It depends upon the inductance and the wire size used for the inductor.

You should include the inductor resistance in your calculations, if the inductor resistance is a significant percentage of the total.

#### georgefrenk

Joined Dec 24, 2023
54
There is no particular correlation between the physical size of an inductor and its resistance.
It depends upon the inductance and the wire size used for the inductor.

You should include the inductor resistance in your calculations, if the inductor resistance is a significant percentage of the total.
I already use equation for calculating AC voltage gain, which incorporates also the inductor DC resistance beside reactance.
In this forum topic I am asking specifically about DC equivalent circuit and belonging equations.

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#### georgefrenk

Joined Dec 24, 2023
54
It's ok to ignore it when calculating your bias resistor values (bearing in mind resistor tolerances and transistor beta tolerance), but it will affect the Q value of your tuned circuit.
I tested circuit by increasing DC resistance of real inductor and clipping of output signal appears at some point.
The solution is to decrease input voltage of amplifier. But only in case where there is large DC resistance of real inductor.
Otherwise no need for it.

So I came to conclusion that DC resistance of real inductor in collector LC tank affects both output AC voltage gain and DC bias (this one causes clipping of waveform).

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