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# Electronic Formulas ### Power

• W = Watt
• J = Joule
• S = Second

, ### Transformer inductance and coupling Where:

• M = Mutual inductance in Henrys
• k = coefficient of coupling
• L_1 Inductance of first winding
• L_2 Inductance of second winding

#### Turns ratio

For Iron Core , Where:

• p subscript refers to Primary winding
• s subscript refers to secondary winding
• N = number of turns

#### Ohms law in magnetics

Where:

• F = repulsive force
• r = distance between poles
• μ permeability ( μ of air = 1)
• H = Magnetizing Force in Oersted
• mmf = Magnetic Motive Force - in Gilberts
• Orested = Gilberts/CM
• β = Flux density in Gauss
• φ = Total flux density in Maxwells
• r = Reluctance
• P = Permeance mmf = φR , ### Impedance

Where

• XL refers to inductive reactance
• XC refers to capacitive reactance  #### Impedance through a transformer #### Series RCL #### Parallel RCL ### Parallel LC Resonance #### Q or Quality factor ### Time constance

Τ = time in seconds to 2/3 rise

Τ = RC ### Power factor #### Standing Wave Ratio (SWR) ### Modulation  Deviation = modulating index Deviation ratio = Deviation / highest modulating frequency

### Transistors

Ic = Ibβ

or • Where

β = beta [/itex] Ib = Base current Ic = Collector current

Transconductance
Transconductance is a contraction of transfer conductance. The old unit of conductance, the mho (ohm spelled backwards), was replaced by the SI unit, the siemens, with the symbol S (1 siemens = 1 ampere per volt). For small signal alternating current, the Transconductance is estimated: Where :

• gm = small signal transconductance
• Thermal voltage
• k Boltzmann constant , • Icq = quiescent point, Q-point, or bias point

Distortion begins somewhere once the base voltage exceeds about 5 to 15mVp-p

If we look at large signals we must use Where :

Gm = Large signal Transconductance (not to be confused with gm
• Small-signal-input-resistance ( Rπ )for a common emitter amp with the emitter AC grounded: Which means that input resistance goes up with β

• Large-signal-Input-resistance ( RΠ )for a common emitter amp with the emitter AC grounded: ### Ebers-Moll equation  (approximately 26 mV at 300 K ≈ room temperature).

where

• VT is the Boltzmann constant kT / q
• q Electronic Charge
• TTemperature in K
• IE is the emitter current
• IC is the collector current
• αF is the common base forward short circuit current gain (0.98 to 0.998)
• IES is the reverse saturation current of the base–emitter diode (on the order of 10−15 to 10−12 amperes)
• VBE is the base–emitter voltage

## ESR formulas

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